Robert T Downs
- Professor Emeritus
Contact
- (520) 621-6024
- Gould-Simpson, Rm. 208
- Tucson, AZ 85721
- rdowns@u.arizona.edu
Bio
No activities entered.
Interests
No activities entered.
Courses
2022-23 Courses
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Directed Research
GEOS 492 (Spring 2023)
2020-21 Courses
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Character+Id Of Minerals
GEOS 560 (Spring 2021) -
Thesis
GEOS 910 (Winter 2020)
2019-20 Courses
-
Thesis
GEOS 910 (Summer I 2020) -
Mineral-Petrol-Geochem
GEOS 596A (Spring 2020) -
Research
GEOS 900 (Spring 2020) -
Thesis
GEOS 910 (Spring 2020) -
Master's Report
GEOS 909 (Fall 2019) -
Mineralogy
GEOS 306 (Fall 2019) -
Research
GEOS 900 (Fall 2019) -
Teaching Geosciences
GEOS 397A (Fall 2019)
2018-19 Courses
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Independent Study
GEOS 499 (Spring 2019) -
Master's Report
GEOS 909 (Spring 2019) -
Research
GEOS 900 (Spring 2019) -
Directed Research
GEOS 492 (Fall 2018) -
Independent Study
GEOS 599 (Fall 2018) -
Mineralogy
GEOS 306 (Fall 2018) -
Research
GEOS 900 (Fall 2018) -
Teaching Geosciences
GEOS 397A (Fall 2018)
2017-18 Courses
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Dissertation
GEOS 920 (Spring 2018) -
Research
GEOS 900 (Spring 2018) -
Directed Research
GEOS 492 (Fall 2017) -
Dissertation
GEOS 920 (Fall 2017) -
Geosciences
GEOS 596H (Fall 2017) -
Mineralogy
GEOS 306 (Fall 2017) -
Research
GEOS 900 (Fall 2017) -
Teaching Geosciences
GEOS 397A (Fall 2017)
2016-17 Courses
-
Dissertation
GEOS 920 (Spring 2017) -
Mineral-Petrol-Geochem
GEOS 596A (Spring 2017) -
Research
GEOS 900 (Spring 2017) -
Dissertation
GEOS 920 (Fall 2016) -
Independent Study
GEOS 599 (Fall 2016) -
Research
GEOS 900 (Fall 2016)
2015-16 Courses
-
Directed Research
GEOS 492 (Spring 2016) -
Dissertation
GEOS 920 (Spring 2016) -
Independent Study
GEOS 599 (Spring 2016) -
Master's Report
GEOS 909 (Spring 2016) -
Mineral-Petrol-Geochem
GEOS 596A (Spring 2016) -
Research
GEOS 900 (Spring 2016)
Scholarly Contributions
Books
- Henderson, G., Neuville, D., & Downs, R. (2014). Spectroscopic Methods in Mineralogy and Materials Sciences, Reviews in Mineralogy and Geochemistry. Washington DC: Mineralogical Society of America.More infoVolume: 78. Editors: Henderson, GS | Neuville, DR | Downs, RT
Chapters
- Lavina, B., Dera, P., & Downs, R. (2014). Modern X-ray diffraction methods in mineralogy and geosciences. In Reviews in Mineralogy & Geochemistry(pp 1-31).More infoVolume: 78
- Hazen, R., Downs, R., Jones, A., & Kah, L. (2013). Carbon mineralogy and crystal chemistry. In Reviews in Mineralogy and Geochemistry (pp 7-46).More infoVolume: 75
- Hazen, R., Downs, R., Kah, L., & Sverjensky, D. (2013). Carbon mineral evolution. In Reviews in Mineralogy and Geochemistry(pp 79-107).More infoVolume: 75
Journals/Publications
- Downs, R. T. (2017). Crystal chemistry of martian minerals from Bradbury Landing through Naukluft Plateau, Gale crater, Mars.. American Mineralogist. doi:10.2138/am-2018-6124
- Downs, R. T. (2017). Hydroxycalciomicrolite, Ca1.5Ta2O6(OH), a new member of the microlite group from Volta Grande pegmatite, Nazareno, Minas Gerais, Brazil.. Mineralogical Magazine, 81, 555-564.
- Downs, R. T. (2017). In memoriam: William Wallace Pinch (1940-2017). Rocks & Minerals, 92, 485-488.
- Downs, R. T. (2017). Mineralogy of an active eolian sediment from the Namib dune, Gale crater, Mars.. Journal of Geophysical Research: Planets, 122. doi:10.1002/2017JE005262
- Downs, R. T. (2017). Mineralogy of an ancient lacustrine mudstone succession from the Murray formation, Gale crater, Mars.. Earth and Planetary Science Letters, 471, 172-185.
- Downs, R. T. (2017). Multiple stages of aqueous alteration along fractures in mudstone and sandstone strata in Gale Crater, Mars. Earth and Planetary Science Letters, 471, 186-198.
- Downs, R. T. (2017). New data on hemihedrite from Arizona.. Mineralogical Magazine, 81, 1021-1030.
- Downs, R. T. (2017). Relationships between unit-cell parameters and composition for rock-forming minerals on Earth, Mars, and other extraterrestrial bodies. American Mineralogist. doi:10.2138/am-2018-6123
- Hazen, R. M., Grew, E. S., Origlieri, M. J., & Downs, R. T. (2017). On the mineralogy of the "Anthropocene Epoch". AMERICAN MINERALOGIST, 102(3), 595-611.
- Hazen, R. M., Hystad, G., Golden, J. J., Hummer, D. R., Liu, C., Downs, R. T., Morrison, S. M., Ralph, J., & Grew, E. S. (2017). Cobalt mineral ecology. AMERICAN MINERALOGIST, 102(1-2), 108-116.
- Hummer, D. R., Noll, B. C., Hazen, R. M., & Downs, R. T. (2017). Crystal structure of abelsonite, the only known crystalline geoporphyrin. AMERICAN MINERALOGIST, 102(5), 1129-1132.
- Hystad, G., Downs, R. T., Hazen, R. M., & Golden, J. J. (2017). Relative Abundances of Mineral Species: A Statistical Measure to Characterize Earth-like Planets Based on Earth's Mineralogy. MATHEMATICAL GEOSCIENCES, 49(2), 179-194.
- Liu, C., Hystad, G., Golden, J. J., Hummer, D. R., Downs, R. T., Morrison, S. M., Ralph, J. P., & Hazen, R. M. (2017). Chromium mineral ecology. AMERICAN MINERALOGIST, 102(3), 612-619.
- Ma, X., Hummer, D., Golden, J. J., Fox, P. A., Hazen, R. M., Morrison, S. M., Downs, R. T., Madhikarmi, B. L., Wang, C., & Meyer, M. B. (2017). Using Visual Exploratory Data Analysis to Facilitate Collaboration and Hypothesis Generation in Cross-Disciplinary Research. ISPRS INTERNATIONAL JOURNAL OF GEO-INFORMATION, 6(11).
- Morrison, S. M., Liu, C., Eleish, A., Prabhu, A., Li, C., Ralph, J., Downs, R. T., Golden, J. J., Fox, P., Hummemmemmer, D. R., Meyer, M. B., & Hazen, R. M. (2017). Network analysis of mineralogical systems. AMERICAN MINERALOGIST, 102(8), 1588-1596.
- Schumer, B. N., Andrade, M. B., Evans, S. H., & Downs, R. T. (2017). A new formula and crystal structure for nickelskutterudite, (Ni,Co,Fe)As-3, and occupancy of the icosahedral cation site in the skutterudite group. AMERICAN MINERALOGIST, 102(1-2), 205-209.
- Schumer, B. N., Yang, H., & Downs, R. T. (2017). Natropalermoite, Na2SrAl4(PO4)(4)(OH)(4), a new mineral isostructural with palermoite, from the Palermo No. 1 mine, Groton, New Hampshire, USA. MINERALOGICAL MAGAZINE, 81(4), 833-840.
- Shannon, R. C., Lafuente, B., Shannon, R. D., Downs, R. T., & Fischer, R. X. (2017). Refractive indices of minerals and synthetic compounds. AMERICAN MINERALOGIST, 102(9), 1906-1914.
- Xu, J., Zhang, D., Fan, D., Downs, R. T., Hu, Y. i., & Dera, P. K. (2017). Isosymmetric pressure-induced bonding increase changes compression behavior of clinopyroxenes across jadeite-aegirine solid solution in subduction zones. JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH, 122(1), 142-157.
- Downs, R. T., Andrade, M., Barton, I. F., & Yang, H. (2016). Crystal structure of a new compound, CuZnCl(OH)3, isostructural with botallackite. American Mineralogist, 101, 986-990.
- Downs, R. T. (2015). Crystal structure of tetrawickmanite, Mn2+Sn4+(OH)6. Acta Crystallographica, E71, 234-237.
- Downs, R. T. (2015). Determining mineralogy on Mars with the CheMin X-ray diffractometer. Elements, 11, 45-50.
- Downs, R. T. (2015). Hydrothermal synthesis and crystal structure of AlSO4(OH): A titanite-group member. American Mineralogist, 100, 330-333.
- Barton, I. F., Jenkins, R., Downs, R. T., Yang, H., & Andrade, M. (2014). Te-rich raspeite, Pb(W0.56Te0.44)O4, from Tombstone, Arizona, USA: the first natural example of Te6+ substitution for W6+. American Mineralogist, 99(7), 1507-1510.
- Downs, R. T. (2014). A new biogenic, struvite-related phosphate, the ammonium-analog of hazenite, (NH4)NaMg2(PO4)2·14H2O.. American Mineralogist, 99, 1761-1766.
- Downs, R. T. (2014). Downs R T (2014) Pauling's rules, in a world of non-spherical atoms. American Mineralogist, 99, 1817-1817.
- Downs, R. T. (2014). Ferrian saponite from the Santa Monica Mountains (California, U.S.A., Earth): Characterization as an analog for clay minerals on Mars with application to Yellowknife Bay in Gale Crater. American Mineralogist, 99, 2234-2250.
- Downs, R. T. (2014). High-pressure single-crystal X-ray diffraction study of jadeite and kosmochlor. Physics and Chemistry of Minerals, 41, 695-707.
- Downs, R. T. (2014). Mineral evolution: Episodic metallogenesis, the supercontinent cycle, and the coevolving geosphere and biosphere. Society of Economic Geologists Special Publication, 18, 1-15.
- Downs, R. T. (2014). Te-rich raspite, Pb(W0.56Te0.44)O4, from Tombstone, Arizona, U.S.A.: The first natural example of Te6+ substitution for W6+.. American Mineralogist, 99, 1507-1510.
- Downs, R. T. (2014). The first X-ray diffraction measurements on Mars. IUCrJ, 1, 514-522.
- Lafuente, B., Downs, R. T., Yang, H., & Jenkins, R. A. (2014). Calcioferrite with composition (Ca3.94Sr0.06)Mg 1.01 (Fe2.93Al1.07)(PO4) 6- (OH)4·12H2O. Acta Crystallographica Section E: Structure Reports Online, 70(3), i16-i17.More infoAbstract: Calcioferrite, ideally Ca4MgFe3+ 4(PO 4)6(OH)4·12H2O (tetracalcium magnesium tetrairon(III) hexakis-phosphate tetrahydroxide dodecahydrate), is a member of the calcioferrite group of hydrated calcium phosphate minerals with the general formula Ca4 AB 4(PO4)6(OH)4·12H2O, where A = Mg, Fe2+, Mn2+ and B = Al, Fe3+. Calcioferrite and the other three known members of the group, montgomeryite (A = Mg, B = Al), kingsmountite (A = Fe2+, B = Al), and zodacite (A = Mn2+, B = Fe3+), usually occur as very small crystals, making their structure refinements by conventional single-crystal X-ray diffraction challenging. This study presents the first structure determination of calcioferrite with composition (Ca3.94Sr0.06)Mg 1.01 (Fe2.93Al1.07)(PO4) 6- (OH)4·12H2O based on single-crystal X-ray diffraction data collected from a natural sample from the Moculta quarry in Angaston, Australia. Calcioferrite is isostructural with montgomeryite, the only member of the group with a reported structure. The calcioferrite structure is characterized by (Fe/Al)O6 octahedra (site symmetries 2 and -1) sharing corners (OH) to form chains running parallel to [101]. These chains are linked together by PO4 tetrahedra (site symmetries 2 and 1), forming [(Fe/Al)3(PO4)3(OH)2] layers stacking along [010], which are connected by (Ca/Sr)2+ cations (site symmetry 2) and Mg 2+ cations (site symmetry 2; half-occupation). Hydrogen-bonding interactions involving the water molecules (one of which is equally disordered over two positions) and OH function are also present between these layers. The relatively weaker bonds between the layers account for the cleavage of the mineral parallel to (010).
- Vaniman, D. T., Bish, D. L., Ming, D. W., Bristow, T. F., Morris, R. V., Blake, D. F., Chipera, S. J., Morrison, S. M., Treiman, A. H., Rampe, E. B., Rice, M., Achilles, C. N., Grotzinger, J. P., McLennan, S. M., Williams, J., III, J. B., Newsom, H. E., Downs, R. T., Maurice, S., , Sarrazin, P., et al. (2014). Mineralogy of a mudstone at Yellowknife Bay, Gale crater, Mars. Science, 343(6169).More infoPMID: 24324271;Abstract: Sedimentary rocks at Yellowknife Bay (Gale crater) on Mars include mudstone sampled by the Curiosity rover. The samples, John Klein and Cumberland, contain detrital basaltic minerals, calcium sulfates, iron oxide or hydroxides, iron sulfides, amorphous material, and trioctahedral smectites. The John Klein smectite has basal spacing of ∼10 angstroms, indicating little interlayer hydration. The Cumberland smectite has basal spacing at both ∼13.2 and ∼10 angstroms. The larger spacing suggests a partially chloritized interlayer or interlayer magnesium or calcium facilitating H2O retention. Basaltic minerals in the mudstone are similar to those in nearby eolian deposits. However, the mudstone has far less Fe-forsterite, possibly lost with formation of smectite plus magnetite. Late Noachian/Early Hesperian or younger age indicates that clay mineral formation on Mars extended beyond Noachian time.
- Yang, H., Downs, R. T., Evans, S. H., & Pinch, W. W. (2014). Lavinskyite, K(LiCu)Cu6(Si4O11) 2(OH)4, isotypic with plancheite, a new mineral from the Wessels mine, Kalahari Manganese Fields, South Africa. American Mineralogist, 99(2-3), 525-530.More infoAbstract: A new mineral species, lavinskyite, ideally K(LiCu2+)Cu 62+(Si4O11)2(OH) 4 (IMA 2012-028), has been found in the Wessels mine, Kalahari Manganese Fields, Northern Cape Province, South Africa. Associated minerals include wesselsite, pectolite, richterite, sugilite, and scottyite. Lavinskyite crystals are tabular [parallel to (010)]. The mineral is light blue, transparent with very pale blue streak and vitreous luster. It is brittle and has a Mohs hardness of 5; cleavage is perfect on {010} and no parting was observed. The measured and calculated densities are 3.61(3) and 3.62 g/cm3, respectively. Optically, lavinskyite is biaxial (+), with α = 1.675(1), β = 1.686(1), γ = 1.715(1), 2Vmeas = 64(2) . An electron microprobe analysis produced an average composition (wt%) of SiO2 42.85(10), CuO 46.13(23), K2O 4.16(2), MgO 1.53(17), Na2O 0.27(4), BaO 0.18(6), and MnO 0.08(1), plus Li2O 1.38 from the LA-ICP-MS measurement and H2O 3.22 (added to bring the analytical total close to 100%), yielding a total of 99.79% and an empirical chemical formula (K0.99Ba0.01)Σ=1.00(Li 1.04Cu0.93Na0.10)Σ=2.07 (Cu5.57Mg0.43Mn0.01) Σ=6.01(Si4.00O11)2(OH) 4. Lavinskyite is isotypic with plancheite, Cu8(Si 4O11)2(OH)4·H2O, an amphibole derivative. It is orthorhombic, with space group Pcnband unit-cell parameters α = 19.046(2), β = 20.377(2), γ = 5.2497(6) Å, and V = 2037.4(4) Å3. The key difference between lavinskyite and plancheite lies in the coupled substitution of K + and Li+ in the former for H2O and Cu 2+ in the latter, respectively. The structure of lavinskyite is characterized by the undulating, brucite-like layers consisting of three distinct octahedral sites occupied mainly by Cu. These layers are sandwiched by the amphibole-type double silicate chains extending along the c axis, forming a sheet structure of compact silicate-Cu-silicate triple layers. Adjacent sheets are linked together by K and M4 (= Cu + Li) cations, as well as hydrogen bonding. The M4 site is split, with Cu and Li occupying two different sites. Lavinskyite exhibits more amphibole-like structural features than plancheite, as a consequence of K in the large cavity between the two back-to-back double silicate chains. © 2014 by Walter de Gruyter GmbH & Co. 2014.
- Andrade, M. B., Doell, D., Downs, R. T., & Yang, H. (2013). Redetermination of katayamalite, KLi3Ca7Ti 2(SiO3)12(OH)2. Acta Crystallographica Section E: Structure Reports Online, 69(7), i41.More infoPMID: 24046542;PMCID: PMC3772399;Abstract: The crystal structure of katayamalite, ideally KLi3Ca7Ti2(SiO3)12(OH)2 (potassium trilithium heptacalcium dititanium dodecasilicate dihydroxide), was previously reported in triclinic symmetry (C-1), with isotropic displacement parameters for all atoms and without the H-atom position [Kato & Murakami (1985). Mineral. J. 12, 206-217]. The present study redetermines the katayamalite structure with monoclinic symmetry (space group C2/c) based on single-crystal X-ray diffraction data from a sample from the type locality, Iwagi Island, Ehime Prefecture, Japan, with anisotropic displacement parameters for all non-H atoms, and with the H atoms located by difference Fourier analysis. The structure of katayamalite contains a set of six-membered silicate rings interconnected by sheets of Ca atoms on one side and by an ordered mixture of Li, Ti and K atoms on the other side, forming layers which are stacked normal to (001). From the eight different metal sites, three are located on special positions, viz. one K and one Li atom on twofold rotation axes and one Ca atom on an inversion center. The Raman spectrum of kataymalite shows a band at 3678 cm-1, similar to that observed for hydroxyl-amphiboles, indicating no or very weak hydrogen bonding.
- Bish, D. L., Blake, D. F., Vaniman, D. T., Chipera, S. J., Morris, R. V., Ming, D. W., Treiman, A. H., Sarrazin, P., Morrison, S. M., Downs, R. T., Achilles, C. N., Yen, A. S., Bristow, T. F., Crisp, J. A., Morookian, J. M., Farmer, J. D., Rampe, E. B., Stolper, E. M., & Spanovich, N. (2013). X-ray diffraction results from mars science laboratory: Mineralogy of rocknest at Gale crater. Science, 341(6153).More infoPMID: 24072925;Abstract: The Mars Science Laboratory rover Curiosity scooped samples of soil from the Rocknest aeolian bedform in Gale crater. Analysis of the soil with the Chemistry and Mineralogy (CheMin) x-ray diffraction (XRD) instrument revealed plagioclase (∼An57), forsteritic olivine (∼Fo62), augite, and pigeonite, with minor K-feldspar, magnetite, quartz, anhydrite, hematite, and ilmenite. The minor phases are present at, or near, detection limits. The soil also contains 27 ± 14 weight percent x-ray amorphous material, likely containing multiple Fe3+- and volatile-bearing phases, including possibly a substance resembling hisingerite. The crystalline component is similar to the normative mineralogy of certain basaltic rocks from Gusev crater on Mars and of martian basaltic meteorites. The amorphous component is similar to that found on Earth in places such as soils on the Mauna Kea volcano, Hawaii.
- Blake, D. F., Morris, R. V., Kocurek, G., Morrison, S. M., Downs, R. T., Bish, D., Ming, D. W., Edgett, K. S., Rubin, D., Goetz, W., Madsen, M. B., Sullivan, R., Gellert, R., Campbell, I., Treiman, A. H., McLennan, S. M., Yen, A. S., Grotzinger, J., Vaniman, D. T., , Chipera, S. J., et al. (2013). Curiosity at Gale Crater, Mars: Characterization and analysis of the rocknest sand shadow. Science, 341(6153).More infoPMID: 24072928;Abstract: The Rocknest aeolian deposit is similar to aeolian features analyzed by the Mars Exploration Rovers (MERs) Spirit and Opportunity. The fraction of sand
- Bowman, M. G., Downs, R. T., & Yang, H. (2013). Penikisite, BaMg2Al2(PO4) 3(OH)3, isostructural with bjarebyite. Acta Crystallographica Section E: Structure Reports Online, 69(2), i4-i5.More infoPMID: 23424395;PMCID: PMC3569169;Abstract: The bjarebyite group of minerals, characterized by the general formula BaX 2 Y 2(PO4)3(OH)3, with X = Mg, Fe2+ or Mn 2+, and Y = Al or Fe3+, includes five members: bjarebyite BaMn2+ 2Al2(PO4)3(OH)3, johntomaite BaFe 2+ 2Fe3+ 2(PO4)3(OH)3, kulanite BaFe2+ 2Al2(PO4)3(OH)3, penikisite BaMg2Al2(PO4)3(OH)3, and perloffite BaMn2+ 2Fe3+ 2(PO4)3(OH)3. Thus far, the crystal structures of all minerals in the group, but penikisite, have been determined. The present study reports the first structure determination of penikisite (barium dimagnesium dialuminium triphosphate trihydroxide) using single-crystal X-ray diffraction data of a crystal from the type locality, Mayo Mining District, Yukon Territory, Canada. Penikisite is isotypic with other members of the bjarebyite group with space group P21/m, rather than triclinic (P1 or P-1), as previously suggested. Its structure consists of edge-shared [AlO 3(OH)3] octahedral dimers linking via corners to form chains along [010]. These chains are decorated with PO4 tetrahedra (one of which has site symmetry m) and connected along [100] via edge-shared [MgO5(OH)] octahedral dimers and eleven-coordinated Ba2+ ions (site symmetry m), forming a complex three-dimensional network. O - H⋯O hydrogen bonding provides additional linkage between chains. Microprobe analysis of the crystal used for data collection indicated that Mn substitutes for Mg at the 1.5% (apfu) level.
- Dera, P., Finkelstein, G. J., Duffy, T. S., Downs, R. T., Meng, Y., Prakapenka, V., & Tkachev, S. (2013). Metastable high-pressure transformations of orthoferrosilite Fs82. Physics of the Earth and Planetary Interiors, 221, 15-21.More infoAbstract: High-pressure single-crystal X-ray diffraction experiments with natural ferrosilite Fs82 (Fe2+0.82Mg0.16Al0.01Ca0.01)(Si0.99Al0.01)O3 orthopyroxene (opx) reveal that at ambient temperature the sample does not transform to the clinopyroxene (cpx) structure, as reported earlier for a synthetic Fs100 end-member (Hugh-Jones et al., 1996), but instead undergoes a series of two polymorphic transitions, first above 10.1(1)GPa, to the monoclinic P21/c phase β-opx (distinctly different from both P21/c and C2/c cpx), also observed in natural enstatite (Zhang et al., 2012), and then, above 12.3(1)GPa to a high-pressure orthorhombic Pbca phase γ-opx, predicted for MgSiO3 by atomistic simulations (Jahn, 2008). The structures of phases α, β and γ have been determined from the single-crystal data at pressures of 2.3(1), 11.1(1), and 14.6(1)GPa, respectively. The two new high-pressure transitions, very similar in their character to the P21/c-C2/c transformation of cpx, make opx approximately as dense as cpx above 12.3(1)GPa and significantly change the elastic anisotropy of the crystal, with the [100] direction becoming almost twice as stiff as in the ambient α-opx phase. Both transformations involve mainly tetrahedral rotation, are reversible and are not expected to leave microstructural evidence that could be used as a geobarometric proxy. The high Fe2+ content in Fs82 shifts the α-β transition to slightly lower pressure, compared to MgSiO3, and has a very dramatic effect on reducing the (meta) stability range of the β-phase. © 2013 Elsevier B.V.
- Dera, P., Zhuravlev, K., Prakapenka, V., Rivers, M. L., Finkelstein, G. J., Grubor-Urosevic, O., Tschauner, O., Clark, S. M., & Downs, R. T. (2013). High pressure single-crystal micro X-ray diffraction analysis with GSE-ADA/RSV software. High Pressure Research, 33(3), 466-484.More infoAbstract: GSE-ADA/RSV is a free software package for custom analysis of single-crystal micro X-ray diffraction (SCμXRD) data, developed with particular emphasis on data from samples enclosed in diamond anvil cells and subject to high pressure conditions. The package has been in extensive use at the high pressure beamlines of Advanced Photon Source (APS), Argonne National Laboratory and Advanced Light Source (ALS), Lawrence Berkeley National Laboratory. The software is optimized for processing of wide-rotation images and includes a variety of peak intensity corrections and peak filtering features, which are custom-designed to make processing of high pressure SCμXRD easier and more reliable. © 2013 Taylor & Francis.
- Golden, J., McMillan, M., Downs, R. T., Hystad, G., Goldstein, I., Stein, H. J., Zimmerman, A., Sverjensky, D. A., Armstrong, J. T., & Hazen, R. M. (2013). Rhenium variations in molybdenite (MoS2): Evidence for progressive subsurface oxidation. Earth and Planetary Science Letters, 366, 1-5.More infoAbstract: Temporal trends in Earth's near-surface mineralogy correlate with major events in Earth's geochemical and tectonic history. New and published analyses of 422 molybdenite (MoS2) specimens from 135 localities with known ages from 2.91 billion years (Ga) to 6.3 million years (Ma) reveal two statistically significant trends. First, systematic increases in average and maximum trace concentrations of Re in molybdenite since 3.0Ga point to enhanced oxidative weathering by subsurface fluids. In addition, episodic molybdenum mineralization correlates with five intervals of supercontinent assembly from ~2.7Ga (Kenorland) to 300Ma (Pangaea). © 2013 Elsevier B.V.
- Hazen, R. M., Downs, R. T., Jones, A. P., & Kah, L. (2013). Carbon mineralogy and crystal chemistry. Reviews in Mineralogy and Geochemistry, 75, 7-46.
- Hazen, R. M., Downs, R. T., Kah, L., & Sverjensky, D. (2013). Carbon mineral evolution. Reviews in Mineralogy and Geochemistry, 75, 79-107.
- McKay, N. P., Dettman, D. L., Downs, R. T., & Overpeck, J. T. (2013). On the potential of Raman-spectroscopy-based carbonate mass spectrometry. Journal of Raman Spectroscopy, 44(3), 469-474.More infoAbstract: The potential for using Raman spectroscopy to measure stable oxygen isotope ratios (18O/16O) in carbonates is evaluated by measuring the Raman spectra and isotope ratios of a suite of 60 synthesized, 18O-enriched calcite crystals ranging in composition from natural abundance (0.2 mole-% 18O) to 1.2 mole-% 18O. We determined the Raman-inferred isotopic ratios (RRaman) by fitting curves to the ν1 symmetric stretching peak at 1086 cm-1 and the smaller satellite peak, associated with the ν1 stretching mode of singly substituted carbonate groups (C16O218O) at 1065 cm-1. The ratio of the two peak areas shows a 1:1 correspondence with the 18O/16O ratios derived from standard mass spectrometry methods, confirming that the relative intensities of the ν1 symmetric stretching peaks is a direct measure of the isotopic ratio in the carbonates. The 1-sigma uncertainties of the R Raman values of the individual crystals were 0.00079 (384‰ PDB) and 0.00043 (210‰ PDB) for the four-crystal sample means. This level of uncertainty is much too high to provide significant estimates of natural variability; however, there are multiple prospects for improving the accuracy and precision of the technique. Carbon isotope ratios in carbonates cannot be measured by our approach, but our results highlight the potential of Raman-based isotope ratio measurement for C and other elements in minerals and organic compounds. Copyright © 2012 John Wiley & Sons, Ltd.
- Morrison, S. M., Andrade, M. B., Wenz, M. D., Domanik, K. J., & Downs, R. T. (2013). Lanthanite-(Nd), Nd2(CO3)3·8H2O. Acta Crystallographica Section E: Structure Reports Online, 69(3), i15-i16.More infoPMID: 23476479;PMCID: PMC3588547;Abstract: Lanthanite-(Nd), ideally Nd2(CO3)3· 8H2O [dineodymium(III) tricarbonate octahydrate], is a member of the lanthanite mineral group characterized by the general formula REE 2(CO3)3·8H2O, where REE is a 10-coordinated rare earth element. Based on single-crystal X-ray diffraction of a natural sample from Mitsukoshi, Hizen-cho, Karatsu City, Saga Prefecture, Japan, this study presents the first structure determination of lanthanite-(Nd). Its structure is very similar to that of other members of the lanthanite group. It is composed of infinite sheets made up of corner- and edge-sharing of two NdO10-polyhedra (both with site symmetry..2) and two carbonate triangles (site symmetries..2 and 1) parallel to the ab plane, and stacked perpendicular to c. These layers are linked to one another only through hydrogen bonding involving the water molecules. © 2013 Morrison et al.
- Morrison, S. M., Domanik, K. J., Origlieri, M. J., & Downs, R. T. (2013). Agardite-(Y), Cu2+6Y(AsO4) 3(OH)6·3H2O. Acta Crystallographica Section E: Structure Reports Online, 69(9), i61-i62.More infoAbstract: Agardite-(Y), with a refined formula of Cu2+5.70(Y0.69Ca0.31)[(As0.83P 0.17)O4]3(OH)6·3H2O [ideally Cu2+6Y(AsO4)3(OH) 6·3H2O, hexacopper(II) yttrium tris(arsenate) hexahydroxide trihydrate], belongs to the mixite mineral group which is characterized by the general formula Cu2+6A(TO4) 3(OH)6·3H2O, where nine-coordinated cations in the A-site include rare earth elements along with Al, Ca, Pb, or Bi, and the T-site contains P or As. This study presents the first structure determination of agardite-(Y). It is based on the single-crystal X-ray diffraction of a natural sample from Jote West mine, Pampa Larga Mining District, Copiapo, Chile. The general structural feature of agardite-(Y) is characterized by infinite chains of edge-sharing CuO5 square pyramids (site symmetry 1) extending down the c axis, connected in the ab plane by edge-sharing YO9 polyhedra (site symmetry -6..) and corner-sharing AsO4 tetrahedra (site symmetry m..). Hydroxyl groups occupy each corner of the CuO5-square pyramids not shared by a neighboring As or Y atom. Each YO9 polyhedron is surrounded by three tubular channels. The walls of the channels, parallel to the c axis, are six-membered hexagonal rings comprised of CuO5 and AsO4 polyhedra in a 2:1 ratio, and contain free molecules of lattice water. © 2013 Morrison et al.
- Origlieri, M. J., & Downs, R. T. (2013). Schaurteite, Ca3Ge(SO4)2(OH) 6·3H2O. Acta Crystallographica Section E: Structure Reports Online, 69(2), i6.More infoPMID: 23424396;PMCID: PMC3569170;Abstract: This report presents the first crystal structure determination of the mineral schaurteite, ideally Ca3Ge(SO4)2(OH)6·3H2O, tricalcium germanium bis(sulfate) hexahydroxide trihydrate. This single-crystal X-ray diffraction study investigated a natural sample from the type locality at Tsumeb, Namibia. Schaurteite is a member of the fleischerite group of minerals, which also includes fleischerite, despujolsite, and mallestigite. The structure of schaurteite consists of slabs of Ca(O,OH,H2O)8 polyhedra (site symmetry mm2) interleaved with a mixed layer of Ge(OH)6 octahedra (-3m.) and SO4 tetrahedra (3m.). There are two H atoms in the asymmetric unit, both located by full-matrix refinement, and both forming O - H⋯O hydrogen bonds.
- Schumer, B. N., Downs, R. T., Domanik, K. J., Andrade, M. B., & Origlieri, M. J. (2013). Pirquitasite, Ag2ZnSnS4. Acta Crystallographica Section E: Structure Reports Online, 69(2), i8-i9.More infoPMID: 23424398;PMCID: PMC3569172;Abstract: Pirquitasite, ideally Ag2ZnSnS4 (disilver zinc tin tetrasulfide), exhibits tetragonal symmetry and is a member of the stannite group that has the general formula A2BCX 4, with A = Ag, Cu; B = Zn, Cd, Fe, Cu, Hg; C = Sn, Ge, Sb, As; and X = S, Se. In this study, single-crystal X-ray diffraction data are used to determine the structure of pirquitasite from a twinned crystal from the type locality, the Pirquitas deposit, Jujuy Province, Argentina, with anisotropic displacement parameters for all atoms, and a measured composition of (Ag1.87Cu0.13)(Zn0.61Fe0.36Cd0.03)SnS4. One Ag atom is located on Wyckoff site Wyckoff 2a (symmetry -4..), the other Ag atom is statistically disordered with minor amounts of Cu and is located on 2c (-4..), the (Zn, Fe, Cd) site on 2d (-4..), Sn on 2b (-4..), and S on general site 8g. This is the first determination of the crystal structure of pirquitasite, and our data indicate that the space group of pirquitasite is I-4, rather than I-42m as previously suggested. The structure was refined under consideration of twinning by inversion [twin ratio of the components 0.91 (6):0.09 (6)].
- Thompson, R. M., Xie, X., Zhai, S., Downs, R. T., & Yang, H. (2013). A comparison of the Ca3(PO4)2 and CaSiO3 systems, with a new structure refinement of tuite synthesized at 15 GPa and 1300 °c. American Mineralogist, 98(8-9), 1585-1592.More infoAbstract: Tuite, the high-pressure γ-form of the Ca3(PO 4)2 system, has been synthesized from chlorapatite at 15 GPa and 1300 °C using a multi-anvil apparatus. Its crystal structure was determined with single-crystal X ray diffraction. It is isostructural with palmierite, with space group R3̄m and unit-cell parameters a = 5.2522(9) and c = 18.690(3) Å. The structure of tuite is characterized by three distinct polyhedra, PO4, Ca1O12, and Ca2O10, that are translationally interconnected in the sequence of PO 4-Ca2O10-Ca1O12-Ca2O10-PO4 along the c axis. Comparison of the CaSiO3 and Ca3(PO 4)2 polymorphic systems shows a striking resemblance in the evolution of atomic packing arrangements as the polymorphic density increases. In both cases, the Ca atoms are progressively incorporated into the (Ca+O) close-packed monolayers, consistent with the hypothesis that close packing is a consequence of volume decrease as density increases. Based on this observation, we predict a possible high-pressure post-tuite phase.
- Yang, H., Downs, R. T., Evans, S. H., & Pinch, W. W. (2013). Scottyite, the natural analog of synthetic BaCu2Si 2O7, a new mineral from the Wessels mine, Kalahari Manganese Fields, South Africa. American Mineralogist, 98(2-3), 478-484.More infoAbstract: A new mineral species, scottyite, ideally BaCu2Si 2O7, has been found in the Wessels mine, Kalahari Manganese Fields, Northern Cape Province, South Africa. The mineral appears to have formed as a result of a hydrothermal event and is associated with wesselsite, pectolite, richterite, sugilite, and lavinskyite. Scottyite forms blocky grains with striations parallel to the c axis. Crystals are found up to 0.4 × 0.3 × 0.3 mm. No twinning is observed. The mineral is dark-blue in transmitted and under incident lights, transparent with pale blue streak and vitreous luster. It is brittle and has a Mohs hardness of 4∼5; cleavage is perfect on {100} and {010} and no parting was observed. The calculated density is 4.654 g/cm3. Optically, scottyite is biaxial (-), with α = 1.750(1), β = 1.761(1), and γ = 1.765(1), 2Vmeas = 66(2)°. It is insoluble in water, acetone, or hydrochloric acid. An electron microprobe analysis produced an average composition (wt%) (8 points) of CuO 36.98(31), BaO 35.12(16), SiO2 27.01(61), SrO 0.28(5), and Na2O 0.06(2), and total = 99.45(65), yielding an empirical formula (based on 7 O apfu) Ba1.00Sr0.01Na0.01Cu 2.04Si1.97O7. Scottyite is the natural analog of synthetic BaCu2(Si,Ge)2O7, which exhibits novel one-dimensional quantum spin-1/2 antiferromagnetic properties with tunable super-exchange interactions. It is orthorhombic, with space group Pnma and unit-cell parameters a = 6.8556(2), b = 13.1725(2), c = 6.8901(1) Å, and V = 622.21(6) Å3. The structure of scottyite is characterized by flattened CuO4 tetrahedra sharing corners with one another to form chains parallel to the c axis. These chains are interlinked by Si2O 7 tetrahedral dimers and Ba2+. The Ba2+ cations are bonded to seven O atoms in an irregular coordination. The average Si-O, Cu-O, and Ba-O bond lengths are 1.630, 1.941, and 2.825 Å, respectively. Scottyite is topologically related to a group of compounds with the general formula BaM22+Si2O7, where M = Be (barylite and clinobarylite), Fe (andrémeyerite), Mg, Mn, Co, and Zn.
- Yang, H., Downs, R. T., Evans, S. H., & Pinch, W. W. (2013). Terrywallaceite, AgPb(Sb,Bi)3S6, isotypic with gustavite, a new mineral from Mina Herminia, Julcani Mining District, Huancavelica, Peru. American Mineralogist, 98(7), 1310-1314.More infoAbstract: A new mineral species, terrywallaceite, ideally AgPb(Sb,Bi) 3S6, has been found in Mina Herminia, Julcani Mining District, Huancavelica, Peru. It is associated with tetrahedrite, gustavite, barite, and pyrite. Terrywallaceite crystals are lath-shaped, metallic-black, with striations parallel to the elongated direction (the c axis). The mineral is opaque with black streak and metallic luster. It is brittle and has a Mohs hardness of ∼4; cleavage is good on {010} and no parting was observed. Twinning is pervasive on (100). The calculated density is 6.005 g/cm 3. Optically, terrywallaceite is grayish white in polished thin section, with weak bireflectance, weak pleochroism (white to pale gray), and weak anisotropy (gray with bluish tint to bluish black in air). An electron microprobe analysis yielded an empirical formula, based on 6 (S+As) apfu, Ag1.02Pb0.87(Sb1.53Bi1.47)Σ=3. 00(S5.94As0.06)Δ=6.00. Terrywallaceite is a member of the lillianite group and isostructural with P21/c gustavite. Its unit-cell parameters are a = 6.9764(4), b = 19.3507(10), c = 8.3870(4) Å, b = 107.519(2)°, and V = 1079.7(1) Å3. The structure of terrywallaceite contains six symmetrically-nonequivalent S sites and five cation sites [Ag, Pb, M1 (=0.82Bi + 0.18Sb), M2 (=0.60Bi + 0.40Sb), and M3 (=0.95Sb + 0.05Bi)]. The pronounced preference of Sb for the M3 site over M2 and M1 in terrywallaceite is consistent with the site occupancy data reported for Sb-bearing gustavite, and suggests an alternative ideal formula for terrywallaceite of AgPb(Sb,Bi)(Bi,Sb)6S Å, instead of AgPb(Sb,Bi)3S6.
- Yang, H., Downs, R. T., Evans, S. H., Jenkins, R. A., & Bloch, E. M. (2013). Rongibbsite, Pb2(Si4Al)O11(OH), a new zeolitic aluminosilicate mineral with an interrupted framework from Maricopa County, Arizona, U.S.A.. American Mineralogist, 98(1), 236-241.More infoAbstract: A new zeolitic aluminosilicate mineral species, rongibbsite, ideally Pb2(Si4Al)O11(OH), has been found in a quartz vein in the Proterozoic gneiss of the Big Horn Mountains, Maricopa County, Arizona, U.S.A. The mineral is of secondary origin and is associated with wickenburgite, fornacite, mimetite, murdochite, and creaseyite. Rongibbsite crystals are bladed (elongated along the c axis, up to 0.70 × 0.20 × 0.05 mm), often in tufts. Dominant forms are {100}, {010}, {001}, and {101̄}. Twinning is common across (100). The mineral is colorless, transparent with white streak and vitreous luster. It is brittle and has a Mohs hardness of ∼5; cleavage is perfect on {100} and no parting was observed. The calculated density is 4.43 g/cm3. Optically, rongibbsite is biaxial (+), with nα = 1.690, nβ = 1.694, nγ = 1.700, cZ = 26°, 2Vmeas = 65(2)°. It is insoluble in water, acetone, or hydrochloric acid. Electron microprobe analysis yielded an empirical formula Pb2.05 (Si 3.89Al1.11)O11 (OH). Rongibbsite is monoclinic, with space group I2/m and unit-cell parameters a = 7.8356(6), b = 13.913(1), c = 10.278(1) Å, β = 92.925(4)°, and V = 1119.0(2) Å3. Its structure features an interrupted framework made of three symmetrically distinct TO4 tetrahedra (T = Si + Al). The framework density is 17.9 T per 1000 Å3. Unlike many known interrupted frameworks in zeolite-type materials, which are usually broken up by OH or F, the framework in rongibbsite is interrupted by O atoms. There are various corner-shared tetrahedral rings in the framework of rongibbsite, including two types of 4-membered, three 6-membered, and one 8-membered rings. The extraframework Pb and OH reside alternately in the channels formed by the 8-membered rings. The Pb cations are disordered over two split sites, Pb and Pb′, with site occupancies of 0.8 and 0.2, respectively, and a Pb-Pb′ distance of 0.229 Å, providing a structural explanation for the two strong Raman bands (at 3527 and 3444 cm-1) attributable to the O-H stretching vibrations. The average bond lengths for the T1, T2, and T3 tetrahedra are 1.620, 1.648, and 1.681 Å, respectively, indicating that the preference of Al for the three tetrahedral sites is T3 ≫ T2 > T1. Rongibbsite represents the first natural aluminosilicate with Pb as the only extraframework cation.
- A.D., L., Yang, H., Downs, R. T., L., M., & Persiano, A. C. (2012). Lithio-tantite, ideally LiTa3O8. Acta Crystallographica Section E: Structure Reports Online, 68(5), i27-i28.More infoAbstract: Lithio-tantite (lithium tritantalum octa-oxide) and lithio-wodg-inite are natural dimorphs of LiTa3O8, corresponding to the laboratory-synthesized L-LiTa3O8 (low-temperature form) and M-LiTa3O8 (inter-mediate-temperature form) phases, respectively. Based on single-crystal X-ray diffraction data, this study presents the first structure determination of lithio-tantite from a new locality, the Murundu mine, Jenipapo District, Itinga, Minas Gerais, Brazil. Lithio-tantite is isotypic with LiNb3O8 and its structure is composed of a slightly distorted hexa-gonal close-packed array of O atoms stacked in the [-101] direction, with the metal atoms occupying half of the octa-hedral sites. There are four symmetrically non-equivalent cation sites, with three of them occupied mainly by (Ta5+ + Nb5+) and one by Li+. The four distinct octa-hedra share edges, forming two types of zigzag chains (A and B) extending along the b axis. The A chains are built exclusively of (Ta,Nb)O6 octa-hedra (M1 and M2), whereas the B chains consist of alternating (Ta,Nb)O6 and LiO6 octa-hedra (M3 and M4, respectively). The average M1 - O, M2 - O, M3 - O and M4 - O bond lengths are 2.011, 2.004, 1.984, and 2.188 Å, respectively. Among the four octa-hedra, M3 is the least distorted and M4 the most. The refined Ta contents at the M1, M2 and M3 sites are 0.641 (2), 0.665 (2), and 0.874 (2), respectively, indicating a strong preference of Ta5+ for M3 in the B chain. The refined composition of the crystal investigated is Li0.96Mn0. 03Na0.01Nb0.82Ta 2.18O8. © Menezes Filho et al. 2012.
- Andrade, M. B., Morrison, S. M., J., A., Feinglos, M. N., & Downs, R. T. (2012). Robertsite, Ca2MnIII3O2(PO 4)3·3H2O. Acta Crystallographica Section E: Structure Reports Online, 68(10), i74-i75.More infoPMID: 23125565;PMCID: PMC3470121;Abstract: Robertsite, ideally Ca2Mn3O2(PO 4)3·3H2O [calcium manganese(III) tris(orthophosphate) trihydrate], can be associated with the arseniosiderite structural group characterized by the general formula Ca2A 3O2(TO4)3·H2O, with A = Fe, Mn; T = As, P; and n = 2 or 3. In this study, single-crystal X-ray diffraction data were used to determine the robertsite structure from a twinned crystal from the type locality, the Tip Top mine, Custer County, South Dakota, USA, and to refine anisotropic displacement parameters for all atoms. The general structural feature of robertsite resembles that of the other two members of the arseniosiderite group, the structures of which have previously been reported. It is characterized by sheets of [MnO6] octahedra in the form of nine-membered pseudo-trigonal rings. Located at the center of each ninemembered ring is a PO4 tetrahedron, and the other eight PO 4 tetrahedra sandwich the Mn-oxide sheets. The six different Ca 2+ ions are seven-coordinated in form of distorted pentagonal bipyramids, [CaO5(H2O)2], if Ca-O distances less than 2.85 A are considered. Along with hydrogen bonding involving the water molecules, they hold the manganese-phosphate sheets together. All nine [MnO6] octahedra are distorted by the Jahn-Teller effect.
- Bindi, L., Downs, R. T., Spry, P. G., Pinch, W. W., & Menchetti, S. (2012). A chemical and structural re-examination of fettelite samples from the type locality, Odenwald, southwest Germany. Mineralogical Magazine, 76(3), 551-566.More infoAbstract: The crystal structure and chemical composition of two samples of fettelite from the type locality, including a portion of the holotype material, was investigated to verify if a previously proposed revision of the chemical formula was applicable, and to study the role of cation substitution for Hg that would suggest new members of the fettelite family. The crystal structure of fettelite from the type locality was found to be equivalent to that reported previously for the Chilean occurrence, and consists of an alternation of two kinds of layers along c: layer A with general composition [Ag 6As 2S 7] 2- and layer B with general composition [Ag 10HgAs 2S 8] 2+. In this structure, the Ag atoms occur in various coordination configurations, varying from quasi-linear to quasi-tetrahedral, the AsS 3 groups form pyramids as are typically observed in sulfosalts, and Hg links two sulfur atoms in a linear coordination. The refined compositions for the crystals in this study, [Ag 6As 2S 7][Ag 10(Fe 0.53Hg 0.47)As 2S 8] (R100124) and [Ag 6As 2S 7][Ag 10(Hg 0.79Cu 0.21)As 2S 8] (R110042), clearly indicate that new mineral species related to fettelite are likely to be found in nature. © 2012 Mineralogical Society.
- Blake, D., Vaniman, D., Achilles, C., Anderson, R., Bish, D., Bristow, T., Chen, C., Chipera, S., Crisp, J., Marais, D. D., Downs, R. T., Farmer, J., Feldman, S., Fonda, M., Gailhanou, M., Hongwei, M. a., Ming, D. W., Morris, R. V., Sarrazin, P., , Stolper, E., et al. (2012). Characterization and calibration of the CheMin mineralogical instrument on Mars Science Laboratory. Space Science Reviews, 170(1-4), 341-399.More infoAbstract: A principal goal of the Mars Science Laboratory (MSL) rover Curiosity is to identify and characterize past habitable environments on Mars. Determination of the mineralogical and chemical composition of Martian rocks and soils constrains their formation and alteration pathways, providing information on climate and habitability through time. The CheMin X-ray diffraction (XRD) and X-ray fluorescence (XRF) instrument on MSL will return accurate mineralogical identifications and quantitative phase abundances for scooped soil samples and drilled rock powders collected at Gale Crater during Curiosity's 1-Mars-year nominal mission. The instrument has a Co X-ray source and a cooled charge-coupled device (CCD) detector arranged in transmission geometry with the sample. CheMin's angular range of 5 to 50 2θ with
- Fejfarová, K., Plášil, J., Yang, H., Cejka, J., Dušek, M., Downs, R. T., Barkley, M. C., & Škoda, R. (2012). Revision of the crystal structure and chemical formula of weeksite, K2(UO 2) 2(Si 5O 13)̇4H 2O. American Mineralogist, 97(4), 750-754.More infoAbstract: The previously published structure determination of weeksite from the Anderson mine, Arizona, U.S.A., suggested that it is orthorhombic, Cmmb, with a = 14.209(2), b = 14.248(2), c = 35.869(4) Å, and V = 7262(2) Å3, and an ideal chemical formula (K,Ba)1-2(UO 2) 2(Si 5O 13)̇H 2O. Using single-crystal X-ray diffraction, electron microprobe analysis, and thermal analysis, we reexamined weeksite from the same locality. Our results demonstrate that weeksite is monoclinic, with the space group C2/m and unit-cell parameters a = 14.1957(4), b = 14.2291(5), c = 9.6305(3) Å, b = 111.578(3)°, V = 1808.96(10) Å3, and an ideal formula K2(UO 2) 2(Si 5O 13)̇4H 2O. The previously reported orthorhombic unit cell is shown to result from twinning of the monoclinic cell. The structure refinement yielded R1 = 2.84% for 1632 observed reflections [Iobs > 3s(I)] and 5.42% for all 2379 reflections. The total H 2O content derived from the structure refinement agrees well with that from the thermal analysis. Although the general topology of our structure resembles that reported previously, all Si sites in our structure are fully occupied, in contrast to the previous structure determination, which includes four partially occupied SiO 4 tetrahedra. From our structure data on weeksite, it appears evident that the orthorhombic cell of the newly discovered weeksite-type mineral coutinhoite, ThxB1-2x(UO 2) 2Si 5O 13̇3H 2O, needs to be reevaluated Copyright © 1997-2012 Mineralogical Society of America.
- Gražulis, S., Daškevič, A., Merkys, A., Chateigner, D., Lutterotti, L., Quirós, M., Serebryanaya, N. R., Moeck, P., Downs, R. T., & Bail, A. L. (2012). Crystallography Open Database (COD): An open-access collection of crystal structures and platform for world-wide collaboration. Nucleic Acids Research, 40(D1), D420-D427.More infoPMID: 22070882;PMCID: PMC3245043;Abstract: Using an open-access distribution model, the Crystallography Open Database (COD, http://www .crystallography.net) collects all known 'small molecule / small to medium sized unit cell' crystal structures and makes them available freely on the Internet. As of today, the COD has aggregated ∼150 000 structures, offering basic search capabilities and the possibility to download the whole database, or parts thereof using a variety of standard open communication protocols. A newly developed website provides capabilities for all registered users to deposit published and so far unpublished structures as personal communications or pre-publication depositions. Such a setup enables extension of the COD database by many users simultaneously. This increases the possibilities for growth of the COD database, and is the first step towards establishing a world wide Internet-based collaborative platform dedicated to the collection and curation of structural knowledge. © The Author(s) 2011.
- Hazen, R. M., Golden, J., Downs, R. T., Hystad, G., Grew, E. S., Azzolini, D., & Sverjensky, D. A. (2012). Mercury (Hg) mineral evolution: A mineralogical record of supercontinent assembly, changing ocean geochemistry, and the emerging terrestrial biosphere. American Mineralogist, 97(7), 1013-1042.More infoAbstract: Analyses of the temporal and geographic distribution of earliest recorded appearances of the 88 IMA-approved mercury minerals plus two potentially valid species exemplify principles of mineral evolution. Metacinnabar (HgS) and native Hg are the only two species reported from meteorites, specifically, the primitive H3 Tieschitz chondrite with an age of 4550 Ma. Since the first terrestrial appearance of cinnabar more than 3 billion years ago, mercury minerals have been present continuously at or near Earth's surface. Mercury mineral evolution is characterized by episodic deposition and diversification, perhaps associated with the supercontinent cycle. We observe statistically significant increases in the number of reported Hg mineral localities and new Hg species at ∼2.8-2.6, ∼1.9-1.8, and ∼0.43-0.25 Ga - intervals that correlate with episodes of presumed supercontinent assembly and associated orogenies of Kenorland (Superia), Columbia (Nuna), and Pangea, respectively. In constrast, few Hg deposits or new species of mercury minerals are reported from the intervals of supercontinent stability and breakup at ∼2.5-1.9, ∼1.8-1.2, and 1.1-0.8 Ga. The interval of Pangean supercontinent stability and breakup (∼250-65 Ma) is also marked by a significant decline in reported mercury mineralization; however, rocks of the last 65 million years, during which Pangea has continued to diverge, is characterized by numerous ephemeral near-surface Hg deposits. The period ∼1.2-1.0 Ga, during the assembly of the Rodinian supercontinent, is an exception because of the absence of new Hg minerals or deposits from this period. Episodes of Hg mineralization reflect metamorphism of Hg-enriched marine black shales at zones of continental convergence. We suggest that Hg was effectively sequestered as insoluble nanoparticles of cinnabar (HgS) or tiemannite (HgSe) during the period of the sulfidic "intermediate ocean" (∼1.85-0.85 Ga); consequently, few Hg deposits formed during the aggregation of Rodinia, whereas several deposits date from 800-600 Ma, a period that overlaps with the rifting and breakup of Rodinia. Nearly all Hg mineral species (87 of 90 known), as well as all major economic Hg deposits, are known to occur in formations ≤400 million years old. This relatively recent diversification arises, in part, from the ephemeral nature of many Hg minerals. In addition, mercury mineralization is strongly enhanced by interactions with organic matter, so the relatively recent pulse of new Hg minerals may reflect the rise of a terrestrial biosphere at ∼400 Ma.
- J., A., Downs, R. T., & Yang, H. (2012). Redetermination of clinobarylite, BaBe2Si2O 7. Acta Crystallographica Section E: Structure Reports Online, 68(10), i78-i79.More infoPMID: 23125568;PMCID: PMC3470124;Abstract: Clinobarylite, ideally BaBe2Si2O7 (chemical name barium diberyllium disilicate), is a sorosilicate mineral and dimorphic with barylite. It belongs to a group of compounds characterized by the general formula BaM2+2Si2O7, with M2+ = Be, Mg, Fe, Mn, Zn, Co, or Cu, among which the Be-, Fe-, and Cu-members have been found in nature. The crystal structure of clinobarylite has been re-examined in this study based on single-crystal X-ray diffraction data collected from a natural sample from the type locality (Khibiny Massif, Kola Peninsula, Russia). The structure of clinobarylite can be considered as a framework of BeO4 and SiO4 tetrahedra, with one of the O atoms coordinated to two Be and one Si, one coordinated to two Si, and two O atoms coordinated to one Si and one Be atom. The BeO4 tetrahedra share corners, forming chains parallel to the c axis, which are interlinked by the Si2O7 units oriented parallel to the a axis. The Ba2+ cations (site symmetry m..) are in the framework channels and are coordinated by eleven O atoms in form of an irregular polyhedron. The Si - Obr (bridging O atom, at site symmetry m..) bond length, the Si - Onbr (non-bridging O atoms) bond lengths, and the Si - O - Si angle within the Si2O7 unit are in marked contrast to the corresponding values determined in the previous study [Krivovichev et al. (2004). N. Jb. Miner. Mh. pp. 373-384].
- Menezes, L., Yang, H., Downs, R., Chaves, L., & Persiano, A. (2012). Lithiotantite, ideally LiTa3O8. Acta Crystallographica, E68, i27-i28.
- Morrison, S. M., Downs, R. T., & Yang, H. (2012). Redetermination of kovdorskite, Mg 2PO 4(OH) ·3H 2O. Acta Crystallographica Section E: Structure Reports Online, 68(2), i12-i13.More infoPMID: 22346789;PMCID: PMC3274836;Abstract: The crystal structure of kovdorskite, ideally Mg 2PO 4(OH)·3H 2O (dimagnesium phosphate hydroxide trihydrate), was reported previously with isotropic displacement paramaters only and without H-atom positions [Ovchinnikov et al. (1980). Dokl. Akad. Nauk SSSR. 255, 351-354]. In this study, the kovdorskite structure is redetermined based on single-crystal X-ray diffraction data from a sample from the type locality, the Kovdor massif, Kola Peninsula, Russia, with anisotropic displacement parameters for all non-H atoms, with all H-atom located and with higher precision. Moreover, inconsistencies of the previously published structural data with respect to reported and calculated X-ray powder patterns are also discussed. The structure of kovdorskite contains a set of four edge-sharing MgO 6 octa-hedra inter-connected by PO 4 tetra-hedra and O - H⋯O hydrogen bonds, forming columns and channels parallel to [001]. The hydrogen-bonding system in kovdorskite is formed through the water mol-ecules, with the OH - ions contributing little, if any, to the system, as indicated by the long H⋯A distances (>2.50 Å) to the nearest O atoms. The hydrogen-bond lengths determined from the structure refinement agree well with Raman spectroscopic data.
- Morrison, S. M., Downs, R. T., Domanik, K. J., Yang, H., & Doell, D. (2012). Nioboaeschynite-(Ce), Ce(NbTi)O6. Acta Crystallographica Section E: Structure Reports Online, 68(8), i64-i65.More infoPMID: 22904701;PMCID: PMC3414094;Abstract: Nioboaeschynite-(Ce), ideally Ce(NbTi)O6 [cerium(III) niobium(V) titanium(IV) hexa-oxide; refined formula of the natural sample is Ca 0.25Ce0.79(Nb1.14Ti0.86)O 6], belongs to the aeschynite mineral group which is characterized by the general formula AB2(O,OH)6, where eight-coordinated A is a rare earth element, Ca, Th or Fe, and six-coordinated B is Ti, Nb, Ta or W. The general structural feature of nioboaeschynite-(Ce) resembles that of the other members of the aeschynite group. It is characterized by edge-sharing dimers of [(Nb,Ti)O6] octa-hedra which share corners to form a three-dimensional framework, with the A sites located in channels parallel to the b axis. The average A-O and B-O bond lengths in nioboaeschynite-(Ce) are 2.471 and 1.993 Å, respectively. Moreover, another eight-coordinated site, designated as the C site, is also located in the channels and is partially occupied by A-type cations. Additionally, the refinement revealed a splitting of the A site, with Ca displaced slightly from Ce (0.266 Å apart), presumably resulting from the crystal-chemical differences between the Ce 3+ and Ca2+ cations.
- Origlieri, M. J., Yang, H., Downs, R. T., Posner, E. S., Domanik, K. J., & Pinch, W. W. (2012). Letter: The crystal structure of bartelkeite, with a revised chemical formula, PbFeGe VI(Ge 2IVO 7) (OH) 2·H 2O, isotypic with high-pressure P2 1/m lawsonite. American Mineralogist, 97(10), 1812-1815.More infoAbstract: Bartelkeite from Tsumeb, Namibia, was originally described by Keller et al. (1981) with the chemical formula PbFeGe 3O 8. By means of electron microprobe analysis, single-crystal X-ray diffraction, and Raman spectroscopy, we examined this mineral from the type locality. Our results show that bartelkeite is monoclinic with space group P2 1/m, unit-cell parameters a = 5.8279(2), b = 13.6150(4), c = 6.3097 (2) Å, β = 127.314(2)°, and a revised ideal chemical formula PbFeGe VIGe 2IVO 7(OH) 2·H 2O (Z = 2). Most remarkably, bartelkeite is isostructural with the high-pressure P2 1/m phase of lawsonite, CaAl 2Si 2O 7(OH)·H 2O, which is only stable above 8.6 GPa and a potential host for H 2O in subducting slabs. Its structure consists of single chains of edge-sharing FeO 6 and Ge1O 6 octahedra parallel to the c-axis, cross-linked by Ge2 2O 7 tetrahedral dimers. The average bond lengths for the GeO 6 and GeO 4 polyhedra are 1.889 and 1.744 Å, respectively. The Pb atoms and H 2O groups occupy large cavities within the framework. The hydrogen bonding scheme in bartelkeite is similar to that in lawsonite. Bartelkeite represents the first known mineral containing both 4- and 6-coordinated Ge atoms and may serve as an excellent analog for further exploration of the temperature-pressure-composition space of lawsonite.
- Posner, E. S., Konzett, J., Frost, D. J., Downs, R. T., & Yang, H. (2012). High-pressure synthetic (Na 0.97Mg 0.03)(Mg 0.43Fe 0.173+Si 0.40)Si 2O 6, with six-coordinated silicon, isostructural with P2/n omphacite. Acta Crystallographica Section E: Structure Reports Online, 68(2), i18.More infoPMID: 22346794;PMCID: PMC3274841;Abstract: The title compound, (sodium magnesium) [magnesium iron(III) silicon] disilicate, Na 0.97Mg 0.03)(Mg 0.43Fe 0.173+Si 0.40)Si 2O 6, is isotypic with ordered P2/n omphacite. Its structure is characterized by single chains of corner-sharing SiO 4 tetra-hedra, extending along the c axis, which are crosslinked by bands of edge-sharing octa-hedra (site symmetry 2), statistically occupied by (Mg 2+ + Fe 3+ + Si 4+). Between the bands built up of the octahedra are two non-equivalent highly distorted six-coordinated sites (site symmetry 2), statistically occupied by (Na + Mg). In contrast to omphacites, the great differences in size and charge between Mg 2+ and Si 4+ result in complete, rather than partial, ordering of Mg and Si into two distinct octa-hedral sites, whereas Fe 3+ is disordered between the two sites. The octa-hedron filled by (Mg + Fe) is larger and markedly more distorted than that occupied by (Si + Fe). The average (Mg + Fe) - O and ( VISi + Fe) - O bond lengths are 2.075 and 1.850 Å, respectively.
- Thompson, R. M., Yang, H., & Downs, R. T. (2012). Packing systematics and structural relationships of the new copper molybdate markascherite and related minerals. American Mineralogist, 97(11-12), 1977-1986.More infoAbstract: The structural relationships between the new mineral markascherite, ideally Cu 3(MoO 4)(OH) 4, and the related minerals szenicsite, antlerite, deloryite, flinkite, retzian, and cahnite are analyzed using hypothetical ideal closest-packed equivalents. Markascherite and the first three related minerals are based on cubic closest-packing (CCP) of anions, flinkite is based on stacking sequence ABAC, and retzian and cahnite are based on hexagonal closest-packing (HCP). However, models that are more realistic than those based on CP can be constructed for retzian and cahnite using small but systematic alterations of CP monolayers. A regular pattern of slight dislocations of some of the spheres in the monolayers creates dodecahedral interstitial sites when the monolayers are stacked, a feature not seen in perfect CP. The use of ideal crystals removes all distortion from polyhedra in closest-packed minerals, allowing for comparison of structural similarities and differences. CCP minerals can have up to four nonequivalent stacking directions. Corresponding stacking directions in the minerals of interest are identified and used to compare the layers of cation coordination polyhedra perpendicular to these zones or face poles (stacking directions are presented in both direct space and reciprocal space). Such layers are natural structural subunits and provide insight into the relationships among these minerals.
- Yang, H., Downs, R. T., Yang, Y. W., & Allen, W. H. (2012). Pyrosmalite-(Fe), Fe 8Si 6O 15(OH,Cl) 10. Acta Crystallographica Section E: Structure Reports Online, 68(1), i7-i8.More infoPMID: 22259315;PMCID: PMC3254272;Abstract: Pyrosmalite-(Fe), ideally Fe II8Si 6O 15(OH,Cl) 10 [refined composition in this study: Fe 8Si 6O 15(OH 0.814Cl 0.186) 10·-0.45H 2O, octairon(II) hexasilicate deca(chloride/hydroxide) 0.45-hydrate], is a phyllosilicate mineral and a member of the pyrosmalite series (Fe,Mn) 8Si 6O 15(OH,Cl) 10, which includes pyrosmalite-(Mn), as well as friedelite and mcgillite, two polytypes of pyrosmalite-(Mn). This study presents the first structure determination of pyrosmalite-(Fe) based on singlecrystal X-ray diffraction data from a natural sample from Burguillos del Cerro, Badajos, Spain. Pyrosmalite-(Fe) is isotypic with pyrosmalite-(Mn) and its structure is characterized by a stacking of brucite-type layers of FeO 6-octahedra alternating with sheets of SiO 4 tetrahedra along [001]. These sheets consist of 12-, six-and four-membered rings of tetrahedra in a 1:2:3 ratio. In contrast to previous studies on pyrosmalite-(Mn), which all assumed that Cl and one of the four OH-groups occupy the same site, our data on pyrosmalite-(Fe) revealed a split-site structure model with Cl and OH occupying distinct sites. Furthermore, our study appears to suggest the presence of disordered structural water in pyrosmalite-(Fe), consistent with infrared spectroscopic data measured from the same sample. Weak hydrogen bonding between the ordered OH-groups that are part of the brucitetype layers and the terminal silicate O atoms is present.
- Yang, H., Jenkins, N. G., & Downs, R. T. (2012). Redetermination of junitoite, CaZn2Si2O 7·H2O. Acta Crystallographica Section E: Structure Reports Online, 68(10), i73.More infoPMID: 23125564;PMCID: PMC3470120;Abstract: The crystal structure of the mineral junitoite, ideally CaZn 2Si2O7·H2O (calcium dizinc disilicate monohydrate), was first determined by Hamilton & Finney [Mineral. Mag. (1985), 49, 91-95] based on the space group Ama2, yielding a reliability factor R of 0.10, with isotropic displacement parameters for all non-H atoms. The present study reports a structure redetermination of junitoite using single-crystal X-ray diffraction data from a natural sample, demonstrating that the space group of this mineral is actually Aea2, which can be attained simply by shifting the origin. Topologically, the structure models in the space groups Aea2 and Ama2 are analogous, consisting of chains of corner-sharing ZnO 4 tetrahedra parallel to the b axis, cross-linked by Si 2O7 tetrahedral dimers (the site symmetry of the bridging O atom is ..2) along a and c, forming a three-dimensional framework. The Ca 2+ cations (site symmetry ..2) are situated in cavities of the framework and are bonded to five O atoms and one H2O molecule (site symmetry ..2) in a distorted octahedral coordination environment. However, some bond lengths, especially for the SiO4 tetrahedron, are noticeably different between the two structure models. Hydrogen bonding in junitoite is found between the water molecule and a framework O atom.
- Yang, H., Jenkins, R. A., Thompson, R. M., Downs, R. T., Evans, S. H., & Bloch, E. M. (2012). Markascherite, Cu 3(MoO 4)(OH) 4, a new mineral species polymorphic with szenicsite, from Copper Creek, Pinal County, Arizona, U.S.A.. American Mineralogist, 97(1), 197-202.More infoAbstract: A new mineral species, markascherite (IMA2010-051), ideally Cu 3(MoO 4)(OH) 4, has been found at Copper Creek, Pinal County, Arizona, U.S A. The mineral is of secondary origin and is associated with brochantite, antlerite, lindgrenite, wulfenite, natrojarosite, and chalcanthite. Markascherite crystals are bladed (elongated along the b axis), up to 0.50 × 0.10 × 0.05 mm. The dominant forms are {001}, {100}, and {010}. Twinning is found with the twofold twin axis along [101]. The mineral is green, transparent with green streak and vitreous luster. It is brittle and has a Mohs hardness of 3.5∼4; cleavage is perfect on {100} and no parting was observed. The calculated density is 4.216 g/cm 3. Optically, markascherite is biaxial (-), with n α >1.8, n β > 1.8, and n γ >1.8. The dispersion is strong (r > v). It is insoluble in water, acetone, or hydrochloric acid. An electron microprobe analysis yielded an empirical formula Cu 2.89(Mo 1.04O 4)(OH) 4. Markascherite, polymorphic with szenicsite, is monoclinic, with space group P2 1/m and unit-cell parameters a = 9.9904(6), b = 5.9934(4), c = 5.5255(4) Å, β = 97.428(4)°, and V = 328.04(4) Å 3. Its structure is composed of three nonequivalent, markedly distorted Cu 2+(O,OH) 6 octahedra and one MoO 4 tetrahedron. The Cu1 and Cu2 octahedra share edges to form brucite-type layers parallel to (100), whereas the Cu3 octahedra share edges with one another to form rutile-type chains parallel to the b axis. These layers and chains alternate along [100] and are interlinked together by both MoO 4 tetrahedra and hydrogen bonds. Topologically, the structure of markascherite exhibits a remarkable resemblance to that of deloryite, Cu 4(UO 2)(MoO 4) 2(OH) 6, given the coupled substitution of [2Cu 2+ + 2(OH-)] 2+ for [(U 6+ + □) + 2O 2-] 2+. The Raman spectra of markascherite are compared with those of two other copper molybdate minerals szenicsite and lindgrenite.
- Yang, Y. W., Evans, S. H., Downs, R. T., & Yang, H. (2012). Lotharmeyerite, Ca(Zn,Mn) 2(AsO 4)2-(H 2O,OH) 2. Acta Crystallographica Section E: Structure Reports Online, 68(1), i9-i10.More infoPMID: 22259316;PMCID: PMC3254273;Abstract: Lotharmeyerite, calcium bis(zinc/manganese) bis(arsenate) bis(hydroxide/hydrate), Ca(Zn,Mn 3+) 2(AsO 4) 2(H 2O,OH) 2, is a member of the natrochalcite group of minerals, which are characterized by the general formula AM 2(XO 4) 2(H 2O,OH) 2, where A may be occupied by Pb 2+, Ca 2+, Na +, and Bi 3+, M by Fe 3+, Mn 3+, Cu 2+, Zn 2+, Co 2+, Ni 2+, Al 3+, and Mg 2+, and X by P V, As V, V V, and S VI. The minerals in the group display either monoclinic or triclinic symmetry, depending on the ordering of chemical components in the M site. Based on single-crystal X-ray diffraction data of a sample from the type locality, Mapimi, Durango, Mexico, this study presents the first structure determination of lotharmeyerite. Lotharmeyerite is isostructural with natrochalcite and tsumcorite. The structure is composed of rutile-type chains of edge-shared MO 6 octahedra (site symmetry 1̄) extending along [010], which are interconnected by XO 4 tetrahedra (site symmetry 2) and hydrogen bonds to form [M 2(XO 4) 2(OH,H 2O) 2] sheets parallel to (001). These sheets are linked by the larger A cations (site symmetry 2/m), as well as by hydrogen bonds. Bond-valence sums for the M cation, calculated with the parameters for Mn 3+ and Mn 2+ are 2.72 and 2.94 v.u., respectively, consistent with the occupation of the M site by Mn 3+. Two distinct hydrogen bonds are present, one with O···O = 2.610(4) Å and the other O···O = 2.595(3) Å. One of the H-atom positions is disordered over two sites with 50% occupancy, in agreement with observations for other natrochalcite-type minerals, such as natrochalcite and tsumcorite.
- Barkley, M. C., Downs, R. T., & Yang, H. (2011). Structure of walstromite, BaCa2Si3O9, and its relationship to CaSiO3-walstromite and wollastonite-II. American Mineralogist, 96(5-6), 797-801.More infoAbstract: The crystal structure of walstromite, ideally BaCa2Si 3O9, was refined with data from single-crystal X-ray diffraction on a natural specimen from the type locality Esquire No. 8 claim, Big Creek, Fresno County, California, U.S.A. It is triclinic, with space group P1 and unit-cell parameters a = 6.7335(2), b = 9.6142(3), c = 6.6859(2) Å, a = 69.638(2)°, β = 102.281(2)°, ? = 96.855(2)°, and V = 396.01(2) Å3. The only previously published structure for walstromite was based on photographic film intensity data collected from synthetic BaCa 2Si3O9 (Dent Glasser and Glasser 1968). Due to uncertainty in oxygen positions, the reported final R-factor was 0.16. The current refinement yielded an R-factor of 0.030 with the inclusion of anisotropic displacement parameters. Walstromite is a Ba-Ca cyclosilicate characterized by Si3O9 three-membered rings. It is related to the important calcium silicate group of minerals, especially to CaSiO 3-walstromite, through the substitution of Ba into one of the three distinct Ca sites. Joswig et al. (2003) suggested that the structural changes caused by the replacement of Ba2+ by Ca2+ are minimal and that walstromite is isomorphic with CaSiO3-walstromite, but topologically different from high-pressure wollastonite-II (Ca 3Si3O9). Our study demonstrates that wollastonite-II and CaSiO3-walstromite are identical phases, and are isostructural with walstromite. This isomorphism implies that the high-pressure CaSiO3 phase may be a potential host for large cations in deep Earth environments.
- Barkley, M. C., Yang, H., Evans, S. H., Downs, R. T., & Origlieri, M. J. (2011). Redetermination of despujolsite, Ca 3Mn 4+(SO 4) 2(OH) 6·3H 2O. Acta Crystallographica Section E: Structure Reports Online, 67(9), i47-i48.More infoPMID: 22064218;PMCID: PMC3200628;Abstract: The crystal structure of despujolsite [tricalcium manganese bis-(sulfate) hexahydroxide tri-hydrate], the Ca/Mn member of the fleischerite group, ideally Ca 3Mn 4+(SO 4) 2(OH) 6·3H 2O, was previously determined based on X-ray diffraction intensity data from photographs, without H-atom positions located [Gaudefroy et al. (1968). Bull. Soc. Fr. Minéral. Crystallogr. 91, 43-50]. The current study redetermines the structure of despujolsite from a natural specimen, with all H atoms located and with higher precision. The structure of despujolsite is characterized by layers of CaO8 polyhedra (m.. symmetry) inter-connected by Mn(OH) 6 octa-hedra (32. symmetry) and SO 4 tetra-hedra (3.. symmetry) along [001]. The average Ca - O, Mn - O and S - O bond lengths are 2.489, 1.915, and 1.472 Å, respectively. There are two distinct hydrogen bonds that stabilize the structural set-up. This work represents the first description of hydrogen bonds in the fleischerite group of minerals.
- Dera, P., Lazarz, J. D., Prakapenka, V. B., Barkley, M., & Downs, R. T. (2011). New insights into the high-pressure polymorphism of SiO2 cristobalite. Physics and Chemistry of Minerals, 38(7), 517-529.More infoAbstract: Single-crystal X-ray diffraction experiments with SiO2 α-cristobalite reveal that the well-known reversible displacive phase transition to cristobalite-II, which occurs at approximately 1.8 GPa, can be suppressed by rapid pressure increase, leading to an overpressurized metastable state, persisting to pressure as high as 10 GPa. In another, slow pressure increase experiment, the monoclinic high-pressure phase-II was observed to form at ~1. 8 GPa, in agreement with earlier in situ studies, and its crystal structure has been unambiguously determined. Single-crystal data have been used to refine the structure models of both phases over the range of pressure up to the threshold of formation of cristobalite X-I at ~12 GPa, providing important constraints on the feasibility of the two competing silica densification models proposed in the literature, based on quantum mechanical calculations. Preliminary diffraction data obtained for cristobalite X-I reveal a monoclinic unit cell that contradicts the currently assumed model. © 2011 Springer-Verlag.
- Gibbs, G. V., Wallace, A. F., Downs, R. T., Ross, N. L., Cox, D. F., & Rosso, K. M. (2011). Thioarsenides: A case for long-range Lewis acid-base-directed van der Waals interactions. Physics and Chemistry of Minerals, 38(4), 267-291.More infoAbstract: Electron density distributions, bond paths, Laplacian and local-energy density properties have been calculated for a number of As4Sn (n = 3, 4 and 5) thioarsenide molecular crystals. On the basis of the distributions, the intramolecular As-S and As-As interactions classify as shared bonded interactions, and the intermolecular As-S, As-As and S-S interactions classify as closed-shell van der Waals (vdW) bonded interactions. The bulk of the intermolecular As-S bond paths link regions of locally concentrated electron density (Lewis-base regions) with aligned regions of locally depleted electron density (Lewis-acid regions) on adjacent molecules. The paths are comparable with intermolecular paths reported for several other molecular crystals that link aligned Lewis base and acid regions in a key-lock fashion, interactions that classified as long-range Lewis acid-base-directed vdW interactions. As the bulk of the intermolecular As-S bond paths (~70%) link Lewis acid-base regions on adjacent molecules, it appears that molecules adopt an arrangement that maximizes the number of As-S Lewis acid-base intermolecular bonded interactions. The maximization of the number of Lewis acid-base interactions appears to be connected with the close-packed array adopted by molecules: distorted cubic close-packed arrays are adopted for alacránite, pararealgar, uzonite, realgar and β-AsS and the distorted hexagonal close-packed arrays adopted by α- and β-dimorphite. A growth mechanism is proposed for thioarsenide molecular crystals from aqueous species that maximizes the number of long-range Lewis acid-base vdW As-S bonded interactions with the resulting directed bond paths structuralizing the molecules as a molecular crystal. © 2010 Springer-Verlag.
- Hazen, R. M., Bekker, R., Bish, D. L., Bleeker, W., Downs, R. T., Farquhar, J., Ferry, J. M., Grew, E. S., Knoll, R. H., Papineau, D., Ralph, J. P., Sverjensky, D. A., & Valley, J. W. (2011). Needs and opportunities in mineral evolution research. American Mineralogist, 96(7), 953-963.More infoAbstract: Progress in understanding mineral evolution, Earth's changing near-surface mineralogy through time, depends on the availability of detailed information on mineral localities of known ages and geologic settings. A comprehensive database including this information, employing the mindat.org web site as a platform, is now being implemented. This resource will incorporate software to correlate a range of mineral occurrences and properties vs. time, and it will thus facilitate studies of the changing diversity, distribution, associations, and characteristics of individual minerals as well as mineral groups. The Mineral Evolution Database thus holds the prospect of revealing mineralogical records of important geophysical, geochemical, and biological events in Earth history.
- Kampf, A. R., Downs, R. T., Housley, R. M., Jenkins, R. A., & Hyrsl, J. (2011). Anorpiment, As 2S 3, the triclinic dimorph of orpiment. Mineralogical Magazine, 75(6), 2857-2867.More infoAbstract: The new mineral anorpiment, As 2S 3, the triclinic dimorph of orpiment, has space group P1 and cell parameters a = 5.7577(2), b = 8.7169(3), c = 10.2682(7) Å, α = 78.152(7), β = 75.817(7), γ = 89.861(6)°, V = 488.38(4) Å 3 and Z = 4. It occurs at the Palomo mine, Castrovirreyna Province, Huancavelica Department, Peru. It is a low-temperature hydrothermal mineral associated with dufrénoysite, muscovite, orpiment, pyrite and realgar. It occurs in drusy crusts of wedge-shaped, transparent, greenish yellow crystals. The streak is yellow. The lustre is resinous on crystal faces, but pearly on cleavage surfaces. The Mohs hardness is about 1½. The mineral is sectile with an irregular fracture and one perfect and easy cleavage on {001}. The measured and calculated densities are 3.33 and 3.321 g cm -3, respectively. All indices of refraction are greater than 2. The mineral is optically biaxial (-) with 2V = 35-40° and no observed dispersion. The acute bisectrix (X) is approximately perpendicular to the {001} cleavage. Electron microprobe analyses yielded the averages and ranges in wt.%: As 58.21 (57.74-59.03), S 38.72 (38.33-39.00), total 96.94 (96.07-97.75), providing the empirical formula (based on 5 atoms) As 1.96S 3.04. The strongest powder X-ray diffraction lines are [d (hkl) I] 4.867(002) 97, 4.519 (110,111) 77, 3.702 (111) 46, 3.609 (022,112) 82, 2.880 (201,022,121,023) 75, 2.552 (113,131,132) 100, 2.469 (114,130,131) 96. The structure of anorpiment [R 1 = 0.021 for1484 reflections with F o > 4σ(F)] consists of layers of covalently bonded As and S atoms. Each S atom bonds to two As atoms at As-S-As angles between 100.45 and 104.15°. Each As atom is strongly bonded to three S atoms at S-As-S angles between 91.28 and 103.59°, forming an AsS 3 pyramid with As at its apex. The As-S linkages within the layers form rings of six AsS 3 pyramids. Interlayer bonding forces are interpreted as van der Waals. The structure of anorpiment is similar to that of orpiment in that it is composed of layers of As 2S 3 macromolecules, but arranged in a different stacking sequence. © 2011 Mineralogical Society.
- Kampf, A. R., Yang, H., Downs, R. T., & Pinch, W. W. (2011). The crystal structures and Raman spectra of aravaipaite and calcioaravaipaite. American Mineralogist, 96(2-3), 402-407.More infoAbstract: The original structure determination for aravaipaite, Pb 3AlF9(H2O), indicated it to be monoclinic, P21/n, with a = 25.048(4), b = 5.8459(8), c = 5.6505(7) Å, β = 94.013(3)°, V = 829.7(2) Å3, and Z = 4. Examination of additional crystal fragments from the same specimen revealed that some have a triclinic cell, P1, with a = 5.6637(1), b = 5.8659(1), c = 12.7041(9) Å, α = 98.725(7)°, β = 94.020(7)°, γ = 90.683(6)°, V = 416.04(3) Å3, and Z = 2. The topology of the structure is the same as that reported previously, but the structure refinement is significantly improved, with R1 = 0.0263 for 1695 observed reflections [Fo > 4σF] and 0.0306 for all 1903 reflections, and with the H atoms located. Twinning may be responsible for the original monoclinic cell or the two structures could be order-disorder (OD) polytypes. New X-ray diffraction data collected on a crystal of calcioaravaipaite, PbCa2Al(F,OH)9, showed it to be triclinic, P1, with a = 5.3815(3), b = 5.3846(3), c = 12.2034(6) Å, α = 91.364(2)°, β = 101.110(3)°, γ = 91.525(3)°, V = 346.72(3) Å3, and Z = 2. This cell is essentially identical to the reduced cell reported in conjunction with an earlier structure solution on a twinned crystal using the OD approach. Our study confirms the findings of the earlier study and significantly improves upon the earlier structure refinement, yielding R1 = 0.0195 for 2257 observed reflections [Fo > 4σF] and 0.0227 for all 2427 reflections. The structures of aravaipaite and calcioaravaipaite are based upon square-packed layers of F atoms on either side of which are bonded Pb atoms (in aravaipaite) or Ca atoms (in calcioaravaipaite) in fluorite-type configurations. These layers parallel to (001) serve as templates to which on both sides are attached AlF6 octahedra and PbF6(H2O)2 polyhedra (in aravaipaite) or PbF12 polyhedra (in calcioaravaipaite). The Pb2+ cations in these structures have stereoactive 6s2 lone-electron-pairs, manifest in off-center coordinations. The very different sizes of the Pb 2+ and Ca2+ cations yield fluorite-type layers with very different metrics, reflected in the a and b cell dimensions of the two structures; but more significantly, the lone-pair effect results in a very irregular template of F atoms peripheral to the fluorite-type layer in aravaipaite, while the F atoms peripheral to the fluorite-type layer in calcioaravaipaite are in a more regular, nearly planar array. As a result, the interlayer AlF6 octahedra and PbF6(H2O) 2 polyhedra in aravaipaite form a relatively open configuration, while the AlF6 octahedra and PbF12 polyhedra in calcioaravaipaite form a more tightly packed configuration containing no H 2O molecules. The Raman spectra of aravaipaite and calcioaravaipaite are consistent with the results of the structure studies, except that the spectrum of calcioaravaipaite exhibits the strong bands typically associated with OH stretching vibrations, while the structure refinement is most consistent with full occupancy of all anion sites by only F.
- Lavina, B., Dera, P., Kim, E., Meng, Y., Downs, R. T., Weck, P. F., Sutton, S. R., & Zhao, Y. (2011). Discovery of the recoverable high-pressure iron oxide Fe 4O 5. Proceedings of the National Academy of Sciences of the United States of America, 108(42), 17281-17285.More infoPMID: 21969537;PMCID: PMC3198347;Abstract: Phases of the iron-oxygen binary system are significant to most scientific disciplines, directly affecting planetary evolution, life, and technology. Iron oxides have unique electronic properties and strongly interact with the environment, particularly through redox reactions. The iron-oxygen phase diagram therefore has been among the most thoroughly investigated, yet it still holds striking findings. Here, we report the discovery of an iron oxide with formula Fe 4O 5, synthesized at high pressure and temperature. The previously undescribed phase, stable from 5 to at least 30 GPa, is recoverable to ambient conditions. First-principles calculations confirm that the iron oxide here described is energetically more stable than FeO + Fe 3O 4 at pressure greater than 10 GPa. The calculated lattice constants, equation of states, and atomic coordinates are in excellent agreement with experimental data, confirming the synthesis of Fe 4O 5. Given the conditions of stability and its composition, Fe 4O 5 is a plausible accessory mineral of the Earth's upper mantle. The phase has strong ferrimagnetic character comparable to magnetite. The ability to synthesize the material at accessible conditions and recover it at ambient conditions, along with its physical properties, suggests a potential interest in Fe 4O 5for technological applications.
- Righter, K., Sutton, S., Danielson, L., Pando, K., Schmidt, G., Yang, H., Berthet, S., Newville, M., Choi, Y., Downs, R. T., & Malavergne, V. (2011). The effect of fO2 on the partitioning and valence of v and Cr in garnet/melt pairs and the relation to terrestrial mantle v and Cr content. American Mineralogist, 96(8-9), 1278-1290.More infoAbstract: Chromium and vanadium are stable in multiple valence states in natural systems, and their distribution between garnet and silicate melt is not well understood. Here, the partitioning and valence state of V and Cr in experimental garnet/melt pairs have been studied at 1.8-3.0 GPa, with variable oxygen fugacity between IW-1.66 and the Ru-RuO2 (IW+9.36) buffer. In addition, the valence state of V and Cr has been measured in several high-pressure (majoritic garnet up to 20 GPa) experimental garnets, some natural megacrystic garnets from the western United States, and a suite of mantle garnets from South Africa. The results show that Cr remains in trivalent in garnet across a wide range of oxygen fugacities. Vanadium, on the other hand, exhibits variable valence state from 2.5 to 3.7 in the garnets and from 3.0 to 4.0 in the glasses. The valence state of V is always greater in the glass than in the garnet. Moreover, the garnet/melt partition coefficient, D(V), is highest when V is trivalent, at the most reduced conditions investigated (IW-1.66 to FMQ). The V2.5+ measured in high P-T experimental garnets is consistent with the reduced nature of those metal-bearing systems. The low V valence state measured in natural megacrystic garnets is consistent with f O2 close to the IW buffer, overlapping the range of fO2 measured independently by Fe2+/Fe3+ techniques on similar samples. However, the valence state of V measured in a suite of mantle garnets from South Africa is constant across a 3 log fO2 unit range (FMQ-1.8 to FMQ-4.5), suggesting that the valence state of V is controlled by the crystal chemistry of the garnets rather than fO2 variations. The compatibility of V and Cr in garnets and other deep mantle silicates indicates that the depletion of these elements in the Earth's primitive upper mantle could be due to partitioning into lower mantle phases as well as into metal.
- Thompson, R. M., Downs, R. T., & Dera, P. (2011). The compression pathway of quartz. American Mineralogist, 96(10), 1495-1502.More infoAbstract: The structure of quartz over the temperature domain (298 K, 1078 K) and pressure domain (0 GPa, 20.25 GPa) is compared to the following three hypothetical quartz crystals: (1) Ideal a-quartz with perfectly regular tetrahedra and the same volume and Si-O-Si angle as its observed equivalent (ideal ß-quartz has Si-O-Si angle fixed at 155.6°). (2) Model a-quartz with the same Si-O-Si angle and cell parameters as its observed equivalent, derived from ideal by altering the axial ratio. (3) BCC quartz with a perfectly body-centered cubic arrangement of oxygen anions and the same volume as its observed equivalent. Comparison of experimental data recorded in the literature for quartz with these hypothetical crys-tal structures shows that quartz becomes more ideal as temperature increases, more BCC as pressure increases, and that model quartz is a very good representation of observed quartz under all conditions. This is consistent with the hypothesis that quartz compresses through Si-O-Si angle-bending, which is resisted by anion-anion repulsion resulting in increasing distortion of the c/a axial ratio from ideal as temperature decreases and/or pressure increases.
- Yang, H., Downs, R. T., & Yang, Y. W. (2011). Lithiomarsturite, LiCa2Mn2Si5O 14(OH). Acta Crystallographica Section E: Structure Reports Online, 67(12), i73.More infoPMID: 22199471;PMCID: PMC3238580;Abstract: Lithio-marsturite, ideally LiCa2Mn2Si 5O14(OH), is a member of the pectolite-pyroxene series of pyroxenoids (hydro-pyroxenoids) and belongs to the rhodonite group. A previous structure determination of this mineral based on triclinic symmetry in space group P1̄ by Peacor et al. [Am. Mineral. (1990), 75, 409-414] converged with R = 0.18 without reporting any information on atomic coordinates and displacement param-eters. The current study redetermines its structure from a natural specimen from the type locality (Foote mine, North Carolina) based on single-crystal X-ray diffraction data. The crystal structure of lithio-marsturite is characterized by ribbons of edge-sharing CaO6 and two types of MnO6 octa-hedra as well as chains of corner-sharing SiO4 tetra-hedra, both extending along [110]. The octa-hedral ribbons are inter-connected by the rather irregular CaO8 and LiO6 polyhedra through sharing corners and edges, forming layers parallel to (1̄11̄ ), which are linked together by the silicate chains. Whereas the coordination environments of the Mn and Li cations can be compared to those of the corresponding cations in nambulite, the bonding situations of the Ca cations are more similar to those in babingtonite. In contrast to the hydrogen-bonding scheme in babingtonite, which has one O atom as the hydrogen-bond donor and a second O atom as the hydrogen-bond acceptor, our study shows that the situation is reversed in lithio-marsturite for the same two O atoms, as a consequence of the differences in the bonding environments around O atoms in the two minerals.
- Yang, H., Downs, R. T., Evans, S. H., Feinglos, M. N., & Tait, K. T. (2011). Crystal structure of uchucchacuaite, AgMnPb3Sb5S 12, and its relationship with ramdohrite and fzélyite. American Mineralogist, 96(7), 1186-1189.More infoAbstract: Uchucchacuaite, ideally AgMnPb3Sb5S12, was originally reported as orthorhombic, with possible space group Pmmm, P222, or Pmm2, and unit-cell parameters a = 12.67, b = 19.32, and c = 4.38 Å obtained from powder X-ray diffraction data (Moëlo et al. 1984a). Using single-crystal X-ray diffraction, we examined two uchucchacuaite samples, one from the type locality, Uchucchacua, Peru, and the other from Hokkaido, Japan (designated as R100213 and R070760, respectively). Our results show that uchucchacuaite is isostructural with ramdohrite and fizélyite, with monoclinic symmetry (P21/n) and the unit-cell parameters a = 19.3645(11), b = 12.7287(8), c = 8.7571(6) Å, β = 90.059(3)° for R100213 and a = 19.3462(7), b = 12.7251(5), c = 8.7472(3) Å, β = 90.017(2)° for R070760. Both samples are pervasively twinned and the twin refinements yielded the final R1 factors of 0.037 and 0.031 for R100213 and R070760, respectively. The chemical compositions determined from electron microprobe analysis are Ag0.99(Mn0.92Pb 0.03Sb0.02Bi0.01)Σ=0.98 Pb3.00Sb5. 00S12.00 for R100213 and Ag 1.00(Mn0.82Sb0.11Ag0.04Bi 0.02)Σ=0.99Pb2.98vSb5.00S 12.00 for R070760. The key structural difference among uchucchacuaite, ramdohrite, and fizélyite lies in the cations occupying the M2 site, which can be expressed with a general structural formula as Ag(M2+2yAg1/2-ySb1/2-y)Pb 3Sb5S12, where M2+ represents divalent cations with 0 ≤ y ≤ 1/2. From the current list of IMA-defined minerals, we consider M = Cd with y = 0.125 for ramdohrite, M = Pb with y = 0.25 for fizélyite, and M = Mn with y = 0.5 for uchucchacuaite. Associated with the variation in the average M2 cation size from fizélyite (1.078 Å) to ramdohrite (0.955 Å) and uchucchacuaite (0.83 Å) is the significant decrease in the average M2-S bond distance from 2.917 to 2.834, and 2.654 Å, respectively, as well as corresponding variations in the unit-cell b dimension from ∼13.23 to 13.06 and 12.73 Å.
- Yang, H., Evans, S. H., Downs, R. T., & Jenkins, R. A. (2011). The crystal structure of vladimirite, with a revised chemical formula, Ca4(AsO4)2(AsO3OH)·4H 2O. Canadian Mineralogist, 49(4), 1055-1064.More infoAbstract: Two samples of vladimirite, one from Bou Azzer, Morocco, and the other from a new occurrence in Copiapó, Chile (designated as R100075 and R080001, respectively), were examined with an electron microprobe, single-crystal X-ray diffraction, and Raman spectroscopy. Our results show that vladimirite is monoclinic with space group P21/c and unit-cell parameters a 5.8279(2), b 10.1802(4), c 22.8944(10) Å, β 96.943(2)°, and V 1348.35(9) Å3 for R100075 and a 5.8220(1), b 10.1750(2), c 22.8816(6) Å, β 96.902(1)°, and V 1345.66(5) Å3 for R080001. The structure determinations, with R1, = 0.022 and 0.023 for R100075 and R080001, respectively, yielded an ideal chemical formula Ca4(AsO4)2(AsO3OH)·4H 2O (Z = 4) for this mineral, in contrast to Ca5(AsO 4)2(AsO3OH)2·5H2O (Z = 3) documented in the literature. The chemical compositions for R100075 and R080001 are Ca4.03(AsO4)2(As 0.99O3OH)·4H2O and Ca 3.97(AsO4)2(As1.01O 3OH)·4H2O, with trace Zn and Mn, respectively. The structure is characterized by undulating layers formed by the four nonequivalent, rather irregular Ca polyhedra [CalO6(H2O), Ca2O6(H2O), Ca3O4(H 2O)3, and Ca4O5(H2O)3] linked through the sharing of edges and vertices. These undulating layers are parallel to (010) and are interconnected by AsO4 and ASO 3OH tetrahedra, as well as hydrogen bonds, along the b axis. Vladimirite is remarkable inasmuch as one of the hydrogen-bonded O-H···O distances (012H-H···O3) is only 2.465(2) Å, which is the shortest donoracceptor distance of all known Ca-bearing arsenate minerals, similar to the short donor-acceptor distances observed in several synthetic compounds containing AsO3OH groups.
- Yang, H., Jenkins, R. A., Downs, R. T., Evans, S. H., & Tait, K. T. (2011). Rruffite, Ca2Cu(AsO4)2·2H 2O, a new member of the roselite group, from Tierra Amarilla, Chile. Canadian Mineralogist, 49(3), 877-884.More infoAbstract: A new mineral species, rruffite, ideally Ca2Cu(AsO 4)2·2H2O, has been found in the oxidation zone of the Cu-As orebody in the Maria Catalina mine, Tierra Amarilla, Chile. It is a secondary mineral and occurs in granular or blocky aggregates and druses. Associated minerals include quartz, barite, mansfieldite, alumopharmacosiderite, conichalcite, metazeunerite, and barahonite-(Al). Rruffite is pale or light blue, transparent with white streak and vitreous luster. It is brittle, has a Mohs hardness of ∼3 and a perfect cleavage on {010}, and is twinned on (100). The measured and calculated densities are 3.79(3) and 3.77(2) g/cm3, respectively. Rruffite is insoluble in water, acetone, and hydrochloric acid. Optically, it is biaxial (-), with α 1.725(1), β 1.734(1), γ 1.740(1), 2Vmeas 80(2)°, Y= b, X ∧ c 49°, and it does not fluoresce under long-or short-wave ultraviolet light. The dispersion is weak, with r < v. An electron-microprobe analysis yielded the empirical formula Ca 2.01Cu1.01(AsO4)2.02·1. 9H2O. Rruffite, isostructural with roselite, is monoclinic, with space group P21/c and unit-cell parameters a 5.8618(2), b 12.7854(5), c 5.7025 (2) Å, β 109.425(2)°, and V 403.05(3) Å3. Its structure is characterized by isolated CuO 4(H2O)2 octahedra that are linked by corner-sharing with ASO4 tetrahedra to form the krohnkite-type chains parallel to the c axis. These chains are linked together by large Ca cations and hydrogen bonding. Owing to the strong Jahn-Teller effect, the MO 4(H2O)2 octahedron in rruffite is the most distorted of all known roselite-group minerals (M = Cu, Co, Mn, Mg, and Zn) in terms of the quadratic elongation of the octahedra. The Raman spectra of rruffite resemble those of arsenate minerals of the roselite group.
- Yang, H., Sun, H. J., & Downs, R. T. (2011). Hazenite, KNaMg2(PO4)2l4H2O, a new biologically related phosphate mineral, from Mono Lake, California, U.S.A.. American Mineralogist, 96(4), 675-681.More infoAbstract: A new biologically related, struvite-type phosphate mineral, hazenite, ideally KNaMg2(PO4)2- 14H2O, has been found in and/or on completely dried-out or decomposed cyanobacteria on porous calciumcarbonate (mainly calcite and aragonite) substrates in Mono Lake, California. The mineral occurs as radiating clusters of prismatic crystals and is colorless, transparent with white streak and vitreous luster. It is brittle, with the Mohs hardness of 2∼2.5; cleavage is good on {001} and no twinning was observed. The measured and calculated densities are 1.91(3) and 1.88(2) g/cm3, respectively. Hazenite is biaxial (+), with nα =1.494(1), nβ = 1.498(1), nγ = 1.503(1), 2Vmeas = 41(2)°, 2Vcalc = 42°, x = b, Y = c, Z = a, and does not fluoresce under longor short-wave ultraviolet rays. The dispersion is strong with r > v. It is soluble in water. The electron microprobe analysis yielded an empirical formula of K0.97(Na0.96Ca0.02) Mg2.07[(P0.98S0.02)O4] 213.90H2O. Hazenite is orthorhombic with space group Pmnb and unit-cell parameters a = 6.9349(4) Å, b = 25.174(2) Å, c = 11.2195(8) Å, and V = 1958.7(3) Å3. There are many structural similarities between hazenite and struvite, as also revealed by their Raman spectra. The hazenite structure contains six symmetrically independent non-hydrogen cation sites, two for Mg2+ (Mg1 and Mg2), two for P 5+ (P1 and P2), one for Na+, and one for K+. It can be viewed as three types of layers stacking along the b-axis, in a repeating sequence of ABCBABCB..., where layer A consists of Mg1(H 2O)6 octahedra and NaO6 trigonal prisms, layer B of P1O4 and P2O4 tetrahedra, and layer C of Mg2(H 2O)6 octahedra and very irregular KO6 polyhedra. These layers are linked together by hydrogen bonds, plus the K-O bonds between layers B and C (K-O5-P2). Interestingly, the combination of layers B and C in hazenite exhibits a configuration analogous to the struvite-(K) structure. Hazenite is believed to form in high pH environments through the involvement of cyanobacterial activities. To our knowledge, hazenite is the first struvite-type compound that contains two structurally distinct monovalent cations (K and Na), pointing to an exclusive role of biological activity in the mineralization process.
- Barkley, M. C., Yang, H., & Downs, R. T. (2010). Kzulite, an Mn-rich alkali amphibole. Acta Crystallographica Section E: Structure Reports Online, 66(12), i83.More infoPMID: 21589206;PMCID: PMC3011386;Abstract: The crystal structure of kôzulite, an Mn-rich alkali amphibole with the ideal formula NaNa2[Mn42+(Fe 3+,Al)]Si8O22(OH)2, tris-odium tetra-manganese iron/aluminium octa-silicate dihydroxide, was refined from a natural specimen with composition (K0.20Na0.80)(Na 1.60Ca0.18Mn2+0.22)(Mn2+2.14Mn3+0.25Mg2.20Fe3+0.27Al0.14)(Si7.92Al0.06Ti 0.02)O22[(OH)1.86F0.14]. The site occupancies determined from the refinements are M1 = 0.453 (1) Mn + 0.547 (1) Mg, M2 = 0.766 (1) Mn + 0.234 (1) Mg, and M3 = 0.257 (1) Mn + 0.743 (1) Mg, where Mn and Mg represent (Mn+Fe) and (Mg+Al), respectively. The average M - O bond lengths are 2.064 (1), 2.139 (1), and 2.060 (1) Å for the M1, M2, and M3 sites, respectively, indicating the preference of large Mn2+ for the M2 site. Four partially occupied amphibole A sites were revealed from the refinement, with A(m) = 0.101 (4) K, A(m)′ = 0.187 (14) Na, A(2) = 0.073 (6) Na, and A(1) = 0.056 (18) Na, in accord with the result derived from microprobe analysis (0.20 K + 0.80 Na), considering experimental uncertainties.
- Deschaines, T., Lowry, S., Wieboldt, D., & Downs, R. (2010). Applying Raman spectrocospy and multivariate statistical analysis to the characterization of minerals. SME Annual Meeting and Exhibit 2010, 581-583.
- Gibbs, G. V., Wallace, A. F., Zallen, R., Downs, R. T., Ross, N. L., Cox, D. F., & Rosso, K. M. (2010). Bond paths and van der waals interactions in orpiment, As2S 3. Journal of Physical Chemistry A, 114(23), 6550-6557.More infoPMID: 20499911;Abstract: The calculated electron density distribution for orpiment, As 2S3, reveals that As-S, S-S, and A-As bond paths are associated with the experimental interlayer directed bonded interactions detected in a combined infrared and Raman study. The successful modeling of the infrared-and Raman-determined interlayer bonded interactions together with bond paths and the structuralization of a variety of inorganic molecules in terms of key-lock bond path mainstays support the argument that van der Waals forces in inorganic molecular crystals are directional. © 2010 American Chemical Society.
- Lavina, B., Dera, P., Downs, R. T., Tschauner, O., Yang, W., Shebanova, O., & Shen, G. (2010). Effect of dilution on the spin pairing transition in rhombohedral carbonates. High Pressure Research, 30(2), 224-229.More infoAbstract: The compressibility of an iron-bearing magnesite (Mg0.87Fe 2+0.12Ca0.01)CO3 was determined by means of single crystal diffraction up to 64GPa. Up to 49 GPa the pressure-evolution of the unit cell volume of the solid solution with 12% of Fe2+ can be described by a third-order Birch-Murnaghan equation of state with parameters V0=281.0(5)Å3, K 0=102.8(3)GPa, K'0=5.44. The spin pairing of the Fe 2+ d-electrons occurs between 49 and 52 GPa, as evidenced by a discontinuous volume change. The transition pressure is increased by about 5GPa compared with the iron end-member; an effect consistent with a cooperative contribution of adjacent clusters to the spin transition. The trend is, however, opposite in the periclase-wüstite solid solution. Differences among the two structures, in particular in the Fe-Fe interactions, that might explain the different behavior are discussed. © 2010 Taylor & Francis.
- Lavina, B., Dera, P., Downs, R. T., Yang, W., Sinogeikin, S., Meng, Y., Shen, G., & Schiferl, D. (2010). Structure of siderite FeCO3 to 56 GPa and hysteresis of its spin-pairing transition. Physical Review B - Condensed Matter and Materials Physics, 82(6).More infoAbstract: The structure of siderite, FeCO3, was determined to 56 GPa, beyond the spin-pairing transition of its iron d electrons. Fe2+ in the siderite structure is in the high-spin state at low pressures and transforms to the low-spin (LS) state over a narrow pressure range, 44 to 45 GPa, that is concomitant with a shrinkage of the octahedral bond distance by 4%, and a volume collapse of 10%. The structural rearrangements associated with the electronic transition are nearly isotropic in contrast with other properties of siderite, which mostly are highly anisotropic. Robust refinements of the crystal structure from single-crystal x-ray diffraction data were performed at small pressure intervals in order to accurately evaluate the variation in the interatomic distances and to define the geometry of the carbonate hosting LS-Fe2 +. Thermal vibrations are remarkably lowered in the LS-Sd as shown by atomic displacement parameters. The formation of like-spin domains at the transition shows a hysteresis of more than 3 GPa, compatible with a strong cooperative contribution of neighboring clusters to the transition. © 2010 The American Physical Society.
- Tait, K. T., Yang, H., Downs, R. T., Chen, L. i., & Pinch, W. W. (2010). The crystal structure of esperite, with a revised chemical formula, PbCa2(ZnSiO4)3, isostructural with beryllonite. American Mineralogist, 95(5-6), 699-705.More infoAbstract: Esperite from Franklin, New Jersey, was first described by Moore and Ribbe (1965) as monoclinic with a well-developed "superlattice" a = 2 × 8.814(2) Å, b = 8.270(3) Å, c = 2 × 15.26(1) Å, β ≈ 90°, space group P21/n (subcell), and the chemical formula PbCa3(ZnSiO4)4. They attributed "superlattice" reflections to the ordered distributions of Pb and Ca cations over four beryllonite-type subcells for esperite with the Ca:Pb ratio greater than 2:1. We examined two esperite fragments from the type sample using single-crystal X-ray diffraction, electron microprobe analysis, and Raman spectroscopy. Although both fragments have Ca:Pb ≈ 1.8, one exhibits the "superlattice" reflections as observed by Moore and Ribbe (1965), whereas the other does not. The sample without "superlattice" reflections has unit-cell parameters a = 8.7889(2), b = 8.2685(2), c = 15.254(3) Å, β = 90.050(1)°, V = 1108.49(4) Å3, and the chemical composition Pb1.00(Ca1.86Fe2+0.07Mn0.04Cr3+0.02) Σ=1.99(Zn1.00Si1.00O4)3. Its crystal structure was solved in space group P21/n (R1 = 0.022). Esperite is isostructural with beryllonite, NaBePO4, and its ideal chemical formula should, therefore, be revised to PbCa 2(ZnSiO4)3, Z = 4. The ZnO4 and SiO4 tetrahedra in esperite share corners to form an ordered framework, with Pb2+ occupying the nine-coordinated site in the large channels and Ca2+ occupying the two distinct octahedral sites in the small channels. The so-called "superlattice" reflections are attributed to triple twins, a trilling of ∼60° rotational twinning around the b axis, similar to those observed in many other beryllonite-type materials. A phase transformation from a high-temperature polymorph to the esperite structure is proposed to be responsible for the twinning formation.
- Thompson, R. M., & Downs, R. T. (2010). Packing system a tics of the silica polymorphs: The role played by O-O nonbonded interactions in the compression of quartz. American Mineralogist, 95(1), 104-111.More infoAbstract: The anion skeleton of quartz is a distorted body-centered cubic (BCC) arrangement. A hypothetical ideal. BCC crystal structure for quartz has been derived and used to locate and describe the unoccupied tetrahedral. sites, quantify the distortion of the quartz anion arrangement from ideal BCC, and characterize the role of tetrahedral distortion, and O-O interactions in the compression of quartz. Quartz has eight crystallographically nonequivalent tetrahedra, one occupied by silicon and seven unoccupied. These tetrahedra completely nil space, something that cannot be done using only regular tetrahedra. In ideal. BCC quartz, the nonequivalent tetrahedra are identical in size and shape with a unique geometry and are referred to as Sommerville tetrahedra. In reality, the unoccupied tetrahedra of quartz are very distorted from both regular and Sommerville tetrahedra. Changes in the unoccupied tetrahedra are responsible for most of the compression in quartz with pressure, as the volume of the Si tetrahedron decreases by
- Aydin, F., Thompson, R. M., Karsli, O., Uchida, H., Burt, J. B., & Downs, R. T. (2009). C2/c pyroxene phenocrysts from three potassic series in the Neogene alkaline volcanics, NE Turkey: Their crystal chemistry with petrogenetic significance as an indicator of P -T conditions. Contributions to Mineralogy and Petrology, 158(1), 131-147.More infoAbstract: Chemical and structural data are reported for C2/c pyroxene phenocrysts collected from three potassic series (Group A: basanite-tephrite, Group B: tephrite-phonolitic tephrite, Group C: alkaline basalt-trachybasalt) of the Neogene alkaline volcanics (NAVs) in northeastern Turkey, in order to investigate the evolution of the magmatic plumbing system and the location of magma chamber(s) with crystallization conditions. The rock series hosting the clinopyroxene phenocrysts show generally porphyritic texture and have a variable phenocryst-rich nature (20-58%), with phenocryst assemblages characterized by cpx ± ol ± plag ± foid ± amp ± bio. The clinopyroxene phenocrysts can be chemically classified as Ti- and Fe3-rich Al-diopsides for Groups A and B (ABcpxs) and Ti- and Fe3-poor Al-diopsides for Group C (Ccpxs). They have poorly variable composition, clustering in the diopside field. Structurally, the diopside groups have nearly similar a (ranging from 9.73 to 9.75 Å), Vcell (437.2-440.9 Å3), and hbetai angle values (106.01°-106.23°), but some differences in polyhedral parameters and geometries of the AB-cpxs and C-cpxs have been observed. For example, the AB-cpxs are characterized by. larger c (5.27-5.30 vs. 5.25-5.28 Å), VT (2.27-2.30 vs. 2.23-2.28 Å3), and VM2 (25.53-25.72 vs. 25.41-25.59 Å3) values and smaller b (8.87-8.88 vs. 8.88-8.91 Å) and VM1(11.49-1.63 vs. 11.64-11.83 Å3) values with respect to the C-cpxs. In addition, the AB-cpxs show higher values of VM2/VM1 (2.20-2.23) due to large VM2 and small VM1 compared to the VM2/ VM1 ratios of the C-cpxs (
- Dera, P., Lavina, B., Borkowski, L. A., Prakapenka, V. B., Sutton, S. R., Rivers, M. L., Downs, R. T., Boctor, N. Z., & Prewitt, C. T. (2009). Structure and behavior of the barringerite Ni end-member, Ni2P, at deep Earth conditions and implications for natural Fe-Ni phosphides in planetary cores. Journal of Geophysical Research B: Solid Earth, 114(3).More infoAbstract: High-pressure and high-temperature behavior of synthetic Ni2P has been studied in a laser-heated diamond anvil cell up to 50 GPa and 2200 K. Incongruent melting associated with formation of pyrite-type NiP2 and amorphous Ni-P alloy was found at an intermediate pressure range, between 6.5 and 40 GPa. Above 40 GPa, Ni2P melts congruently. At room conditions, Ni2P has hexagonal C22-type structure, and without heating it remains in this structure to at least 50 GPa. With a bulk modulus .K0 = 201(8) GPa and K' = 4.2(6 Ni2P is noticeably less compressible than hep Fe, as well as all previously described iron phosphides, and its presence in the Earth core would favorably lower the core density. In contrast to Fe2P, the c/a ratio in Ni2P decreases on compression because of the lack of ferromagnetic interaction along the c direction. Lack of the C22→C23 transition in Ni2P rules out a stabilizing effect of Ni on the orthorhombic phase of natural (Fe1-xNix)P allabogdanite. Copyright 2009 by the American Geophysical Union.
- Downs, R. (2009). Notes from trie editors. Mineralogical Record, 40(4), 267-269.
- Gibbs, G. V., Wallace, A. F., Cox, D. F., Dove, P. M., Downs, R. T., Ross, N. L., & Rosso, K. M. (2009). Role of directed van der Waals bonded interactions in the determination of the structures of Molecular arsenate solids. Journal of Physical Chemistry A, 113(4), 736-749.More infoPMID: 19123777;Abstract: Bond paths, local energy density properties, and Laplacian, L(r) = -∇ 2p(r), composite isosurfaces of the electron density distributions were calculated for the intramolecular and intermolecular bonded interactions for molecular solids of As 2O 3 and AsO 2 composition, an As 2O 5 crystal, a number of arsenate molecules, and the arsenic metalloid, arsenolamprite. The directed intermolecular van der Waals As-O, O-O, and As-As bonded interactions are believed to serve as mainstays between the individual molecules in each of the molecular solids. As-O bond paths between the bonded atoms connect Lewis base charge concentrations and Lewis acid charge depletion domains, whereas the O-O and As-As paths connect Lewis base pair and Lewis acid pair domains, respectively, giving rise to sets of intermolecular directed bond paths. The alignment of the directed bond paths results in the periodic structures adopted by the arsenates. The arrangements of the As atoms in the claudetite polymorphs of As 2O 3 and the As atoms in arsenolamprite are similar. Like the As203 polymorphs, arsenolamprite is a molecular solid connected by relatively weak As-As intermolecular directed van der Waals bond paths between the layers of stronger As-As intramolecular bonded interactions. The bond critical point and local energy density properties of the intermolecular As-As bonded interactions in arsenolamprite are comparable with the As-As interactions in claudetite I. As such, the structure of claudetite I can be viewed as a stuffed derivative of the arsenolamprite structure with O atoms between pairs of As atoms comprising the layers of the structure. The cubic structure adopted by the arsenolite polymorph can be understood in terms of sets of directed acid-base As-O and base-base O-O pair domains and bond paths that radiate from the tetrahedral faces of its constituent molecules, serving as face-to-face key-lock mainstays in forming a periodic tetrahedral array of molecules rather than one based on some variant of close packing. The relatively dense structure and the corrugation of the layers in claudetite I can also be understood in terms of directed van der Waals As-O bonded interactions. Our study not only provides a new basis for understanding the crystal chemistry and the structures of the arsenates, but it also calls for a reappraisal of the concept of van der Waals bonded interactions, how the structures of molecular solids are viewed, and how the molecules in these solids are bonded in a periodic structure. © 2009 American Chemical Society.
- Gibbs, G. V., Wallace, A. F., Cox, D. F., Downs, R. T., Ross, N. L., & Rosso, K. M. (2009). Bonded interactions in silica polymorphs, silicates, and siloxane molecules. American Mineralogist, 94(8-9), 1085-1102.More infoAbstract: Experimental model electron density distributions obtained for the silica polymorphs coesite and stishovite are comparable with electron density distributions calculated for various silicates and siloxane molecules. The Si-O bond lengths and Si-O-Si angles calculated with first-principles density functional theory methods as a function of pressure are also comparable with the bond lengths and angles observed for coesite and quartz within the experimental error. The similarity of the topological properties of the Si-O bonded interactions and the experimental and the geometry-optimized structures for the silica polymorphs provide a basis for understanding the properties and crystal chemistry of silica. The agreement supports the argument that the bulk of the structural and physical properties of the silica polymorphs are intrinsic properties of molecular-like coordination polyhedra such that the silica polymorphs can be pictured as "supermolecules" of silica bound by virtually the same forces that bind the Si and O atoms in simple siloxane molecules. The topology of the electron density distribution is consistent with the assertion that the Si-O bonded interaction arises from the net electrostatic attraction exerted on the nuclei by the electron density accumulated between the Si and O atoms. The correlation between the Si-O bond length and Si-O-Si angle is ascribed to the progressive local concentration of the electron density in the nonbonded lone pair region of the O atom rather than to a bonded interaction that involves the d-orbitals on Si. The accumulation of deformation electron density, Δ?(r), in the bonded and nonbonded regions of the Si-O bond, the close proximity of the bond critical point, rc, of the bond with the nodal surface of the Laplacian and the negative value of the total energy density are taken as evidence that the bond has a nontrivial component of shared character. For M-O bonded interactions for first and second row metal atoms bonded to O, ∇2?(rc) is positive and increases linearly as ?(rc) and G(rc)/?(rc) both increase and as the value of H(rc) decreases; the greater the shared character of the interaction, the larger the values of both ∇2?(rc) and G(rc)/?(rc). In addition, a mapping of ∇2?(r) serves to highlight those Lewis base domains that are susceptible to electrophilic attack by H, like the O atoms in coesite involved in bent Si-O-Si angles; the narrower the angle, the greater the affinity for H. On the basis of the net charges conferred on the Si and O atoms and the bonded radii of the two atoms, the Si-O bond for stishovite, with six-coordinated Si and three-coordinated O, is indicated to be more ionic in character than that in quartz with four-coordinated Si and two-coordinated O. Unlike the conclusion reached for ionic and crystal radii, it is the bonded radius of the O atom that increases with the increasing coordination number of Si, not the radius of the Si atom. The modeling of the electron density distributions for quartz, coesite, and beryl as a function of pressure suggests that the shared character of the bonded interactions in these minerals increases slightly with increasing pressure. The insight provided by the calculations and the modeling of the electron density distributions and the structures of the silica polymorphs bodes well for future Earth materials studies that are expected to improve and clarify our understanding of the connection between properties and structure within the framework of quantum mechanical observables, to find new and improved uses for the materials and to enhance our understanding of crystal chemistry and chemical reactions of materials in their natural environment at the atomic level.
- Graulis, S., Chateigner, D., Downs, R. T., Yokochi, A. F., Quirós, M., Lutterotti, L., Manakova, E., Butkus, J., Moeck, P., & Bail, A. L. (2009). Crystallography Open Database - An open-access collection of crystal structures. Journal of Applied Crystallography, 42(4), 726-729.More infoAbstract: The Crystallography Open Database (COD), which is a project that aims to gather all available inorganic, metal-organic and small organic molecule structural data in one database, is described. The database adopts an open-access model. The COD currently contains 80 000 entries in crystallographic information file format, with nearly full coverage of the International Union of Crystallography publications, and is growing in size and quality. © 2009 International Union of Crystallography.
- Lavina, B., Cesare, B., Alvarez-Valero, A. M., Uchida, H., Downs, R. T., Koneva, A., & Dera, P. (2009). Closure temperatures of intracrystalline ordering in anatectic and metamorphic hercynite, fe 2+al 2o 4. American Mineralogist, 94(5-6), 657-665.More infoAbstract: The closure temperature, T c, of the intracrystalline ordering of Mg-hercynite is estimated with a comparative ciystal-chemical approach. The single crystals were selected from two distinct geological environments that represent extremely different cooling rates. The fast cooled setting refers to anatectic metapelitic enclaves that occur in the high-K calc-alkaline lavas of the Neogene Volcanic Province of SE Spain. The slow cooled setting refers to metabauxite from the Anga metamorphic complex. Lake Baikal. Parameters sensitive to T c include the oxygen fractional coordinate (u) and the inversion parameter (i). Experimental equilibration data on the spinel and hercynite end-members and on their solid solution are fitted to equations where T c is given as a function of the hercynite content (Hc) of the solid solution and of u or i. The unavoidable simplifications made in this empirical approach are discussed. A reasonable value for T c, ~400 °C, was obtained for the slow cooled metamorphic hercynite from the oxygen fractional coordinates. In contrast, an unreasonably high value of T c, ~600 °C, was obtained from the inversion parameters. In the case of the fast cooled anatectic samples, T ccalculated from the two structural parameters are comparable; the five crystals show a range in the calculated values for T c over ~250 °C, from ~700 to ~950 °C, which is reasonable considering the known diversity of cooling rates exhibited by their volcanic host-rocks.
- Lavina, B., Dera, P., Downs, R. T., Prakapenka, V., Rivers, M., Sutton, S., & Nicol, M. (2009). Siderite at lower mantle conditions and the effects of the pressure-induced spin-pairing transition. Geophysical Research Letters, 36(23).More infoAbstract: Siderite (FeCO3) forms a complete solid solution with magnesite (MgCO3), the most likely candidate for a mantle carbonate. Our experiments with natural siderite reveal spin pairing of d-orbital electrons of Fe2+ at 43 GPa, as evidenced by a sharp volume collapse of about 10%. The initially colorless crystals assume an intense green color after the transition, which progressively turns to red above 60 GPa. We present clear evidence for the instability of an intermediate spin state in siderite at ambient temperature. At the transition pressure, domains of high and low spin siderite coexist. The unit cell volume difference between magnesite and siderite is significantly decreased by the spin transition, enhancing the solubility between the two calcite-type minerals. A siderite component in magnesite at lower mantle pressure would significantly increase its density and slightly increase the carbonate bulk modulus. Copyright 2009 by the American Geophysical Union.
- Lowry, S., Wiebolt, D., Dalrymple, D., Jasinevicius, R., & Downs, R. T. (2009). The use of a raman spectral database of minerals for the rapid verification of semiprecious gemstones. Spectroscopy (Santa Monica), 24(5).More infoAbstract: The use of a Raman spectral database of minerals for the rapid verification of semiprecious gemstones was demonstrated. Ninety-six semiprecious gemstones were placed in a glass-bottomed 96 micro well plate for this demonstration. Raman spectra were obtained from 96 semiprecious gemstones using a Thermo Scientific DXR Smart Raman System. The system was configured with an automated x-y stage designed to hold SBS standard micro well plates. Oriented spectra were obtained using a special system designed for polarization studies with a 514-nm laser, while high-resolution spectra were obtained using Raman instrument. It was observed that different samples produced different levels of Raman signal, which proved that autoexposure feature could be used to obtain a desired signal level. The use of 'full range' option on the Raman instruments provided a tool to verify if a sample was treated with polymer or epoxy.
- Origlieri, M. J., Downs, R. T., Pinch, W. W., & Zito, G. L. (2009). Stibioclaudetite ASSB03 a new mineral from tsumeb, namibia. Mineralogical Record, 40(3), 209-213.More infoAbstract: Stibioclaudetite is a new mineral species with ideal chemistry AsSb0 3. The mineral has monoclinic symmetry, P21/n, with a = 4.5757(4)Å,b = 13.1288(13) A, c = 5.4216(5) Å, ß = 95.039(4)°, V = 324.44(5) Å3, Z = 4, and dcaic = 5.009 g/cm3. The strongest X-ray lines (calculated) are 3.512 (100), 3.282 (82), 3.238 (71), 2.279 (34), and 4.995 (32). The average of ten microprobe analyses is 45.15% As203 and 55.77% Sb 203, total 100.92, corresponding to As 1.088Sb0.912O3. Stibioclaudetite forms adamantine, colorless transparent bladed crystals to 6 mm, bound by {010}, {110}, {111}, and {101}. The mineral is flexible with perfect cleavage on {010}. The hardness is 2.00. Stibioclaudetite occurs with leiteite, ludlockite, smithsonite and quartz in a vug within massive tennantite from the Tsumeb mine, Tsumeb, Namibia. Stibioclaudetite is isostructural with claudetite, specifically an Sb-substituted ordered analog, and the name denotes the relationship. The crystal structure consists of corrugated sheets of corner-sharing As03 and Sb03 trigonal pyramids arranged in an ordered, alternating pattern. Raman spectra of stibioclaudetite, claudetite, and leiteite are presented and compared.
- Yang, H., Downs, R. T., Burt, J. B., & Costin, G. (2009). Structure refinement of an untwinned single crystal of Ag-excess fizélyite, Ag5.94Pb13.74Sb20.84S 48. Canadian Mineralogist, 47(5), 1257-1264.More infoAbstract: The crystal structure of Ag-excess fizélyite from the Van Silver mine, just north of Squamish, British Columbia, Canada, was studied on the basis of X-ray-diffraction data collected from an untwinned single crystal. The structure was refined to R1 = 0.041 in space group P21/n, with unit-cell parameters a 19.2767(6), b 13.2345(4), c 8.7230(3) Å, β 90.401(2)°, and V 2225.3(1) Å3. The chemical composition of the sample, determined by electron-microprobe analysis, is Ag5.94Pb13.74Sb20.84S48, which reveals nearly one more Ag atom per formula unit than in the ideal composition, Ag5Pb14Sb21S48, presently proposed by the IMA for fizélyite. Except for an additional partially occupied Ag site (Ag2) and two split cation sites (Ag1 and M2) in our sample, fizélyite is isostructural with ramdohrite, CdAg5.5Pb 12Sb21.5S48. The structure of fizélyite contains chains of edge-sharing SbS6 octahedra connected by chains of alternating M1S6, (M1 = Pb + Sb) and SbS6 octahedra, forming slabs parallel to (100). The slabs of octahedra are linked by distorted AgS4 tetrahedra, M2S6 octahedra, and PbS8 polyhedra. The excess Ag in our fizélyite is mostly situated at the Ag2 site, and is likely responsible for the splitting of the Ag1 and M2 sites.
- Yang, H., Konzett, J., & Downs, R. T. (2009). Crystal structure and compressibility of a high-pressure Ti-rich oxide, (Ti0.50Zr0.26Mg0.14Cr0.10)O1.81, isomorphous with cubic zirconia. Journal of Physics and Chemistry of Solids, 70(9), 1297-1301.More infoAbstract: A Ti-rich oxide, (Ti0.50Zr0.26Mg0.14Cr0.10)∑=1.0O1.81, was synthesized at 8.8 GPa and 1600 °C using a multi-anvil apparatus. Its crystal structure at ambient conditions and compressibility up to 10.58 GPa were determined with single-crystal X-ray diffraction. This high-pressure phase is isomorphous with cubic zirconia (fluorite-type) with space group Fm3̄m and unit-cell parameters a=4.8830(5) Å and V=116.43(4) Å3. Like stabilized cubic zirconia, the structure of (Ti0.50Zr0.26Mg0.14Cr0.10)O1.81 is also relaxed, with all O atoms displaced from the (frac(1, 4), frac(1, 4), frac(1, 4)) position along 〈1 0 0〉 by 0.319 Å and all cations from the (0, 0, 0) position along 〈1 1 1〉 by 0.203 Å. No phase transformation was detected within the experimental pressure range. Fitting the high-pressure data (V vs. P) to a third-order Birch-Murnaghan EOS yields K0=164(4) GPa, K′=4.3(7), and V0=116.38(3) Å3. The bulk modulus of (Ti0.50Zr0.26Mg0.14Cr0.10)O1.81 is significantly lower than that (202 GPa) determined experimentally for cubic TiO2 or that (~210 GPa) estimated for cubic ZrO2. This study demonstrates that cubic TiO2 may also be obtained by introducing various dopants, similar to the way cubic zirconia is stabilized below 2370 °C. Furthermore, (Ti0.50Zr0.26Mg0.14Cr0.10)O1.81 has the greatest ratio of Ti4+ content vs. vacant O2- sites of all doped cubic zirconia samples reported thus far, making it a more promising candidate for the development of electrolytes in solid oxide fuel cells. © 2009 Elsevier Ltd. All rights reserved.
- Yang, H., Konzett, J., Downs, R. T., & Frost, D. J. (2009). Crystal structure and Raman spectrum of a high-pressure Li-rich majoritic garnet, (Li2Mg)Si2(SiO4)3. American Mineralogist, 94(4), 630-633.More infoAbstract: A Li-rich majoritic garnet (LiMGt), (Li2Mg)Si 2(SiO4)3, was synthesized at 15 GPa and 1500 °C and its structure studied with single-crystal X-ray diffraction and Raman spectroscopy. It is cubic with space group Ia3̄Bd and unit-cell parameters a = 11.2660(2) å and V = 1429.91(1) å3. The 8-, 6-, and 4-coordinated cation sites in LiMGt are occupied by (Li+ + Mg 2+), Si4+, and Si4+, respectively. Whereas the SiO6 octahedron is nearly regular, the XO8 dodecahedron is the most distorted of all known silicate garnets in terms of the bond-length distortion index. All Raman peaks of LiMGt are broader than those of pyrope, due to the substitution of Li+ for Mg2+ at the dodecahedral site. Furthermore, both Si-O symmetric stretching (A1g-v1) and O-Si-O symmetric bending (A1g-v2) modes of LiMGt shift significantly to higher frequencies relative to the corresponding ones of pyrope. In contrast, the A1g-(SiO4) rotational mode of LiMGt displays a much lower frequency than that of pyrope. This study represents the first structural report on a garnet with an all-silicate framework and suggests that, like Na incorporation in garnets, the pressure-dependent coupled substitution of (Li+ + Si4+) for (Mg2+ + Al3+) is likely one of the primary mechanisms for Li enrichment in garnets in the mantle and the transition zone.
- Yang, H., Konzett, J., Frost, D. J., & Downs, R. T. (2009). X-ray diffraction and Raman spectroscopic study of clinopyroxenes with six-coordinated Si in the Na(Mg0.5Si0.5)Si 2O6-NaAlSi2O6 system. American Mineralogist, 94(7), 942-949.More infoAbstract: Five clinopyroxenes containing various amounts of six-coordinated Si ( VISi) in the Na(Mg0.5Si0.5) Si 2O6 (NaPx)-NaAlSi2O6 (jadeite) system have been synthesized at 15 GPa and 1600 °C and their structures studied with single-crystal X-ray diffraction and Raman spectroscopy. The results show that clinopyroxenes with VISi ≤ 0.33 atoms per formula unit (apfu) possess C2/c symmetry, whereas those with VISi ≥ 0.45 apfu crystallize with P2/n symmetry. There is an obvious discontinuity in the unit-cell parameters a, β, and V as the VISi content increases from 0.33 to 0.45 apfu, suggesting that the C2/c-P2/n transition is first-order in character, rather than tricritical as reported for the transition from jadeite/diopside/augite to omphacite. The Mg and VISi cations in the P2/n structure are completely ordered into two nonequivalent octahedral sites, M1 and M1(1), respectively, with M1 being appreciably more distorted than M1(1). With increasing mean distance, the mean tetrahedral distance increases, whereas the O3-O3-O3 angle of the tetrahedral chain decreases systematically, consistent with the structural variation trends found in the jadeite-diopside system. A comparison of the Raman spectra reveals that the C2/c-P2/n transition is characterized by the splitting of many Raman bands in C2/c clinopyroxenes into doublets in P2/n, and such splitting becomes more pronounced with increasing VISi. For C2/c clinopyroxenes, all Raman bands become progressively broader with the increased substitution of (Si4+ + Mg2+) for Al3+. In addition, several new Raman bands, attributable to the presence of VISi, are observed. Together with previous data, we suggest that the maximum VISi content allowed for an Mg/Al dominated octahedral site is close to ∼35%, above which VISi and Mg/Al are likely to be ordered into distinct sites.
- Yang, H., Pinch, W. W., & Downs, R. T. (2009). Crystal structure of argentopyrite, AgFe2S3, and its relationship with cubanite. American Mineralogist, 94(11-12), 1727-1730.More infoAbstract: The structure of argentopyrite, AgFe2S3, was determined for the first time with single-crystal X-ray diffraction. In contrast to the previously reported orthorhombie symmetry, our data show that argentopyrite is monoclinic with space group P1121/n (non-standard setting) and unit-cell parameters a = 6.6902(2), b = 11.4497(4), c = 6.4525(2) Å, γ = 90.2420(8)°, and V= 494.26(3) Å3. Similar to cubanite (CuFe2S3), the structure of argentopyrite is also based on approximately hexagonal close-packed S atoms, with cations ordered over one half of the tetrahedral sites, forming corner-shared AgS4 and FeS4 tetrahedral sheets parallel to (001). The two structures differ chiefly in the linkage between the two adjacent tetrahedral sheets and the ordering patterns of cations within a tetrahedral sheet. Topologically, the structure of argentopyrite can be obtained by a displacement of a tetrahedral sheet in the cubanite structure along the (a/2 + b/6) direction relative to the sheet beneath, giving rise to a cluster of four edge-shared FeS4 tetrahedra in argentopyrite, as compared to two in cubanite. There are two distinct Fe sites (Fel and Fe2) in argentopyrite, rather than only one, as in other MFe2S3 sulfide minerals (M = monovalent cations). Together with published Mössbauer data, we suggest that there exists some degree Of Fe2+-Fe3+ order-disorder in argentopyrite, with Fe2+ favoring the more distorted Fe2 tetrahedral site. Argentopyrite appears to possess all the features proposed by Putnis (1977) for a high-temperature ordered form, of cubanite.
- Chen, L. i., Yang, H., & Downs, R. T. (2008). Redetermination of olivenite from an untwinned single-crystal. Acta Crystallographica Section E: Structure Reports Online, 64(9), i60-i61.More infoPMID: 21201567;PMCID: PMC2960593;Abstract: The crystal structure of olivenite, ideally Cu2(AsO4)(OH) [dicopper(II) arsenate(V) hydroxide], was redetermined from an untwinned and phosphate-containing natural sample, composition Cu2(As0.92P0.08O4), from Majuba Hill (Nevada, USA). Olivenite is structurally analogous with the important rock-forming mineral andalusite, Al2OSiO4. Its structure consists of chains of edge-sharing, distorted [CuO4(OH)2] octa-hedra extending parallel to [001]. These chains are cross-linked by isolated AsO4 tetra-hedra through corner-sharing, forming channels in which dimers of edge-sharing [CuO4(OH)] trigonal bipyramids are located. The structure is stabilized by medium to weak O - H⋯O hydrogen bonds. In contrast to the previous refinements from powder and single crystal X-ray data, all non-H atoms were refined with anisotropic displacement parameters and the H atom was located.
- Dera, P., Lavina, B., Borkowski, L. A., Prakapenka, V. B., Sutton, S. R., Rivers, M. L., Downs, R. T., Boctor, N. Z., & Prewitt, C. T. (2008). High-pressure polymorphism of Fe2P and its implications for meteorites and Earth's core. Geophysical Research Letters, 35(10).More infoAbstract: Minerals with composition (Fe,Ni)2P, are rare, though important accessory phases in iron and chondritic meteorites. The occurrence of these minerals in meteorites is believed to originate either from the equilibrium condensation of protoplanetary materials in solar nebulae or from the later accretion and condensation processes in the cores of parent bodies. Fe-Ni phosphides are considered a possible candidate for a minor phase present in the Earth's core, and at least partially responsible for the observed density deficit with respect to pure iron. We report results of high-pressure high-temperature X-ray diffraction experiments with synthetic barringerite (Fe2P) up to 40 GPa and 1400 K. A new phase transition to the C02Si-type structure has been found at 8.0 GPa, upon heating. The high-pressure phase can be metastably quenched to ambient conditions at room temperature, and then, if heated again, transforms back to barringerite, providing an important constraint on the thermodynamic history of meteorite. Copyright 2008 by the American Geophysical Union.
- Gibbs, G. V., Downs, R. T., Cox, D. F., Ross, N. L., Boisen Jr., M. B., & Rosso, K. M. (2008). Shared and closed-shell O-O interactions in silicates. Journal of Physical Chemistry A, 112(16), 3693-3699.More infoPMID: 18324795;Abstract: Bond paths of maximum electron density spanning O-O edges shared between equivalent or quasiequivalent MOn (n > 4) coordination polyhedra are not uncommon electron density features displayed by silicates. On the basis of the positive values for the local electronic energy density, H(r c), at the bond critical points, rc, they qualify as weak "closed-shell" interactions. As observed for M-O bonded interactions (M = first and second row metal atoms), the electron density, ρ(r c), and the Laplacian of the electron density increase in a regular way as the separation between the O atoms, R(O-O), decreases. A simple model, based on R(O-O) and the distances of the Si atoms from the midpoint between adjacent pairs of O atoms, partitions the O-O bond paths in the high-pressure silica polymorph coesite into two largely disjoint domains, one with and one without bond paths. The occurrence of O-O bond paths shared in common between equivalent coordination polyhedra suggests that they may be grounded in some cases on factors other man bonded interactions, particularly since they are often displayed by inert procrystal representations of the electron density. In these cases, it can be argued that the accumulation of the electron density along the paths has its origin, at least in part, in the superposition of the peripheral electron density distributions of the metal M atoms occupying the edge-sharing polyhedra. On the other hand, the accumulation of electron density along the paths may stabilize a structure by shielding the adjacent M atoms in the edge-sharing polyhedra. For closed-shell Li-O, Na-O, and Mg-O interactions, H(rc) is positive and increases as the value of ρ(rc) increases, unlike the "shared" Be-O, B-O, C-O, Al-O, Si-O, P-O, and S-O interactions, where H(rc) is negative and decreases as ρ(rc) increases. The H(rc) values for the weak closed-shell O-O interactions also increase as ρ(rc) increases, as observed for the closed-shell M-O interactions. On the basis of the bond critical point properties and the negative H(rc) value, the O-O interaction comprising the O2 molecule in silica III qualifies as a shared interaction. © 2008 American Chemical Society.
- Gibbs, G. V., Downs, R. T., Cox, D. F., Ross, N. L., Prewitt, C. T., Rosso, K. M., Lippmann, T., & Kirfel, A. (2008). Bonded interactions and the crystal chemistry of minerals: A review. Zeitschrift fur Kristallographie, 223(1-2), 1-40.More infoAbstract: Connections established during last century between bond length, radii, bond strength, bond valence and crystal and molecular chemistry are briefly reviewed followed by a survey of the physical properties of the electron density distributions for a variety of minerals and representative molecules, recently generated with first-principles local energy density quantum mechanical methods. The structures for several minerals, geometry-optimized at zero pressure and at a variety of pressures were found to agree with the experimental structures within a few percent. The experimental Si-O bond lengths and the Si-O-Si angle, the Si-O bond energy and the bond critical point properties for crystal quartz are comparable with those calculated for the H6Si2O 7 disilicic acid molecule, an indication that the bonded interactions in silica are largely short ranged and local in nature. The topology of model experimental electron density distributions for first and second row metal M atoms bonded to O, determined with high resolution and high energy synchrotron single crystal X-ray diffraction data are compared with the topology of theoretical distributions calculated with first principles methods. As the electron density is progressively accumulated between pairs of bonded atoms, the distributions show that the nuclei are progressively shielded as the bond lengths and the bonded radii of the atoms decrease. Concomitant with the decrease in the M-O bond lengths, the local kinetic energy, G(rc), the local potential energy, V(rc), and the electronic energy density, H(rc) = G(rc) + V(rc), evaluated at the bond critical points, rc, each increases in magnitude with the local potential energy dominating the kinetic energy density in the internuclear region for intermediate and shared interactions. The shorter the bonds, the more negative the local electronic energy density, the greater the stabilization and the greater the shared character of the intermediate and shared bonded interactions. In contrast, the local kinetic energy density increases with decreasing bond length for closed shell interactions with G(rc) dominating V(rc) in the internuclear region, typical of an ionic bond. Notwithstanding its origin in Pauling's electrostatic bond strength rule, the Brown-Shannon bond valence for Si-O bonded interactions agrees with the value of the electron density, ρ(rc), on a one-to-one basis, indicating that the Pauling bond strength is a direct measure of ρ(r c), the greater the bond strength, the more shared the interaction. Mappings of the Laplacian, the deformation electron density distribution and the electron localization function for several silicates are reviewed. The maps display hemispherical domains ascribed to bond pair electrons along the bond vectors and larger kidney-shaped domains ascribed to lone pair electrons on the reflex sides of the Si-O-Si angles. In the case of the nonbridging Si-O bonded interactions, the O atoms are capped by mushroom shaped domains. With few exceptions, the domains agree in number and location with those embodied in the VSEPR model for closed-shell molecules, defining reactive sites of potential electrophilic attack and centers of protonation. The electrophilicity of the O atoms comprising the Si-O-Si bonded interactions in coesite is indicated to increase with decreasing angle, providing a basis for understanding the protonization of the structure. The shapes and arrangements of the bond and lone pair features displayed by the bridging O atoms in quartz and and the nonbridging O atoms in forsterite are transferable on an one-to-one basis to sheet and chain magnesiosilicates that possess both bridging and nonbridging O atoms. The G(rc)/ρ(rc) ratio increases for each of the M-O bonds along separate trends with decreasing bond length and the coordination number of the M atom, suggesting that the ratio is a measure of bond character. An examination of the interactions in terms of the \V(r c)\/G(rc) ratio indicates that the Li-O, Na-O and Mg-O bonds are closed shell ionic interactions, that the C-O bond and one of the S-O bonds is shared covalent and that the Be-O, A1-O, Si-O, B-O, P-O and S-O bonds are intermediate in character. It is noteworthy that the classification closely parallels Pauling's classification based on the electronegativity differences between the M and O atoms. Bond critical point properties calculated for Ni bearing sulfides and high and low spin Fe bearing sulfides are discussed. The properties correlate linearly, as observed for the M-O bonds, with the experimental bond lengths, the shorter the bond lengths, the greater the ρ(rc) and ∇2ρ(rc) values. The high and low spin Fe-S data scatter along parallel but separate trends with the values of ρ(rc) and ∇2ρ(rc) for a given low spin Fe-S bond length being larger than those calculated for a given comparable high spin Fe-S bond length. The properties of the Ni-Ni bonded interactions calculated and observed for the Ni sulfides are virtually the same as those calculated for bulk Ni metal. No bond paths were found between the Fe atoms of the face sharing octahedra of troilite. The experimental bond critical point properties for the Ni sulfide heazlewoodite, Ni3S2, are in close agreement with those calculated. The |V(rc)|/G(rc) ratio indicates that the Fe-S, Ni-S and Ni-Ni bonded interactions are intermediate in character. The successful reproduction of the bond lengths and angles for several silicates, the comparable properties of the electron density distributions and the location of sites of potential chemical reactivity recounted in the review bodes well for the exploitation of the properties of minerals and the deciphering of crystal chemical problems, using first principles computational quantum chemical strategies. © by Oldenbourg Wissenschaftsverlag.
- Gibbs, G. V., Downs, R. T., Cox, D. F., Rosso, K. M., Ross, N. L., Kirfel, A., Lippmann, T., Morgenroth, W., & Crawford, T. D. (2008). Experimental bond critical point and local energy density properties determined for Mn-O, Fe-O, and Co-O bonded interactions for tephroite, Mn 2SiO4, fayalite, Fe2SiO4, and Co2SiO4 olivine and selected organic metal complexes: Comparison with properties calculated for non-transition and transition metal M-O bonded interactions. Journal of Physical Chemistry A, 112(37), 8811-8823.More infoPMID: 18714960;Abstract: Bond critical point (bcp) and local energy density properties for the electron density (ED) distributions, calculated with first-principle quantum mechanical methods for divalent transition metal Mn-, Co-, and Fe-containing silicates and oxides are compared with experimental model ED properties for tephroite, Mn2SiO4, fayalite, Fe2SiO 4, and Co2SiO4 olivine, each determined with high-energy synchrotron single-crystal X-ray diffraction data. Trends between the experimental bond lengths, R(M-O), (M = Mn, Fe, Co), and the calculated bcp properties are comparable with those observed for non-transition M-O bonded interactions. The bcp properties, local total energy density, H(rc), and bond length trends determined for the Mn-O, Co-O, and Fe-O interactions are also comparable. A comparison is also made with model experimental bcp properties determined for several Mn-O, Fe-O, and Co-O bonded interactions for selected organometallic complexes and several oxides. Despite the complexities of the structures of the organometallic complexes, the agreement between the calculated and model experimental bcp properties is fair to good in several cases. The G(rc)/ρ(rc) versus R(M-O) trends established for non-transition metal M-O bonded interactions hold for the transition metal M-O bonded interactions with G(rc)/ρ(r c) increasing in value as H(rc) becomes progressively more negative in value, indicating an increasing shared character of the interaction as G(rc)/ρ(rc) increases in value. As observed for the non-transition metal M-O bonded interactions, the Laplacian, ∇2ρ(rc), increases in value as ρ(r c) increases and as H(rc) decreases and becomes progressive more negative in value. The Mn-O, Fe-O, and Co-O bonded interactions are indicated to be of intermediate character with a substantial component of closed-shell character compared with Fe-S and Ni-S bonded interactions, which show greater shared character based on the |V(rc)|/G(rc) bond character indicator. The atomic charges conferred on the transition metal atoms for the three olivines decrease with increasing atomic number from Mn to Fe to Co as the average M-O bond lengths decrease from 2.219 to 2.168 to 2.128 Å, respectively. © 2008 American Chemical Society.
- Hazen, R. M., Papineau, D., Bleeker, W., Downs, R. T., Ferry, J. M., McCoy, T. J., Sverjensky, D. A., & Yang, H. (2008). Mineral evolution. American Mineralogist, 93(11-12), 1693-1720.More infoAbstract: The mineralogy of terrestrial planets evolves as a consequence of a range of physical, chemical, and biological processes. In pre-stellar molecular clouds, widely dispersed microscopic dust particles contain approximately a dozen refractory minerals that represent the starting point of planetary mineral evolution. Gravitational clumping into a protoplanetary disk, star formation, and the resultant heating in the stellar nebula produce primary refractory constituents of chondritic meteorites, including chondrules and calcium-aluminum inclusions, with ∼60 different mineral phases. Subsequent aqueous and thermal alteration of chondrites, asteroidal accretion and differentiation, and the consequent formation of achondrites results in a mineralogical repertoire limited to ∼250 different minerals found in unweathered meteorite samples. Following planetary accretion and differentiation, the initial mineral evolution of Earth's crust depended on a sequence of geochemical and petrologic processes, including volcanism and degassing, fractional crystallization, crystal settling, assimilation reactions, regional and contact metamorphism, plate tectonics, and associated large-scale fluid-rock interactions. These processes produced the first continents with their associated granitoids and pegmatites, hydrothermal ore deposits, metamorphic terrains, evaporites, and zones of surface weathering, and resulted in an estimated 1500 different mineral species. According to some origin-of-life scenarios, a planet must progress through at least some of these stages of chemical processing as a prerequisite for life. Biological processes began to affect Earth's surface mineralogy by the Eoarchean Era (∼3.85-3.6 Ga), when large-scale surface mineral deposits, including banded iron formations, were precipitated under the influences of changing atmospheric and ocean chemistry. The Paleoproterozoic "Great Oxidation Event" (∼2.2 to 2.0 Ga), when atmospheric oxygen may have risen to > 1 % of modern levels, and the Neoproterozoic increase in atmospheric oxygen, which followed several major glaciation events, ultimately gave rise to multicellular life and skeletal biomineralization and irreversibly transformed Earth's surface mineralogy. Biochemical processes may thus be responsible, directly or indirectly, for most of Earth's 4300 known mineral species. The stages of mineral evolution arise from three primary mechanisms: (1) the progressive separation and concentration of the elements from their original relatively uniform distribution in the pre-solar nebula; (2) an increase in range of intensive variables such as pressure, temperature, and the activities of H2O, CO2, and O2; and (3) the generation of far-from-equilibrium conditions by living systems. The sequential evolution of Earth's mineralogy from chondritic simplicity to Phanerozoic complexity introduces the dimension of geologic time to mineralogy and thus provides a dynamic alternate approach to framing, and to teaching, the mineral sciences.
- Henderson, R. R., Yang, H., Downs, R. T., & Jenkins, R. A. (2008). Redetermination of conichalcite, CaCu(AsO4)(OH). Acta Crystallographica Section E: Structure Reports Online, 64(9), i53-i54.More infoPMID: 21201563;PMCID: PMC2960568;Abstract: The crystal structure of conichalcite [calcium copper(II) arsenate(V) hydroxide], with ideal formula CaCu(AsO4)(OH), was redetermined from a natural twinned specimen found in the Maria Catalina mine (Chile). In contrast to the previous refinement from photographic data [Qurashi & Barnes (1963). Can. Mineral. 7, 561-577], all atoms were refined with anisotropic displacement parameters and with the H atom located. Conichalcite belongs to the adelite mineral group. The Jahn-Teller-distorted [CuO6] octa-hedra share edges, forming chains running parallel to [010]. These chains are cross-linked by eight-coordinate Ca atoms and by sharing vertices with isolated AsO4 tetra-hedra. Of five calcium arsenate minerals in the adelite group, the [MO6] (M = Cu, Zn, Co, Ni and Mg) octa-hedron in conichalcite is the most distorted, and the donor-acceptor O - H⋯O distance is the shortest.
- McCarthy, A. C., Downs, R. T., & Thompson, R. M. (2008). Compressibility trends of the clinopyroxenes, and in-situ high-pressure single-crystal X-ray diffraction study of jadeite. American Mineralogist, 93(1), 198-209.More infoAbstract: The crystal structure of a natural jadeite, NaAlSi2O6, was studied at room temperature over the pressure range 0-9.17 GPa using single-crystal X-ray diffraction. Unit-cell data were determined at 16 pressures, and intensity data were collected at nine of these pressures. A third-order Birch-Murnaghan equation of state fit to the P-V data yielded V0 = 402.03(2) Å3, K0 = 136.5(14) GPa, and K′0 = 3.4(4). Jadeite exhibits strongly anisotropic compression with unit stain axial ratios of 1.00:1.63:2.10. Silicate chains become more O-rotated with pressure, reducing ∠O3-O3-O3 from 174.7(1)° at ambient pressure to 169.2(6)° at 9.17 GPa and bringing the anions of jadeite closer to a cubic closest-packed arrangement. No evidence of a phase transition was observed over the studied pressure range. In an effort to understand pyroxene compressibilities, selected clinopyroxene bulk moduli were plotted against ambient unit-cell volumes. Two trends were identified and are explained in terms of differences in M2-O3 bonding topologies and the geometric relationship of the bonds with tetrahedral rotation in the silicate chains. Bonds positioned to favor the tetrahedral rotation upon compression are termed "sympathetic," whereas bonds positioned to resist the rotation are termed "antipathetic." Examination of the different pyroxene structures indicates that structures containing antipathetic M2-O3 bonds are less compressible than those with only sympathetic M2-O3 bonds. This behavior has not been previously recognized.
- McCarthy, A. C., Downs, R. T., Thompson, R. M., & Redhammer, G. J. (2008). In situ high-pressure single-crystal X-ray study of aegirine, NaFe3+Si2O6, and the role of M1 size in clinopyroxene compressibility. American Mineralogist, 93(11-12), 1829-1837.More infoAbstract: The crystal structure of a synthetic aegirine crystal, NaFe3+ Si2O6, was studied at room temperature, under hydrostatic conditions, over the pressure range 0-11.55 GPa using single-crystal X-ray diffraction. Unit-cell data were determined at 16 pressures, and intensity data were collected at eight of these pressures. A third-order Birch-Murnaghan equation of state fit to the P-V data from 0-11.55 GPa yielded K0 = 117(1) GPa, K3′ 3.2(2), and V0 = 429.40(g) Å 3. Aegirine, like the other Na-clinopyroxenes that have been examined at high pressure, exhibits strongly anisotropic compression, with unit strain axial ratios ε1:ε 2:ε3 of 1.00:2.38:2.63. Silicate chains in aegirine become more O-rotated with pressure, reducing ∠O3-O3-O3 from 174.1(1)° at ambient pressure to 165.5(5)° at 10.82 GPa. No evidence of a phase transition was observed over the studied pressure range. The relationship between M1 cation radius and bulk modulus is examined for 14 clinopyroxenes, and two distinct trends are identified in a plot of these values, The distinction between these trends can be explained by the presence or absence of antipathetic bonds around M2, a feature first described by McCarthy et al. (2008). Aegirine, with Fe3+, has nearly the same bulk modulus, within error, as hedenbergite, with Fe2+, despite the difference in M2 bonding topology, M2 (Fe) valence and ambient unit-cell volume. Several explanations for this apparent paradox are considered.
- Nestola, F., Ballaran, T. B., Liebske, C., Thompson, R., & Downs, R. T. (2008). The effect of the hedenbergitic substitution on the compressibility of jadeite. American Mineralogist, 93(7), 1005-1013.More infoAbstract: Four synthetic crystals belonging to the jadeite (Jd, NaAlSi2O6)-hedenbergite (Hd, CaFeSi2O6) solid solution were investigated by X-ray diffraction in situ at high pressure using a diamond anvil cell to Pmax = 10.6 GPa. The samples exhibited space group symmetry C2/c throughout the investigated pressure range and did not show any phase transformations. V0, KT0, and K' were simultaneously refined by fitting a third-order Birch-Murnaghan equation of state to pressure-volume data for all samples. The following relationship between bulk modulus and molar fraction of jadeite is observed: KT0 = 108.7(2) (GPa) + 0.191(9) × [% molar Jd] + 0.0006(1) × [% molar Jd]2 The bulk modulus of hedenbergite is 19% lower than jadeite with a strong axial anisotropy that increases with increasing the Hd content. In particular, the compressibility along the b axis (the most compressible in pyroxenes) increases by about 35% going from Jd to Hd while along the c axis the increase in compressibility is about 24%. The a axis does not show any variation in the deformation rate along the join. The analysis of the crystal structure behavior with pressure for all samples clearly indicates that the main cause of the strong anisotropy on the b-c plane is related to the narrowing of the M1 octahedral chain and to anion-anion interactions increasing the packing efficiency of the anion skeletons of the crystals going from Jd to Hd.
- Righter, K., Yang, H., Costin, G., & Downs, R. T. (2008). Oxygen fugacity in the Martian mantle controlled by carbon: New constraints from the nakhlite MIL 03346. Meteoritics and Planetary Science, 43(10), 1709-1723.More infoAbstract: Pyroxene structural data, along with analyses of titanomagnetite, fayalite and mesostasis of the new nakhlite Miller Range (MIL) 03346, define equilibration near 1 bar, 1100°C, and oxygen fugacity near the FMQ buffer. There is a clear progression of oxygen fugacity (fO2) in Martian meteorites from reduced Allan Hills (ALH) 84001 to intermediate shergottites to oxidized nakhlites. This trend can be explained by polybaric graphite-CO-CO2 equilibria in the Martian mantle. Shergottites would have formed at pressures between 1.2 and 3.0 GPa, and nakhlite parent liquids formed at pressures >3.0 GPa, consistent with geochemical and petrologic data for the shergottites and nahklites. Carbon buffering in the Martian mantle could be responsible for variation in fO2 in Martian meteorites (rather than assimilation or crustal interaction), as well as C-H-O fluids that could be the source of ∼30 ppb CH4 detected by recent spacecraft missions. The conundrum of an oxidized current mantle and basalts, but reduced early mantle during core-mantle equilibrium exists for both the Earth and Mars. A polybaric buffering role for graphite can explain this discrepancy for Mars, and thus it may not be necessary to have an oxidation mechanism like the dissociation of MgFe-perovskite to account for the oxidized terrestrial mantle. © The Meteoritical Society, 2008.
- Thompson, R. M., & Downs, R. T. (2008). The crystal structure of diopside at pressure to 10 GPa. American Mineralogist, 93(1), 177-186.More infoAbstract: The crystal structure of diopside has been determined at various pressures to 10.16 GPa. The results to 5 GPa are consistent with Levien and Prewitt (1981). The crystal structures have been analyzed using the geometric pyroxene model of Thompson and Downs (2004), the anion packing algorithm of Thompson and Downs (2001), and a new algorithm that quantifies the distortion of observed pyroxenes from their geometric model equivalents. Diopside is shown to compress via three main mechanisms: isotropic scaling, kinking of the tetrahedral chains, and collapse of the M1 chain toward its axis. The kinking of the chains accounts for most of the anisotropy of compression observed in the a-c plane, and the collapse of the M1 chain explains the anisotropy seen in the b-c plane. Model behavior is shown to reproduce many of the observations of previous workers. Anion-anion interactions are shown to be important in the distortion of observed diopside from its geometric model equivalent.
- Wang, Z., Zhao, Y., Zha, C., Xue, Q., Downs, R. T., Duan, R., Caracas, R., & Liao, X. (2008). X-ray induced synthesis of 8H diamond. Advanced Materials, 20(17), 3303-3307.More infoAbstract: The samples of predominantly 8H diamond polytype can be synthesized at ambient pressure by intense 20 kW synchrotron X-ray radiation at CHESS (Cornell High Energy Synchrotron Source) wiggler beam line was investigated. Low-magnification Transmission Electron Microscopy (TEM) images revealed that the X-ray-induced carbon displays a particular lamellar morphology, and selected area electron diffraction (SAED) patterns display 12 symmetrical spots with d-spacings that range from 1.99 to 2.14 Å. Synchrotron X-ray diffraction measurements together with simulations of the patterns were used to confirm the hexagonal structure. It was observed that an energetic high-temperature environment favors the nucleation of 3C and 2H diamond with a critical particle size of 2 nm, in which dislocations and stacking faults are not stable.
- Yang, H., & Downs, R. T. (2008). Crystal structure of glaucodot, (Co,Fe) AsS, and its relationship to marcasite and arsenopyrite. American Mineralogist, 93(7), 1183-1186.More infoAbstract: The crystal structure of glaucodot, (Co,Fe)AsS, an important member of the FcAsS-CoAsS-NiAsS system, was determined with single-crystal X-ray diffraction. It is orthorhombic with space group Pn21m and unit-cell parameters a = 14.158(1), b = 5.6462(4), c = 3.3196(2) Å, and V= 265.37(5) Å3. The structure is closely related to that of arsenopyrite or alloclasite, and represents a new derivative of the marcasite-type structure. The As and S atoms in glaucodot, which are ordered into six distinct sites (As1, As2, As3, S1, S2, and S3), form three types of layers [S, As, and mixed (S + As) layers] that are stacked along a in the sequence of (S + As)-(S + As)-S-(S + As)-(S + As)-As-(S + As)-(S + As)... In contrast, arsenopyrite contains the mixed (S + As) layers only and alloclasite consists of isolated S and As layers only. There are no As-As or S-S bonds in glaucodot; all dianion units are formed between S and As, like those in arsenopyrite and alloclasite. The (Co + Fe) cations in glaucodot occupy three nonequivalent octahedral sites (M1, M2, and M3), with M1(As5S), M2(As3S3), and M3(AsS5), which form three distinct edge-shared octahedral chains, A, B, and C, parallel to c, respectively. These chains are arranged along a in the sequence of A-A-B-C-C-B-A-A.... Whereas the configurations of the A and C chains are analogous to those in safflorite and marcasite, respectively, the configuration of the B chain matches that in alloclasite, leading us to propose that the M1, M2, and M3 sites are predominately occupied by Co, (Co + Fe), and Fe, respectively. Our study, together with previous observations, suggests that glaucodot is likely to have an ideal stoichiometry of (Co0.5 Fe0.5)AsS, with a limited tolerance for the variation of the Co/Fe ratio.
- Yang, H., Dembowski, R. F., Conrad, P. G., & Downs, R. T. (2008). Crystal structure and Raman spectrum of hydroxyl-bästnasite-(Ce), CeCO3(OH). American Mineralogist, 93(4), 698-701.More infoAbstract: Hydroxyl-bästnasite-(Ce), ideally CeCO3(OH), had been regarded isostructural with bästnasite-(Ce), CeCO3F, the dominant member of the bästnasite family that produces ∼70% of the world's supply of rare-earth elements. Using single-crystal X-ray diffraction and Raman spectroscopy, our structural analysis on hydroxyl-bästnasite-(Ce) shows that the previous assumption is incorrect. The crystal structure of hydroxyl-bästnasite-(Ce) possesses P6̄5 symmetry with unit-cell parameters a = 12.4112(2), c = 9.851](3) Å, and V= 1314.2(1) Å3, in contrast to the space group A6̄2c and a ≈ 7.10, c ≈ 9.76 Å, and V ≈ 430 Å3 for bästnasite-(Ce). Moreover, there are 6, 3, and 5 symmetrically-distinct CO3 groups, Ce cations, and (OH/F) ions, respectively, in hydroxyl-bästnasite-(Ce), but 1, 1, and 2 in bästnasite-(Ce). The two structures, nevertheless, are similarly characterized by the layers of CO3 groups alternating with the Ce-(OH/F) layers along the c direction. The Raman spectrum of hydroxyl-bästnasite-(Ce) is dominated by three strong bands at 1080, 1087, and 1098 cm-1 in the CO3 symmetrical stretching region, along with at least four bands in the OH stretching region. Our study further suggests that natural hydroxyl-bästnasite-(Nd) is most likely isotypic with hydroxyl-bästnasite-(Ce), rather than with bästnasite-(Ce), as previously proposed.
- Yang, H., Downs, R. T., & Eichler, C. (2008). Safflorite, (Co,Ni,Fe)As2, isomorphous with marcasite. Acta Crystallographica Section E: Structure Reports Online, 64(9), i62.More infoPMID: 21201568;PMCID: PMC2960506;Abstract: Safflorite, a naturally occurring cobalt-nickel-iron diarsenide (Co,Ni,Fe)As2, possesses the marcasite-type structure, with cations (M = Co + Ni + Fe) at site symmetry 2/m and As anions at m. The MAs6 octa-hedra share two edges, forming chains parallel to c. The chemical formula for safflorite should be expressed as (Co,Ni,Fe)As2, rather than the end-member format CoAs2, as its structure stabilization requires the simultaneous inter-action of the electronic states of Co, Ni, and Fe with As2 2- dianions.
- Burt, J. B., Downs, R. T., & Costin, G. (2007). Single-crystal X-ray refinement of wilkinsonite, Na2Fe 42+Fe23+Si6O 20. Acta Crystallographica Section E: Structure Reports Online, 63(5), i122-i124.More infoAbstract: This is the first structural refinement of the mineral wilkinsonite, Na2Fe4 2+Fe2 3+Si6O20. Wilkinsonite is a member of the aenigmatite group and consists of bands of Fe octa-hedra connected to eight-coordinated Na polyhedra forming sheets parallel to (011), which are inter-connected by chains of [Si6O18] and Fe octa-hedra. The site occupancies of the different valence states of Fe have been determined from bond distances and bond valence. © 2007 International Union of Crystallography. All rights reserved.
- Fritz, E. A., Laurs, B. M., Downs, R. T., & Costin, G. (2007). Yellowish green diopside and tremolite from Merelani, Tanzania. Gems and Gemology, 43(2), 146-148.More infoAbstract: Four similar-appearing yellowish green samples from Block D at Merelani, Tanzania, were identified as diopside and tremolite. The gems are identical in color, but their standard gemological properties are typical for calcic pyroxene and amphibole. The identification of the diopside was made with Raman spectroscopy, while single-crystal X-ray diffraction and electron-microprobe analyses were used to confirm the amphibole species as tremolite. Absorption spectroscopy (in the visible-mid-infrared range) revealed that the two gem materials are colored by V 3+, Cr 3+, or both. © 2007 Gemological Institute of America.
- Gibbs, G. V., Cox, D. F., Rosso, K. M., Ross, N. L., Downs, R. T., & Spackman, M. A. (2007). Theoretical electron density distributions for Fe- and Cu-sulfide earth materials: A connection between bond length, bond critical point properties, local energy densities, and bonded interactions. Journal of Physical Chemistry B, 111(8), 1923-1931.More infoPMID: 17274642;Abstract: Bond critical point and local energy density properties together with net atomic charges were calculated for theoretical electron density distributions, ρ(r), generated for a variety of Fe and Cu metal-sulfide materials with high- and low-spin Fe atoms in octahedral coordination and high-spin Fe atoms in tetrahedral coordination. The electron density, ρ(rc), the Laplacian, ∇2ρ(rc), the local kinetic energy, G(rc), and the oxidation state of Fe increase as the local potential energy density, V(rc), the Fe-S bond lengths, and the coordination numbers of the Fe atoms decrease. The properties of the bonded interactions for the octahedrally coordinated low-spin Fe atoms for pyrite and marcasite are distinct from those for high-spin Fe atoms for troilite, smythite, and greigite. The Fe-S bond lengths are shorter and the values of ρ(rc) and ∇2ρ(rc) are larger for pyrite and marcasite, indicating that the accumulation and local concentration of ρ(r) in the internuclear region are greater than those involving the longer, high-spin Fe-S bonded interactions. The net atomic charges and the bonded radii calculated for the Fe and S atoms in pyrite and marcasite are also smaller than those for sulfides with high-spin octahedrally coordinated Fe atoms. Collectively, the Fe-S interactions are indicated to be intermediate in character with the low-spin Fe-S interactions having greater shared character than the high-spin interactions. The bond lengths observed for chalcopyrite together with the calculated bond critical point properties are consistent with the formula Cu+Fe3+S2. The bond length is shorter and the ρ(rc) value is larger for the FeS4 tetrahedron displayed by metastable greigite than those displayed by chalcopyrite and cubanite, consistent with a proposal that the Fe atom in greigite is tetravalent. S-S bond paths exist between each of the surface S atoms of adjacent slabs of FeS6 octahedra comprising the layer sulfide smythite, suggesting that the neutral Fe3S4 slabs are linked together and stabilized by the pathways of electron density comprising S-S bonded interactions. Such interactions not only exist between the S atoms for adjacent S8 rings in native sulfur, but their bond critical point properties are similar to those displayed by the metal sulfides. © 2007 American Chemical Society.
- Hexiong, Y., Costin, G., Keogh, J., Ren, L. u., & Downs, R. T. (2007). Cobaltaustinite, CaCo(AsO4)(OH). Acta Crystallographica Section E: Structure Reports Online, 63(2), i53-i55.More infoAbstract: This study presents the first structural report of a natural cobaltaustinite sample (calcium cobalt arsenate hydroxide) based on single-crystal X-ray diffraction data. Cobaltaustinite, with the ideal formula CaCo(AsO4)(OH), belongs to the adelite mineral group. The CoO 6 octahedra share edges to form chains extending parallel to the c axis, which are cross-linked by Ca2+ ions and by sharing vertices with isolated AsO4 tetrahedra. The Ca2+ ions are situated in square antiprisms formed by eight O atoms. The major structural difference between the five calcium arsenates in the adelite group is shown in the bonding environments around the octahedrally coordinated M2+ cations (M = Cu, Zn, Co, Ni and Mg), with the average M-O distance decreasing from Cu-O in conichalcite, CaCu(AsO4)(OH) to Zn-O in austinite, CaZn(AsO 4)(OH), Co-O in cobaltaustinite, Ni-O in nickelaustinite, CaNi(AsO4)(OH), and Mg-O in adelite, CaMg(AsO4)(OH). The donor-acceptor O-H⋯O distance [2.721 (7) Å] in cobaltaustinite is similar to those in austinite and nickelaustinite, but different from those in adelite and conichalcite. © 2007 International Union of Crystallography All rights reserved.
- Ikuta, D., Kawame, N., Banno, S., Hirajima, T., Ito, K., Rakovan, J. F., Downs, R. T., & Tamada, O. (2007). First in situ X-ray identification of coesite and retrograde quartz on a glass thin section of an ultrahigh-pressure metamorphic rock and their crystal structure details. American Mineralogist, 92(1), 57-63.More infoAbstract: To ensure the presence of coesite and its transformed polymorph, quartz, in ultrahigh-pressure (UHP) rocks and to examine the relic of the phase transformation, crystal structures were analyzed by single-crystal X-ray diffraction (XRD) directly using the rock thin section mounted on a slide glass. The rock sample used is a coesite-bearing eclogite from the Sulu UHP terrain, eastern China. The crystal structures were determined successfully by this new method and the presence of coesite and quartz in UHP rocks was identified for the first time by XRD. The R-factor [R(F)] converged to 0.046 for coesite and 0.087 for quartz. The displacement ellipsoids for coesite and quartz are larger than those previously reported for these two phases, which is consistent with expected effects of trapped strain due to the phase transformation from coesite to quartz during exhumation from the Earth's mantle. This paper is the first report of single-crystal XRD of a rock thin section on a glass slide and establishes the technique, and provides proof-of-concept of the method. Although the mineral species included in a thin section can often be identified by other methods, such as Raman spectroscopy, an advantage of the reported method is that it can be applied to any mineral in a thin section, and not just to the UHP minerals. Moreover, it is applicable to an unknown or new mineral in a thin section, discarding the spots of known minerals and constructing a lattice from the residual spots to find the structure of the unknown phase.
- Origlieri, M. J., Laetsch, T. A., & Downs, R. T. (2007). A note: On the paragenesis of ottensite. Mineralogical Record, 38(1), 83-84.
- Tait, K. T., Trouw, F., Zhao, Y., Brown, C. M., & Downs, R. T. (2007). Inelastic neutron scattering study of hydrogen in d8 -THF D2 O ice clathrate. Journal of Chemical Physics, 127(13).More infoAbstract: In situ neutron inelastic scattering experiments on hydrogen adsorbed into a fully deutrated tetrahydrofuran-water ice clathrate show that the adsorbed hydrogen has three rotational excitations (transitions between J=0 and 1 states) at approximately 14 meV in both energy gain and loss. These transitions could be unequivocally assigned since there was residual orthohydrogen at low temperatures (slow conversion to the ground state) resulting in an observable J=1→0 transition at 5 K (kT=0.48 meV). A doublet in neutron energy loss at approximately 28.5 meV is interpreted as J=1→2 transitions. In addition to the transitions between rotational states, there are a series of peaks that arise from transitions between center-of-mass translational quantum states of the confined hydrogen molecule. A band at approximately 9 meV can be unequivocally interpreted as a transition between translational states, while broad features at 20, 25, 35, and 50-60 meV are also interpreted to as transitions between translational quantum states. A detailed comparison is made with a recent five-dimensional quantum treatment of hydrogen in the smaller dodecahedral cage in the SII ice-clathrate structure. Although there is broad agreement regarding the features such as the splitting of the J=1 degeneracy, the magnitude of the external potential is overestimated. The numerous transitions between translational states predicted by this model are in poor agreement with the experimental data. Comparisons are also made with three simple exactly solved models, namely, a particle in a box, a particle in a sphere, and a particle on the surface of a sphere. Again, there are too many predicted features by the first two models, but there is reasonable agreement with the particle on a sphere model. This is consistent with published quantum chemistry results for hydrogen in the dodecahedral 512 cage, where the center of the cage is found to be energetically unfavorable, resulting in a shell-like confinement for the hydrogen molecule wave function. These results demonstrate that translational quantum effects are very significant and a classical treatment of the hydrogen molecule dynamics is inappropriate under such conditions. © 2007 American Institute of Physics.
- Tenner, T. J., Lange, R. A., & Downs, R. T. (2007). The albite fusion curve re-examined: New experiments and the high-pressure density and compressibility of high albite and NaAlSi3O8 liquid. American Mineralogist, 92(10), 1573-1585.More infoAbstract: Experimental brackets on the melting temperature of high albite (NaAlSi3O8) were determined at 2.33 ± 0.03 GPa (1360-1370 °C) and 2.79 ± 0.03 GPa (1370-1389 °C) in a piston-cylinder apparatus. All run products that quenched to a glass were analyzed by Fourier-transform infrared spectroscopy and found to contain ≤500 ppm H2O. In addition, new X-ray diffraction experiments on fully disordered albite are reported to 7.6 GPa; the fitted results lead to a zero-pressure bulk modulus (K0) of 56.4 ± 0.7 and a pressure derivative (K'0) of 3.9 ± 0.3 in a third-order Birch-Murnaghan equation of state. Revised values for the enthalpy and entropy of fusion of high albite at one bar and 1100 °C [ΔHTf = 64.5 ± 2.1 kJ/mol and ΔSTf = 47.0 J/(mol·K)] are recommended on the basis of improved heat capacity equations for NaAlSi3O8 glass and liquid. On the basis of these new results on the fusion curve and thermodynamic data for high albite, the pressure dependence of the NaAlSi3O8 liquid compressibility (K'0) is constrained to be 10.8 ± 1.5 in a third-order Birch-Murnaghan equation of state. The uncertainty in K'0 of ± 1.5 contributes an error to melt density at 3 GPa (2.543 ± 0.010 g/cm3 at 1500 °C) of ±0.4%.
- Uchida, H., Downs, R. T., & Thompson, R. M. (2007). Reinvestigation of eakerite, Ca2SnAl2Si 6O18(OH)2·2H2O: H-atom positions by single-crystal X-ray diffraction and correlation with Raman spectroscopic data. Acta Crystallographica Section E: Structure Reports Online, 63(2), i47-i49.More infoAbstract: The crystal structure of natural eakerite [Kossiakoff & Leavens (1976). Am. Mineral. 61, 956-962] has been reinvestigated, the H-atom positions determined and the hydrogen-bonding scheme elucidated. The O⋯O separations of the O-H⋯O hydrogen bonds correlate well with the frequencies of the three corresponding Raman peaks according to Libowtzky's regression curve. The Sn atom has site symmetry 1̄. © 2007 International Union of Crystallography All rights reserved.
- Uchida, H., Righter, K., Lavina, B., Nowell, M. M., Wright, S. I., Downs, R. T., & Yang, H. (2007). Investigation of synthetic Mg1.3V1.7O4 spinel with MgO inclusions: Case study of a spinel with an apparently occupied interstitial site. American Mineralogist, 92(7), 1031-1037.More infoAbstract: A magnesium vanadate spinel crystal, ideally MgV2O4, synthesized at 1 bar, 1200 °C and equilibrated under FMQ + 1.3 log fo2 condition, was investigated using single-crystal X-ray diffraction, electron microprobe, and electron backscatter diffraction (EBSD). The initial X-ray structure refinements gave tetrahedral and octahedral site occupancies of T(Mg0.966□0.034) and M(V0.7113+ V0.1094+Mg0.180), respectively, along with the presence of 0.053 apfu Mg at an interstitial octahedral site (16c). Back-scattered electron (BSE) images and electron microprobe analyses revealed the existence of an Mg-rich phase in the spinel matrix, which was too small (≤3 μm) for an accurate chemical determination. The EBSD analysis combined with X-ray energy dispersive spectroscopy (XEDS) suggested that the Mg-rich inclusions are periclase oriented coherently with the spinel matrix. The final structure refinements were optimized by subtracting the X-ray intensity contributions (∼9%) of periclase reflections, which eliminated the interstitial Mg, yielding a structural formula for spinel TMgM(V1.3683+ V0.3164+Mg0.316)O4. This study provides insight into possible origins of refined interstitial cations reported in the literature for spinel, and points to the difficulty of using only X-ray diffraction data to distinguish a spinel with interstitial cations from one with coherently oriented MgO inclusions.
- Yang, H., & Downs, R. T. (2007). Synthesis and crystal structure of Li0.52Mg0.96 Sc0.52Si2O6 orthopyroxene. American Mineralogist, 92(1), 225-228.More infoAbstract: Li0.52Mg0.96Sc0.52 Si2O6 orthopyroxene was synthesized by the flux method and its structure studied with single-crystal X-ray diffraction. The crystal is orthorhombic with space group Pbca and unit-cell parameters a = 18.259(5), b = 8.883(2), c = 5.271(1) Å, and V = 854.9(3) Å3. The structure refinement shows that the M1 and M2 sites are occupied by (0.48 Mg + 0.52 Sc) and (0.48 Mg + 0.52 Li), respectively. While the O3-O3-O3 kinking angle (165.40°) of the silicate tetrahedral A chain appears to be normal when compared with reported data, the kinking angle (151.92°) of the B chain is the largest of all orthopyroxenes examined at ambient conditions. This is the first orthopyroxene structure that contains more than 50% trivalent and monovalent cations in the M1 and M2 sites, respectively, and displays a kinking angle of the tetrahedral B chain that is greater than 150°. Our study demonstrates the stability of the new pyroxene structure type predicted by Pannhorst (1979) at room temperature.
- Yang, H., Chen, L. i., Jenkins, R. A., Downs, R. T., & Costin, G. (2007). Kolbeckite, ScPO4.2H2O, isomorphous with metavariscite. Acta Crystallographica Section C: Crystal Structure Communications, 63(10), i91-i92.More infoPMID: 17917207;Abstract: This study presents the first structural report of kolbeckite, with the ideal formula ScPO4·2H2O (scandium phosphate dihydrate), based on single-crystal X-ray diffraction data. Kolbeckite belongs to the metavariscite mineral group, in which each PO4 tetra-hedron shares four vertices with four ScO4(H2O)2 octa-hedra and vice versa, forming a three-dimensional network of polyhedra. © International Union of Crystallography 2007.
- Yang, H., Downs, R. T., Costin, G., & Eichler, C. M. (2007). The crystal structure of tvalchrelidzeite, Hg3SbAsS3, and a revision of its chemical formula. Canadian Mineralogist, 45(6), 1529-1533.More infoAbstract: We have determined the crystal structure of tvalchrelidzeite, Hg3SbAsS3, for the first time with single-crystal X-ray diffraction. It is monoclinic, with space group P21In and unit-cell parameters a 11.5526(4), b 4.3852(1), c 15.6373(5) Å, β 91.845(2)°, V 791.79(5) Å3. There are eight symmetrically distinct sites in the structure, three occupied by Hg, one by Sb, one by As, and three by S. There is no disorder between Sb and As. Each Sb is surrounded by six S2- anions, with three at distances shorter than 2.51 Å and three at distances longer than 3.20 Å. In contrast, each As is coordinated by six Hg2+ cations, with three at distances shorter than 2.51 Å and three at distances longer than 3.31 Å. All three independent Hg ions are situated in considerably distorted octahedral sites, with two opposite bonds (one Hg-S and one Hg-As) shorter than 2.51 Å and four equatorial bonds longer than 2.98 Å. The structure of tvalchrelidzeite can be viewed as a sequence of sheets parallel to (′101). These sheets are composed of [Hg6Sb2 As2S6] ribbon-like units (extending along the b axis) linked together by the short Hg 1-As bonds (2.494 Å). The linkage between sheets is achieved through the weak Hg-S (>3.0 Å) and Sb-S (>3.2 Å) bonds, accounting for the observed perfect cleavage in one direction. Tvalchrelidzeite represents one of very few sulfosalt minerals that contain both Sb and As, with the latter behaving as an anion.
- Yang, H., Hubler, D. K., Lavina, B., Downs, R. T., & Costin, G. (2007). Tyrrellite, Cu(Co0.68Ni0.32)2Se 4, isostructural with spinel. Acta Crystallographica Section C: Crystal Structure Communications, 63(8), i73-i74.More infoPMID: 17675677;Abstract: Tyrrellite, a naturally occurring Co-Ni-Cu selenide, has been studied by single-crystal X-ray diffraction. It possesses the normal spinel-type structure, with Cu occupying the tetra-hedral site and (Co+Ni) the octa-hedral site. The average Cu - Se distance of 2.3688 (2) Å is close to that of 2.3703 (8) Å in CuCr2Se4, whereas the average (Co+Ni) - Se distance of 2.3840 (1) Å appears to be slightly shorter than most octa-hedral Co - Se or Ni - Se distances (2.40-2.50 Å) in other selenides. The refined structure provides a basis for a redefinition of the ideal chemical formula of tyrrellite, which should be Cu(Co,Ni)2Se4, rather than the previously suggested (Cu,Co,Ni)3Se4. © International Union of Crystallography 2007.
- Yang, H., Sano, J. L., Eichler, C., Downs, R. T., & Costin, G. (2007). Iranite, CuPb10(CrO4)6(SiO4)2(OH) 2, isomorphous with hemihedrite. Acta Crystallographica Section C: Crystal Structure Communications, 63(12), i122-i124.More infoPMID: 18057588;Abstract: This study presents the first structural report of iranite, ideally CuPb10(CrO4)6(SiO4)2(OH)2 [copper deca-lead hexa-chromate bis-(orthosilicate) dihydroxide], based on single-crystal X-ray diffraction data. Iranite is isomorphous with hemihedrite, with substitution of Cu for Zn and OH for F. The Cu atom is situated at the special position with site symmetry . The CrO4 and SiO4 tetra-hedra and CuO4(OH)2 octa-hedra form layers that are parallel to (120) and are linked together by five symmetrically independent Pb2+ cations displaying a rather wide range of bond distances. The CuO4(OH)2 octa-hedra are corner-linked to two CrO4 and two SiO4 groups, while two additional CrO4 groups are isolated. The mean Cr - O distances for the three nonequivalent CrO4 tetra-hedra are all slightly shorter than the corresponding distances in hemihedrite, whereas the CuO4(OH)2 octa-hedron is more distorted than the ZnO4F2 octa-hedron in hemihedrite in terms of octa-hedral quadratic elongation. © International Union of Crystallography 2007.
- Yang, H., Zwick, J., Downs, R. T., & Costin, G. (2007). Isokite, CaMg(PO4)F0.8(OH)0.2, isomorphous with titanite. Acta Crystallographica Section C: Crystal Structure Communications, 63(10), i89-i90.More infoPMID: 17917206;Abstract: This study presents the first structural report of natural isokite (calcium magnesium phosphate fluoride), with the formula CaMg(PO4)F0.8(OH)0.2 (i.e. some substitution of OH for F), based on single-crystal X-ray diffraction data. Isokite belongs to the C2/c titanite mineral group, in which Mg is on an inversion centre and the Ca, P and F/OH atoms are on twofold axes. The structure is composed of kinked chains of corner-sharing MgO4F2 octa-hedra that are crosslinked by isolated PO4 tetra-hedra, forming a three-dimensional polyhedral network. The Ca2+ cations occupy the inter-stitial sites, coordinated by six O atoms and one F anion. © International Union of Crystallography 2007.
- Bindi, L., Downs, R. T., Harlow, G. E., Safonov, O. G., Litvin, Y. A., Perchuk, L. L., Uchida, H., & Menchetti, S. (2006). Compressibility of synthetic potassium-rich clinopyroxene: In-situ high-pressure single-crystal X-ray study. American Mineralogist, 91(5-6), 802-808.More infoAbstract: The crystal structure of a synthetic potassium-rich clinopyroxene, (Ca0.88K0.12)(Mg0.83Al0.17) (Si1.98Al0.02)O6, was studied using high-pressure single-crystal X-ray diffraction methods. A four-pin diamond anvil cell with 4:1 methanol:ethanol pressure medium was used to achieve pressures to 9.72 GPa. Unit-cell data were measured at 17 pressures, and intensity data were collected at 6 pressures. Fitting the P-V data to the third-order Birch-Murnaghan equation of state yields V0 = 435.49(3) Å3, K0=129(1) GPa, K' = 2.7(3). Anisotropic compression was observed with unit strain axial ratios of 1:1.94:1.90. Unit-cell parameters decrease gradually as a function of pressure with axial compressibilities βb > βc ∼ βa. They match those found for kosmochlor but are stiffer than those observed for synthetic diopside and hedenbergite. Compressibilities of the bond distances within the M2, M1, and T polyhedra show significant anisotropy. The incorporation of K into the clinopyroxene structure has little effect on its compressibility, although the concomitant substitution of Al in M1 from the K-Jd component reduces its compressibility. The K atom is softer than the M2 polyhedron and thus shrinks enough at high pressure to fit into the pyroxene structure.
- Downs, R. T., & Singh, A. K. (2006). Analysis of deviatoric stress from nonhydrostatic pressure on a single crystal in a diamond anvil cell: The case of monoclinic aegirine, NaFeSi2O6. Journal of Physics and Chemistry of Solids, 67(9-10), 1995-2000.More infoAbstract: A synthetic crystal of aegirine was compressed in a 4-pin diamond anvil cell with 4:1 methanol:ethanol+water mixture as a pressure-transmitting medium. The pressure was monitored using the shift of ruby R1-R2 lines. X-ray diffraction patterns were recorded at 17 pressures to 13.52 GPa using unfiltered Mo radiation. The pressure dependencies of the monoclinic cell parameters showed normal behavior to 11.55 GPa. The zero-pressure axial compressibilities were χa = 0.0029, χb = 0.0031, and χc = 0.0027 GPa- 1. Fitting the third-order Birch-Murnaghan equation to the pressure-volume data yielded K0 = 117 (1) GPa, K0′ = 3.2 (2), and V0 = 429.40 (9) Å3. The average peak widths in all the runs to 11.55 GPa were ∼0.08° in ω with small spread (standard deviation). At 12.62 and 13.52 GPa, a monoclinic unit cell did not fit well, as the standard deviations in cell parameters were 5-20 times the values obtained below 11.55 GPa. The cell parameters showed marked deviations from the trends exhibited by the data up to 11.55 GPa. Axial compressibilities over the range from 12.62 to 13.52 GPa were dramatically changed; χa = - 0.0008, χb = 0.0026, and χc = 0.0006 GPa- 1. The resulting discontinuity at 12.62 GPa in the pressure-volume plot was reminiscent of a pressure-induced phase transition. The average peak widths at these two pressures increased to ∼0.24° with large spread. The analyses of the stress and strain tensors associated with the deviations in the cell parameter versus pressure plots at 12.62 GPa indicate the onset of nonhydrostatic stresses. At 13.52 GPa, just 2 GPa above the onset of nonhydrostatic strain, the magnitude of the nonhydrostatic stress was computed to be 1.2 GPa. Such a situation is likely to arise in high-pressure studies on single crystals. It is important to recognize in order to avoid incorrect interpretation of the data. The analysis presented in this article can be used in detecting the onset of nonhydrostatic stresses due either to freezing of the pressure-transmitting medium or to the sample bridging the anvils. © 2006 Elsevier Ltd. All rights reserved.
- Gibbs, G. V., Cox, D. F., Crawford, T. D., Rosso, K. M., Ross, N. L., & Downs, R. T. (2006). Classification of metal-oxide bonded interactions based on local potential- and kinetic-energy densities. Journal of Chemical Physics, 124(8).More infoAbstract: A classification of the hydrogen fluoride H-F-bonded interactions comprising a large number of molecules has been proposed by Espinosa [J. Chem. Phys. 117, 5529 (2002)] based on the ratio ∫V (rc) ∫ G (rc) where ∫V (rc) ∫ is the magnitude of the local potential-energy density and G (rc) is the local kinetic-energy density, each evaluated at a bond critical point rc. A calculation of the ratio for the M-O bonded interactions comprising a relatively large number of oxide molecules and earth materials, together with the constraints imposed by the values of ∇2 ρ (rc) and the local electronic energy density, H (rc) =G (rc) +V (rc), in the H-F study, yielded practically the same classification for the oxides. This is true despite the different trends that hold between the bond critical point and local energy density properties with the bond lengths displayed by the H-F and M-O bonded interactions. On the basis of the ratio, Li-O, Na-O, and Mg-O bonded interactions classify as closed-shell ionic bonds, Be-O, Al-O, Si-O, B-O, and P-O interactions classify as bonds of intermediate character with the covalent character increasing from Be-O to P-O. N-O interactions classify as shared covalent bonds. C-O and S-O bonded interactions classify as both intermediate and covalent bonded interactions. The C-O double- and triple-bonded interactions classify as intermediate-bonded interactions, each with a substantial component of covalent character and the C-O single-bonded interaction classifies as a covalent bond whereas their local electronic energy density values indicate that they are each covalent bonded interactions. The ratios for the Be-O, Al-O, and Si-O bonded interactions indicate that they have a substantial component of ionic character despite their classification as bonds of intermediate character. The trend between the ratio and the character of the bonded interactions is consistent with trends expected from electronegativity considerations. The ratio increases as the net charges and the coordination numbers for the atoms for several Ni-sulfides decrease. On the contrary, the ratio for the Si-O bonded interactions for the orthosilicate, forsterite, Mg2 SiO4, and the high-pressure silica polymorph, stishovite, decreases as the observed net atomic charges and the coordination numbers of Si and O increase in value. The ratio for the Ni-Ni bonded interactions for the Ni-sulfides and bulk Ni metal indicate that the interactions are intermediate in character with a substantial component of ionic character. © 2006 American Institute of Physics.
- Hexiong, Y., Ren, L., Downs, R. T., & Costin, G. (2006). Goethite, α-FeO(OH), from single-crystal data. Acta Crystallographica Section E: Structure Reports Online, 62(12), i250-i252.More infoAbstract: This is the first reported structure refinement of goethite, α-FeO(OH), on the basis of a single-crystal X-ray diffraction study. The structure of goethite, isostructural with diaspore, AlO(OH), and groutite, MnO(OH), can be described in terms of a slightly distorted hexagonal close-packed O-atom arrangement with Fe atoms occupying one-half of the octahedral interstices, and with all atoms located on mirror planes. There are two distinct O sites, O1 and O2, each bonded to three Fe atoms, with O2 additionally bonded to an H atom. The O2-H⋯O1 donor-acceptor distance in goethite is significantly longer than that in diaspore or groutite, indicating that the hydrogen bonding in goethite is the weakest of the three minerals. Analysis of refinement data for the three isostructural compounds reveals rigid-body thermal motion behavior of the octahedral groups. © 2006 International Union of Crystallography All rights reserved.
- Laurs, B. M., Fritz, E. A., Origlieri, M. J., & Downs, R. T. (2006). Gem news international: Triploidite from China. Gems and Gemology, 42(2), 183-184.
- Liermann, H., Downs, R. T., & Yang, H. (2006). Site disorder revealed through Raman spectra from oriented single crystals: A case study on karooite (MgTi2O5). American Mineralogist, 91(5-6), 790-793.More infoAbstract: Raman spectroscopic data were collected from five oriented single crystals of karroite (MgTi2O5) with different ordering states obtained by quenching crystals from 600, 700, 800, 1000, and 1400 °C. The Raman spectra were normalized and treated as vectors. The inner products among the Raman spectra are shown to correlate linearly with the ordered state of the crystals, suggesting that such an analysis of Raman spectra can, in principle, be used to rapidly estimate the ordering state of a mineral, and thus many other crystal properties related to the atomic order-disorder.
- Manoun, B., Downs, R. T., & Saxena, S. K. (2006). A high-pressure Raman spectroscopic study of hafnon, HfSiO4. American Mineralogist, 91(11-12), 1888-1892.More infoAbstract: Raman spectra of synthetic HfSiO4 were determined to pressures of 38.2 GPa. Changes in the spectra indicate that HfSiO4 undergoes a room-temperature phase transition from the hafnon structure (I41/amd space group) to the scheelite structure (I41/a space group) at a pressure of ∼19.6 GPa. Upon release of pressure to ambient conditions, the spectra indicate that the sample retains the scheelite structure. Zircon has been classified previously as the least compressible tetrahedrally coordinated silicate known. However, pressure derivatives of the peak positions in hafnon are smaller than those in zircon, and suggest that hafnon is more incompressible than zircon. Furthermore, the pressure derivatives also suggest that the high-pressure, scheelite-structured HfSiO4 phase is more incompressible than the scheelite-structured ZrSiO4 (reidite). Thus, the post-hafnon phase appears to be even more incompressible than hafnon, which would make it the least compressible tetrahedrally coordinated silicate known to date.
- Rajan, H., Uchida, H., Bryan, D. L., Swaminathan, R., Downs, R. T., & Hall-Wallace, M. (2006). Building the American mineralogist crystal structure database: A recipe for construction of a small Internet database. Special Paper of the Geological Society of America, 397, 73-80.More infoAbstract: Crystal structure data represent one of the most important resources for developing scientific knowledge and should be archived in ways that make them easy to access and preserve. The American Mineralogist Crystal Structure Database currently contains every crystal structure published in American Mineralogist, The Canadian Mineralogist, European Journal of Mineralogy, and Physics and Chemistry of Minerals. It is maintained by the American and Canadian mineralogical societies and is freely accessible through the Internet. The database consists of the data, server-side search and retrieval software and user-side analysis software. It is managed through a partnership of PHP and MySQL programming that provide dynamic construction of Web pages and search procedures. The purpose of this paper is to describe the database and its implementation and to illustrate how to construct similar small, interactive Internet databases. © 2006 Geological Society of America. All rights reserved.
- Richter, K., Sutton, S. R., Newville, M., Le, L., Schwandt, C. S., Uchida, H., Lavina, B., & Downs, R. T. (2006). An experimental study of the oxidation state of vanadium in spinel and basaltic melt with implications for the origin of planetary basalt. American Mineralogist, 91(10), 1643-1656.More infoAbstract: The distribution of V in magmatic rocks is controlled primarily by spinel stability. Extensive previous experimental work at oxidized conditions on doped (V-rich) compositions has led to the recognition of the importance of temperature, oxygen fugacity, and spinel composition, but also left ambiguity with respect to the relative importance of these variables in controlling D spinelmelt. One major uncertainty has been the valence of V in the spinel and glass. Spinel-melt pairs were equilibrated at low and variable oxygen fugacities, with a range of V and Ti contents. XANES spectra were measured on the spinel and glass products, and pre-edge peaks measured and calibrated against valence with the use of glass and oxide standards. The valence of V is always greater in the glass than in the spinels. In spinel, V is dominantly 3+ at oxygen fugacities near the FMQ (fayalite magnetite quartz) buffer, but we find evidence for mixed 3+, 4+, and 5+ at oxidized conditions (FMQ to air), and 2+ and 3+ at very reduced conditions [FMQ to IW-1 (1 log fo2 unit below the iron wüstite buffer)]. Increased V contents in spinels are correlated with increased DSpinel-melt, at constant temperature and oxygen fugacity. However, increased Ti content causes only a slight decrease in DSpinel-melt and a shift to more reduced V (smaller pre-edge peak), which may be related to Fe-V exchange equilibria. Using the new partition coefficients, together with published results and valence information, expressions have been derived to predict DSpinel-melt for basaltic systems. Application of these expressions to natural suites illustrate their utility and also the great range of DSpinel-melt values relevant to natural systems. Calculation of V depletions in planetary mantles from basalt suites must take silicate, oxide, and metal fractionation into account, as is demonstrated using terrestrial, lunar, martian, and eucritic samples.
- Schmidt, G. R., Jacqueline, R., Yang, H., & Downs, R. T. (2006). Tychite, Na6Mg2(SO4)(CO3) 4: Structure analysis and Raman spectroscopic data. Acta Crystallographica Section E: Structure Reports Online, 62(10), i207-i209.More infoAbstract: Tychite, hexasodium dimagnesium sulfate tetracarbonate, is a member of the northupite [Na3Mg(CO3)2Cl] group and has a framework structure consisting of four basic units: CO32- groups (3 symmetry), SO42- tetrahedra (23 symmetry), MgO6 (3 symmetry) and NaO6 octahedra (2 symmetry). Among them, the CO32-, SO42-, and MgO 6 units are regular in shape, but the NaO6 octahedron, showing three different Na-O bond lengths, is considerably distorted. The Na atoms in tychite are slightly overbonded relative to those in northupite. The Raman spectroscopic data show three and four vibration modes for the CO 32- and SO42- groups, respectively; these are comparable to the corresponding Raman modes observed in other carbonates and sulfates. © 2006 International Union of Crystallography. All rights reserved.
- Wajima, T., Haga, M., Kuzawa, K., Ishimoto, H., Tamada, O., Ito, K., Nishiyama, T., Downs, R. T., & Rakovan, J. F. (2006). Zeolite synthesis from paper sludge ash at low temperature (90 °C) with addition of diatomite. Journal of Hazardous Materials, 132(2-3), 244-252.More infoPMID: 16271830;Abstract: Paper sludge ash was partially converted into zeolites by reaction with 3 M NaOH solution at 90 °C for 24 h. The paper sludge ash had a low abundance of Si and significant Ca content, due to the presence of calcite that was used as a paper filler. Diatomite was added to the NaOH solution to increase its Si content in order to synthesize zeolites with high cation exchange capacity. Diatomite residue was filtered from solution before addition of ash. The original ash without addition of diatomite yielded hydroxysodalite with a cation exchange capacity ca. 50 cmol/kg. Addition of Si to the solution yielded Na-P1 (zeolite-P) with a higher cation exchange capacity (ca. 130 cmol/kg). The observed concentrations of Si and Al in the solution during the reaction explain the crystallization of these two phases. The reaction products were tested for their capacity for PO43- removal from solution as a function of Ca2+ content, suggesting the formation of an insoluble Ca-phosphate salt. The product with Na-P1 exhibits the ability to remove NH4+ as well as PO43- from solution in concentrations sufficient for application in water purification. Both NH4+ and PO43- removal showed little variation with pH between 5 and 9. Alternative processing methods of zeolite synthesis, including the addition of ash to an unfiltered Si-NaOH solution and addition of a dry ash/diatomite mixture to NaOH solution, were tested. The third process yielded materials with lower cation exchange capacity due to formation of hydroxysodalite. The second process results in a product with relatively high cation exchange capacity, and reduces the number of processing steps necessary for zeolite synthesis. © 2005 Elsevier B.V. All rights reserved.
- Yang, H., Manoun, B., Downs, R. T., Ganguly, A., & Barsoum, M. W. (2006). Crystal chemistry of layered carbide, Ti3(Si0.43Ge0.57)C2. Journal of Physics and Chemistry of Solids, 67(12), 2512-2516.More infoAbstract: The crystal structure of a layered ternary carbide, Ti3(Si0.43Ge0.57)C2, was studied with single-crystal X-ray diffraction. The compound has a hexagonal symmetry with space group P63/mmc and unit-cell parameters a=3.0823(1) Å, c=17.7702(6) Å, and V=146.21(1) Å3. The Si and Ge atoms in the structure occupy the same crystallographic site surrounded by six Ti atoms at an average distance of 2.7219 Å, and the C atoms are octahedrally coordinated by two types of symmetrically distinct Ti atoms, with an average C-Ti distance of 2.1429 Å. The atomic displacement parameters for C and Ti are relatively isotropic, whereas those for A (=0.43Si+0.57Ge) are appreciably anisotropic, with U11 (=U22) being about three times greater than U33. Compared to Ti3SiC2, the substitution of Ge for Si results in an increase in both A-Ti and C-Ti bond distances. An electron density analysis based on the refined structure shows that each A atom is bonded to 6Ti atoms as well as to its 6 nearest neighbor A site atoms, whether the site is occupied by Si or Ge, suggesting that these bond paths may be significantly involved with electron transport properties. © 2006 Elsevier Ltd. All rights reserved.
- Gibbs, G. V., Cox, D. F., Ross, N. L., Crawford, T. D., Downs, R. T., & Burt, J. B. (2005). Comparison of the electron localization function and deformation electron density maps for selected earth materials. Journal of Physical Chemistry A, 109(44), 10022-10027.More infoPMID: 16838920;Abstract: The electron localization function (ELF) and experimental and theoretical deformation electron density maps are compared for several earth materials and one representative molecule. The number and arrangement of the localized one-electron probability density domains generated in a mapping of the ELF correspond to the number and arrangement of the localized electron density domains generated in a mapping of the deformation electron density distribution, a correspondence that suggests that the two fields are homeomorphically related. As a homeomorphic relationship has been established previously between the Laplacian of the electron density distribution and the ELF, the relationship suggests that the deformation electron density distribution is also homeomorphically related to the Laplacian of the distribution. © 2005 American Chemical Society.
- Gibbs, G. V., Downs, R. T., Prewitt, C. T., Rosso, K. M., Ross, N. L., & Cox, D. F. (2005). Electron density distributions calculated for the nickel sulfides millerite, vaesite, and heazlewoodite and nickel metal: A case for the importance of Ni-Ni bond paths for electron transport. Journal of Physical Chemistry B, 109(46), 21788-21795.More infoPMID: 16853830;Abstract: Bond paths and the bond critical point properties (the electron density (ρ) and the Hessian of ρ at the bond critical points (bcp's)) have been calculated for the bonded interactions comprising the nickel sulfide minerals millerite, NiS, vaesite, NiS 2, and heazlewoodite, Ni 3S 2, and Ni metal. The experimental Ni-S bond lengths decrease linearly as the magnitudes of the properties each increases in value. Bond paths exist between the Ni atoms in heazlewoodite and millerite for the Ni-Ni separations that match the shortest separation in Ni metal, an indicator that the Ni atoms are bonded. The bcp properties of the bonded interactions in Ni metal are virtually the same as those in heazlewoodite and millerite. Ni-Ni bond paths are absent in vaesite where the Ni-Ni separations are 60% greater than those in Ni metal. The bcp properties for the Ni-Ni bonded interactions scatter along protractions of the Ni-S bond length-bcp property trends, suggesting that the two bonded interactions have similar characteristics. Ni-Ni bond paths radiate throughout Ni metal and the metallic heazlewoodite structures as continuous networks whereas the Ni-Ni paths in millerite, a p,d-metal displaying ionic and covalent features, are restricted to isolated Ni 3 rings. Electron transport in Ni metal and heazlewoodite is pictured as occurring along the bond paths, which behave as networks of atomic size wires that radiate in a contiguous circuit throughout the two structures. Unlike heazlewoodite, the electron transport in millerite is pictured as involving a cooperative hopping of the d-orbital electrons from the Ni 3 rings comprising Ni 3S 9 clusters to Ni 3 rings in adjacent clusters via the p-orbitals on the interconnecting S atoms. Vaesite, an insulator at low temperatures and a doped semiconductor at higher temperatures, lacks Ni-Ni bond paths. The net charges conferred on the Ni and S atoms are about a quarter of their nominal charges for the atoms in millerite and vaesite with the net charge on Ni increasing with increasing Ni-S bond length. Reduced net charges are observed on the Ni atoms in heazlewoodite and are related to its Ni-Ni metal bonded interactions and to the greater covalent character of its bonds. Local energy density and bond critical point properties of the electron density distributions indicate that the Ni-S and Ni-Ni bonded interactions are intermediate in character between ionic and covalent. © 2005 American Chemical Society.
- Gunter, M. E., Downs, R. T., Bartelmehs, K. L., Evans, S. H., Pommier, C. J., Grow, J. S., Sanchez, M. S., & Bloss, F. D. (2005). Optic properties of centimeter-sized crystals determined in air with the spindle stage using EXCALIBRW. American Mineralogist, 90(10), 1648-1654.More infoAbstract: Extinction data sets for four centimeter-sized anisotropic crystals were collected in air with standard spindle-stage methods and submitted to a new Windows-based version of EXCALIBR, termed EXCALIBRW. EXCALIBRW solved these data sets with varying degrees of accuracy related to the external shape of the crystal: the more rounded the crystal, the more precise the results. For an olivine crystal ground into a sphere, the results were similar to those obtained for a crystal immersed in an index-matching fluid. However, even for samples bounded with growth or cleavage faces, the program determined the orientation of the optical indicatrix and 2V with an error of only 1-2°. Thus, this logical extension of spindle-stage methods is helpful: (1) to orient centimeter-size single crystals for various types of mineralogical measurements (e.g., spectroscopy or diffusion studies in which it might be undesirable to place the sample in a liquid); (2) as a non-destructive means of identifying gemstones based upon a determination of their optical class (i.e., isotropic vs. uniaxial vs. biaxial); and (3) for optical characterization by determination of 2V. In addition, the newest version of EXCALIBR is easier to use, mathematically more robust in its solution algorithms, and provides solutions for crystals in less favorable orientations than the earlier versions of EXCALIBR.
- Lager, G. A., Marshall, W. G., Liu, Z., & Downs, R. T. (2005). Re-examination of the hydrogarnet structure at high pressure using neutron powder diffraction and infrared spectroscopy. American Mineralogist, 90(4), 639-644.More infoAbstract: Time-of-flight neutron powder data and synchrotron infrared absorption spectra were collected for katoite hydrogarnet [Ca3Al 2(O4D4)3] at pressures to 9.4 and 9.8 GPa, respectively. The phase transition from space group Ia3d to I4̄3d was observed in the neutron spectrum at ∼7.5 GPa, as indicated by the presence of two weak reflections (730 and 530) that violate the hkl conditions (hk0, h ≠ 2n) imposed by the a-glide operation. However, attempts to refine the high-pressure structure in space group I4̄3d did not significantly improve the fit and produced a chemically unreasonable O-D bond length at the second D position. Structure refinements in Ia3d indicate that (1) the O-D bond length, corrected for the effects of thermal motion, remains essentially constant (∼0.95 Å) with increasing pressure; (2) hydrogen bond lengths shorten with increasing pressure; however, the variation in O-D···O angles indicates a preferential strengthening of H bonds; and (3) the compression mechanism is characterized by bond shortening rather than bond bending. The new results are in excellent agreement with both high-pressure X-ray diffraction experiments and ab initio calculations, and illustrate the need to eliminate peak broadening in high-pressure neutron powder experiments. IR spectra collected for the same sample showed discontinuities in both O-H and O-D vibrational frequencies at ∼5 GPa, suggesting that deuteration does not significantly affect the pressure of the transition. The higher pressure observed for the transition in the neutron data is probably due to lower signal-to-noise levels, which mask the weaker, symmetry-forbidden reflections at lower pressure.
- Thompson, R. M., Downs, R. T., & Redhammer, G. J. (2005). Model pyroxenes III: Volume of C2/c pyroxenes at mantle P, T, and x. American Mineralogist, 90(11-12), 1840-1851.More infoAbstract: Variations in unit-cell volumes of mantle minerals as functions of P and T are important parameters in the description of the interior of the Earth and the behavior of materials, Recently, Thompson and Downs (2004) presented a model for the crystal structures of pyroxenes parameterized in terms of the O3-O3-O3 angle, χ, and the oxygen radius, r. This model has proven useful in the analysis of compression and expansion mechanisms in pyroxenes, providing an understanding of χ and r as functions of P and T. However, it did not provide a basis for analyzing changes in some properties that are strongly dependent on composition. In this paper, we show that ambient unit-cell volumes of the C2/c pyroxenes are correlated with M1 cation radius. This relationship can be used to calculate model ambient unit-cell volumes as a function of chemistry. From this starting point, pyroxene unit-cell volume variation with P and T can be modeled as a function of χ(P,T) and r(P,T). These relationships are investigated for diopside, heden-bergite, acmite, jadeite, and kosmochlor. The model reproduces observed unit-cell volumes of these phases recorded at P to within 0.09% and at T to within 0.10%, at simultaneous P and T for jadeite to within 0.57%, and at simultaneous P and T for diopside to within 1.20%. Ko and K1 from third-order Birch-Murnaghan fits to the observed volume vs. pressure relationships and those calculated from the Thompson-Downs model are statistically the same. The fit of the Thompson-Downs EOS to the observed data is compared to the fit of the third order Birch-Murnaghan. The model is used to create an algorithm that estimates volumes for C2/c pyroxenes as a function of P, T, and x.
- Uchida, H., Lavina, B., Downs, R. T., & Chesley, J. (2005). Single-crystal X-ray diffraction of spinels from the San Carlos Volcanic Field, Arizona: Spinel as a geothermometer. American Mineralogist, 90(11-12), 1900-1908.More infoAbstract: Fourteen spinels from two types of mantle xenoliths from the San Carlos Volcanic Field in Arizona were characterized using single-crystal X-ray diffraction and electron microprobe analysis. The dominant feature seen in the chemistry of spinels from the Group I xenoliths is the extensive substitution of Cr for Al (Cr0.20Al1.76 to Cr0.83Al1.10) correlated with Mg for Fe2+ (Mg0.69Fe0.312+ to Mg0.80Fe0.192+). Although Group II spinels display consistently low Cr values, they also show a well-correlated substitution of Mg for Fe2+ (Mg0.63Fe0.372+ to Mg0.69Fe0.312+). Unit-cell parameters for spinels from the Group I xenoliths range from 8.1259 to 8.2167 Å, while those from the Group II xenoliths range from 8.1247 to 8.1569 Å. The cell parameters are linearly correlated with Fe2+ and Cr contents. Cation distributions were determined from experimental bond lengths and refined site occupancies using the algorithm of Lavina et al. (2002). The San Carlos spinels display variable degrees of order, with inversion parameters ranging from 0.10 to 0.16 for Group I and from 0.17 to 0.22 for Group II. Closure temperatures were computed with the Princivalle equation, giving averages of 808(37) °C for spinels from Group I xenoliths and 822(62) °C for samples from Group II xenoliths. We show that these results are reasonable, and thus extend the use of the Princivalle equation, or at least its functional form, to samples with significant Cr and Fe2+ contents. This study demonstrates that, in spite of the extensive chemical variability of the San Carlos spinels, and given that the origins of the two groups of xenoliths are different, the oxygen coordinates remain fixed, suggesting that the oxygen coordinate is a function of thermal history.
- Wang, Z., Daemen, L. L., Zhao, Y., Zha, C. S., Downs, R. T., Wang, X., Wang, Z. L., & Hemley, R. J. (2005). Morphology-tuned wurtzite-type ZnS nanobelts.. Nature materials., 4(12), 922-927.More infoPMID: 16284620;Abstract: Nanometre-sized inorganic dots, wires and belts have a wide range of electrical and optical properties, and variable mechanical stability and phase-transition mechanisms that show a sensitive dependency on size, shape and structure. The optical properties of the semiconductor ZnS in wurtzite structures are considerably enhanced, but the lack of structural stability limits technological applications. Here, we demonstrate that morphology-tuned wurtzite ZnS nanobelts show a particular low-energy surface structure dominated by the +/-[210] surface facets. Experiments and calculations show that the morphology of ZnS nanobelts leads to a very high mechanical stability to approximately 6.8 GPa, and also results in an explosive mechanism for the wurtzite-to-sphalerite phase transformation together with in situ fracture of the nanobelts. ZnS wurtzite nanobelts provide a model that is useful not only for understanding the morphology-tuned stability and transformation mechanism, but also for improving synthesis of metastable nanobelts with quantum effects for electronic and optical devices.
- Clark, C. M., & Downs, R. T. (2004). Using the American Mineralogist Crystal Structure Database in the classroom. Journal of Geoscience Education, 52(1), 76-80.More infoAbstract: The American Mineralogist Crystal Structure Database is a compilation of every crystal structure potentially of mineralogic or geologic interest. The database, seen as an outreach service, is funded and maintained by NSF, the Mineralogical Society of America and the Mineralogical Association of Canada. This database, when linked with visualization software, provides an invaluable resource for instructors, allowing direct access to crystal structures of almost any mineral, and many at various compositions, pressures or temperatures. Associated with the database is a freeware visualization program, XtalDraw with ancillary software modules, Xpow (for visualization of powder diffraction patterns) and SPEEDEN (for visualization of electron density maps). These software modules are user-friendly, allowing students, instructors, and researchers to easily access various crystal structures. Suggestions for using the database and software in an instructional setting are given; full lesson plans and instructional guides are being developed as Internet resources.
- Downs, R. T., & Hazen, R. M. (2004). Chiral indices of crystalline surfaces as a measure of enantioselective potential. Journal of Molecular Catalysis A: Chemical, 216(2), 273-285.More infoAbstract: Chiral crystal surfaces lack mirror or glide plane symmetry. Nevertheless, some chiral surfaces deviate more significantly from an achiral configuration, and thus possess greater enantioselective potential, than others. We describe a procedure to calculate chiral indices, IC (in Å), of any two-dimensional (2D) periodic atomic surface based on atomic displacements from ideal mirror or glide plane symmetry. We define a 2D unit cell parallel to the surface, identify coordinates of atoms associated with that surface unit cell, and employ minimization procedures to determine the positions and orientations of best-fit pseudo-mirror and pseudo-glide plane operators perpendicular to that surface. Achiral surfaces invariably have IC=0, but we find that surfaces of intrinsically chiral crystals [e.g., quartz (1 0 1)] may also display IC=0, depending on the surface atoms selected. Of 14 surfaces modeled, IC is greatest for chiral faces of achiral crystals: the (2 1 4) scalenohedral faces of calcite (IC=2.60 Å), the (1 1 0) faces of diopside (IC=1.54 Å), and the (6 4 3) faces of FCC metals such as copper and platinum (IC=1.29 Å). © 2004 Elsevier B.V. All rights reserved.
- Thompson, R. M., & Downs, R. T. (2004). Model pyroxenes II: Structural variation as a function of tetrahedral rotation. American Mineralogist, 89(4), 614-628.More infoAbstract: Model pyroxenes with regular tetrahedral and MI octahedral coordination polyhedra have been derived. The M2 polyhedron is not constrained to be regular. These models are parameterized in terms of the O3-O3-O3 angle, θ, and the model O atom radius, r. Crystallographic parameters such as interatomic distances, unit cell volume, and packing distortion are determined as a function of the O3-O3-O3 angle. Results are compared with observed pyroxenes, providing insight into which interatomic interactions are important in determining pyroxene topology and behavior. Temperature is shown to favor polyhedral regularity in orthopyroxene and protopyroxene. Compression and expansion strain ellipsoids for observed and model pyroxenes are compared, demonstrating that a combination of tetrahedral rotation and isotropic compression approximately reproduces the compression ellipsoids of pyroxenes. but not the expansion ellipsoids.
- Wang, Z., Tait, K., Zhao, Y., Schiferl, D., Zha, C., Uchida, H., & Downs, R. T. (2004). Size-induced reduction of transition pressure and enhancement of bulk modulus of AlN nanocrystals. Journal of Physical Chemistry B, 108(31), 11506-11508.More infoAbstract: An in situ X-ray-diffraction study of AlN nanocrystals under hydrostatic (or quasihydrostatic) conditions was performed to pressures of 36.9 GPa, using an energy dispersive synchrotron-radiation technique in a diamond-anvil cell (DAC). Hexagonal AlN nanocrystals have a particle size of 10 nm on average, and display an apparent volumetric expansion as compared to the bulk AlN polycrystals. Upon compression to 14.5 GPa, AlN nanocrystals start to transform to a rocksalt structure phase. This pressure is significantly lower than the transition pressure of 22.9 GPa observed from the bulk AlN by using the same technique. The nanosized wurtzite phase has a bulk modulus (B 0) of 321 ± 19 GPa, larger than that of the bulk AlN crystals with a B 0 of 208 GPa. There is a large volumetric decrease of 20.5% upon the wurtzite-to-rocksalt phase transformation. Combination of the size-induced volumetric expansion and resulting softening of the Poisson ratio and shear modulus may explain the reduction of transition pressure in this type of nanomaterials.
- Wang, Z., Zhao, Y., Schiferl, D., Zha, C. S., & Downs, R. T. (2004). Pressure induced increase of particle size and resulting weakening of elastic stiffness of CeO 2 nanocrystals. Applied Physics Letters, 85(1), 124-126.More infoAbstract: The increase in particle size due to pressure effects in CeO 2 nanocrystals was analyzed. The synchroton x-ray diffraction technique was used to explore the size-induced compressional effects in nanocrystalline CeO 2 to the pressure of 38 GPa. At ambient pressure CeO 2 exhibited larger cell parameters than the micro-sized samples. The results show that at critical pressure of ~20 GPa, there was weakening of the size-induced effect in CeO 2 that resulted into the weakening of elastic stiffness.
- Wang, Z., Zhao, Y., Tait, K., Liao, X., Schiferl, D., Zha, C., Downs, R. T., Qian, J., Zhu, Y., & Shen, T. (2004). A quenchable superhard carbon phase synthesized by cold compression of carbon nanotubes. Proceedings of the National Academy of Sciences of the United States of America, 101(38), 13699-13702.More infoPMID: 15361581;PMCID: PMC518820;Abstract: A quenchable superhard high-pressure carbon phase was synthesized by cold compression of carbon nanotubes. Carbon nanotubes were placed in a diamond anvil cell, and x-ray diffraction measurements were conducted to pressures of ≈100 GPa. A hexagonal carbon phase was formed at ≈75 GPa and preserved at room conditions. X-ray and transmission electron microscopy electron diffraction, as well as Raman spectroscopy at ambient conditions, explicitly indicate that this phase is a sp3-rich hexagonal carbon polymorph, rather than hexagonal diamond. The cell parameters were refined to a0 = 2.496(4) Å, c0 = 4.123(8) Å, and V0 = 22.24(7) Å3. There is a significant ratio of defects in this nonhomogeneous sample that contains regions with different stacking faults. In addition to the possibly existing amorphous carbon, an average density was estimated to be 3.6 ± 0.2 g/cm3, which is at least compatible to that of diamond (3.52 g/cm3). The bulk modulus was determined to be 447 GPa at fixed K′=4, slightly greater than the reported value for diamond of ≈440-442 GPa. An indented mark, along with radial cracks on the diamond anvils, demonstrates that this hexagonal carbon is a superhard material, at least comparable in hardness to cubic diamond.
- Downs, R. T. (2003). Topology of the pyroxenes as a function of temperature pressure, and composition as determined from the procrystal electron density. American Mineralogist, 88(4), 556-566.More infoAbstract: The distribution of bonds associated with the M2 sites in various well-ordered pyroxene minerals is determined using a topological analysis of electron density in the manner proposed by Bader (1998). Each M2 atom is bonded to 2 O1 and 2 O2 atoms, and to zero, one, two, or four bridging O3 atoms. Each of the symmetries displayed by pyroxenes have their own bonding systematics, and each pyroxene-to-pyroxene phase transition involves a change in bonding to M2. As a function of temperature or pressure, the bonding changes appear as a well-defined sequence of steps that can be related to the degree of distortion from the ideal closest packing of anions. It is proposed that the condition at which an individual phase transition occurs is related to M2-Si repulsion through a shared edge. The bonding analysis should provide a qualitative means to interpret the behavior of all pyroxene structures over T, P, and x, and may guide the interpretation of the changes in properties observed by techniques other than X-ray diffraction, such as Raman spectroscopy.
- Downs, R. T., & Hall-Wallace, M. (2003). The American Mineralogist crystal structure database. American Mineralogist, 88(1), 247-250.More infoAbstract: A database has been constructed that contains all the crystal structures previously published in the American Mineralogist. The database is called "The American Mineralogist Crystal Structure Database" and is freely accessible from the websites of the Mineralogical Society of America at http://www.minsocam.org/MSA/Crystal_Database.html and the University of Arizona. In addition to the database, a suite of interactive software is provided that can be used to view and manipulate the crystal structures and compute different properties of a crystal such as geometry, diffraction patterns, and procrystal electron densities. The database is set up so that the data can be easily incorporated into other software packages. Included at the website is an evolving set of guides to instruct the user and help with classroom education.
- Gibbs, G. V., Cox, D. F., Boisen, J., Downs, R. T., & Ross, N. (2003). The electron localization function: A tool for locating favorable proton docking sites in the silica polymorphs. Physics and Chemistry of Minerals, 30(5), 305-316.More infoAbstract: The ELF electron localization function was used to locate regions ascribed to localized nonbonding electrons as favorable docking sites for hydrogen atoms in coesite, quartz, and stishovite. A mapping of the function for a series of representative hydroxyacid disilicate and cyclosiloxane molecules indicates that the nonbonding regions on the bridging oxide anions involved in narrow SiOSi angles are more favorable as docking sites than those involved in wide angles. It also indicates that the nonbonding regions associated with the nonbridging oxide anions comprising surface SiOH dangling bonded interactions are likewise favorable docking sites. ELF descriptions of the nonbonding and bonding regions for coesite and quartz were found to be similar to those obtained for the molecules. For a protonated coesite crystal, the function was also used to deduce the positions of the model H atoms of a defect v(OH)4 group where v is the vacancy left by Si. The positions of the H atoms were found to be in reasonable agreement with those determined in an infrared study, particularly for those bonded to anions involved in narrow SiOSi angles. A mapping of the ELF indicates that the OH vector reported for stishovite is oriented nearly perpendicular (92.5°) to [001], as found in infrared studies. The nonbonding region displayed by the function was used to locate the position of the H atom at a distance of 0.96 Å from the nonequivalent oxide anion comprising the shared edge of the silicate octahedra. The positions of model hydrogen atoms comprising a potential defect v(OH)4 group in quartz were also deduced on the basis of the positions of the local maxima of the nonbonding regions. When used in conjunction with spectroscopic methods, the strategy used in this study should be useful in locating protons in Earth materials with trace amounts of H. Maxima ascribed to regions of bonding and nonbonding electrons displayed in the ELF maps were found to be in close correspondence with those displayed in valence, deformation, and Laplacian electron-density distributions. Collectively, these results indicate that the electron density is localized in the nonbonded regions of the anions involved in the narrow SiOSi angles (angles less than ∼ 150°) in silica polymorphs like quartz and coesite and much less so for the anions involved in wider angles.
- Gibbs, G. V., Whitten, A. E., Spackman, M. A., Stimpfl, M., Downs, R. T., & Carducci, M. D. (2003). An Exploration of Theoretical and Experimental Electron Density Distributions and SiO Bonded Interactions for the Silica Polymorph Coesite. Journal of Physical Chemistry B, 107(47), 12996-13006.More infoAbstract: The use of Hirshfeld type radial functions to explore electron density distribution for silica polymorph coesite was discussed. The results show a local concentration and localization of electron density in both the bonding and nonbonding regions of coesite. The preference of hydrogen for the oxide anions proves that the anions are not spherical in shape and possess lone pair features. The SiO bond was shown to be more ionic than a bond of intermediate character, because of its electronegative behavior.
- Thompson, R. M., & Downs, R. T. (2003). Model pyroxenes I: Ideal pyroxene topologies. American Mineralogist, 88(4), 653-666.More infoAbstract: Ideal pyroxenes are hypothetical structures based on ideal closest-packed arrangements of O anions. They are modeled after observed pyroxene structures and have the general formula M2MIT2O6, where M2 and M1 represent octahedrally coordinated cations, and T represents tetrahedrally coordinated cations. An algorithm has been created to construct all possible ideal pyroxenes based on closest-packed stacking sequences of length 12 or less. These structures are reported. The only significant structural parameters that vary between different ideal pyroxenes are the M1-T and M2-T distances. We show that the repulsive forces between these pairs of cations distinguishes the energetics of the ideal pyroxenes and may be important in determining the topologies of observed pyroxenes.
- Wang, Z., Downs, R. T., Pischedda, V., Shetty, R., Saxena, S. K., Zha, C. S., Zhao, Y. S., Schiferl, D., & Waskowska, A. (2003). High-pressure x-ray diffraction and Raman spectroscopic studies of the tetragonal spinel CoFe2O4. Physical Review B - Condensed Matter and Materials Physics, 68(9), 941011-941016.More infoAbstract: In situ x-ray diffraction and Raman spectroscopy have been carried out to pressures of 93.6 and 63.2 GPa, respectively, to explore the pressure-induced phase transformation of CoFe2O4 spinel. CoFe 2O4 adopts a distorted tetragonal spinel structure at one atmosphere. At a pressure of ∼32.5 GPa, both x-ray diffraction and Raman spectroscopy indicate that CoFe2O4 transforms to the orthorhombic CaFe2O4 structure, which remains stable to at least 93.6 GPa. The bulk modulus (K0) of the tetragonal and the high-pressure polymorphs were calculated to be 94(12) and 145(16) GPa, respectively, with K′ ≡ 4. Upon release of pressure the orthorhombic phase persists and appears to be structurally metastable. At zero pressure, laser induced heating leads to a significant transformation back to the tetragonal phase. The high-pressure orthorhombic phase at one atmosphere is 14.7% denser than the tetragonal phase.
- Wang, Z., Zhao, Y., Schiferl, D., Qian, J., Downs, R. T., Mao, H., & Sekine, T. (2003). Threshold Pressure for Disappearance of Size-Induced Effect in Spinel-Structure Ge 3N 4 Nanocrystals. Journal of Physical Chemistry B, 107(51), 14151-14153.More infoAbstract: We demonstrate that the incompressibility of spinel Ge 3N 4 nanocrystals decreases when the pressure is elevated above ∼20 GPa. Ge 3N 4 nanocrystals initially exhibit a higher bulk modulus of 381(2) GPa. But, above 20 GPa, the bulk modulus is apparently reduced to 268(4) GPa, which is similar to the reported bulk modulus of 208-296 GPa for the bulk Ge 3N 4. Thus, a threshold pressure of ∼20 GPa was determined that signifies the onset of size-induced disappearance of elastic stiffness in nanocrystalline Ge 3N 4. Enhanced surface energy contributions to the shell layers of nanoparticles and resulting effect on the corresponding large d spacing planes are used to elucidate the observed phenomenon. This study provides a reasonable explanation for the different compressibility properties of numerous nanocrystals.
- Wang, Z., Zhao, Y., Schiferl, D., Zha, C. S., Downs, R. T., & Sekine, T. (2003). Critical pressure for weakening of size-induced stiffness in spinel-structure Si3N4 nanocrystals. Applied Physics Letters, 83(15), 3174-3176.More infoAbstract: A study was performed on the critical pressure for weakening of size-induced stiffness in spinel-structure Si3N4 nanocrystals. Raman spectroscopy, x-ray diffraction analysis and transmission electron microscopy were used for the study. The results showed that upon decompression, the bulk modulus of the spinel remained at its lower value of 415 GPa.
- Downs, R. T., Gibbs, G. V., Boisen Jr., M. B., & Rosso, K. M. (2002). A comparison of procrystal and ab initio model representations of the electron-density distributions of minerals. Physics and Chemistry of Minerals, 29(5), 369-385.More infoAbstract: The procrystal calculation of the electron density is a very rapid procedure that offers a quick way to analyze various bonding properties of a crystal. This study explores the extent to which the positions, number, and properties of bond-critical points determined from the procrystal representations of the electron density for minerals are similar to those of first-principles ab initio model distributions. The purpose of the study is to determine the limits imposed upon interpretation of the procrystal electron density. Procrystal calculations of the electron density for more than 300 MO bonds in crystals were compared with those previously calculated using CRYSTAL98 and TOPOND software. For every bond-critical point found in the ab initio calculations, an equivalent one was also found in the procrystal model, with similar magnitudes of electron density, and at similar positions along the bonds. The curvatures of the electron densities obtained from the ab initio and the procrystal distributions are highly correlated. It is concluded that the procrystal distributions are capable of providing good estimates of the bonded radii of the atoms and the properties of the electron-density distributions at the bond-critical points. Because the procrystal model is so fast to compute, it is especially useful in addressing the question as to whether a pair of atoms is bonded or not. If the Bader criteria for bonding are accepted, then the successful generation of the bond- critical points by the procrystal model demonstrates that bonding is an atomic feature. The main difference between the critical-point properties of the procrystal and the ab initio model is that the curvature in the electron density perpendicular to the bond path of the ab initio model is sharper than for the procrystal model. This is interpreted as indicating that the electrons that migrate into a bond originate from its sides, and not from the regions closer to the nuclei. This observation also suggests that ab initio optimization routines could see an improvement in speed if the parameters relating to the angular components of atomic wave functions were to vary before the radial components.
- Lager, G. A., Downs, R. T., Origlieri, M., & Garoutte, R. (2002). High-pressure single-crystal X-ray diffraction study of katoite hydrogarnet: Evidence for a phase transition from Ia3d → I4̄3d symmetry at 5 GPa. American Mineralogist, 87(5-6), 642-647.More infoAbstract: The crystal structure of katoite hydrogarnet has been refined at 0.0001, 2.15, 4.21, 5.09, 6.00, 7.09, and 7.78 GPa from single-crystal X-ray diffraction data using a 4:1 methanol: ethanol mixture as pressure medium in a Merrill-Bassett diamond-anvil cell. Below ~5 GPa, the katoite structure has la3d symmetry and compresses by bond shortening rather than bond bending, in agreement with recent quantum mechanical calculations. An unconstrained third-order Birch-Murnaghan fit to the unit-cell volumes and pressures for la3d symmetry gave the following equation of state parameters: Vo = 1987.6(1) Å3, Ko = 58(1) GPa and K' = 4.0(7). Above this pressure, the structure undergoes a phase transition to space group I43d, a non-centric subgroup of la3d. In the I43d structure. there are two non-equivalent (O4H4) groups. Both the Ca and Al atoms are displaced along a relative to their positions in la3d. It is proposed that compression of the short H-H distance between (O4H4) groups destabilizes the structure and may initiate the observed phase transition. Corroboration of this model will require accurate information on the hydrogen atom positions at pressures above 5 GPa.
- Righter, K., & Downs, R. T. (2001). The crystal structures of synthetic Re- and PGE-bearing magnesioferrite spinels: Implications for impacts, accretion and the mantle. Geophysical Research Letters, 28(4), 619-622.More infoAbstract: Re- and PGE-bearing magnesioferrite spinels were synthesized at oxidizing conditions between 0.1 MPa and 5.0 GPa, and 1150 to 1600 °C in equilibrium with Re- or PGE-bearing metals and/or oxides. Electron microprobe analysis and single crystal X-ray diffraction techniques were employed to determine the composition, crystal structures and site occupancies, indicating that the magnesioferrite structure can accommodate wt% levels of Re and the platinum group elements (PGE) at oxidized conditions. These results suggest that magnesioferrite spinels found in K-T boundary samples worldwide could be an important host phase for the Ir, Ru, Os, Re, and Rh anomalies found in the boundary layer. Higher Ru/Ir values in Pacific magnesioferrite-bearing samples may be a reflection of higher condensation temperatures of the oxides. The distribution of PGE's in mantle and magmatic rocks may depend on the stability of spinel-structure oxides, especially those with a high magnesioferrite component. Finally, magnetite-rich meteorite fusion crusts may hold PGEs in oxidized form.
- Thompson, R. M., & Downs, R. T. (2001). Quantifying distortion from ideal closest-packing in a crystal structure with analysis and application. Acta Crystallographica Section B: Structural Science, 57(2), 119-127.More infoPMID: 11262425;Abstract: A parameter, Ucp, that quantifies the distortion of the anion skeleton in a crystal from ideal closest-packing has been developed. It is a measure of the average isotropic displacement of the observed anions from their ideal equivalents. An ideal closest-packed structure can be fit to an observed structure by varying the radius of the ideal spheres, orientation and translation, such that Ucp is minimized. Ideal structures were fit to the M1M2TO4 polymorphs, pyroxenes and kyanite. The distortions of these crystals were analyzed in terms of the two parameters, Ucp and the ideal radius. Changes in structures due to temperature, pressure and compositional effects were characterized in terms of these parameters.
- Thompson, R. M., & Downs, R. T. (2001). Systematic generation of all nonequivalent closest-packed stacking sequences of length N using group theory. Acta Crystallographica Section B: Structural Science, 57(6), 766-771.More infoPMID: 11717475;Abstract: An algorithm has been developed that generates all of the nonequivalent closest-packed stacking sequences of length N. There are 2N + 2(-1)N different labels for closest-packed stacking sequences of length N using the standard A, B, C notation. These labels are generated using an ordered binary tree. As different labels can describe identical structures, we have derived a generalized symmetry group. Q ≃ DN × S3, to sort these into crystallographic equivalence classes. This problem is shown to be a constrained version of the classic three-colored necklace problem.
- Holl, C. M., Smyth, J. R., Laustsen, H. M., Jacobsen, S. D., & Downs, R. T. (2000). Compression of witherite to 8 GPa and the crystal structure of BaCO3II. Physics and Chemistry of Minerals, 27(7), 467-473.More infoAbstract: Natural witherite (Ba0.99Sr0.01CO3) has been studied by single-crystal X-ray diffraction in the diamond anvil cell at eight pressures up to 8 GPa. At ambient pressure, cell dimensions are a = 5.3164(12) Å, b = 8.8921(19) Å, c = 6.4279(16) Å, and the structure was refined in space group Pmcn to R(F) = 0.020 from 2972 intensity data. The unit cell and atom position parameters for the orthorhombic cell were refined at pressures of 1.2, 2.0, 2.9, 3.9, 4.6, 5.5, 6.2, and 7.0 GPa. The volume-pressure data are used to calculate equation of state parameters K(T0) = 50.4(12) GPa and K' = 1.9(4). At approximately 7.2 GPa, a first-order transformation to space group P31c was observed. Cell dimensions of the high-pressure phase at 7.2 GPa are a = 5.258(6) Å, c = 5.64(1) Å. The high pressure structure was determined and refined to R(F) = 0.06 using 83 intensity data, of which 15 were unique. This high-pressure phase appears to be more compressible than the orthorhombic phase with an estimated initial bulk modulus (K(7.2GPa)) of 10 GPa.
- Sprague, A. L., Roush, T. L., Downs, R. T., & Righter, K. (2000). Response to Comment on "Comparison of Laboratory Emission Spectra with Mercury Telescopic Data" by Melissa Lane. Icarus, 143(2), 409-411.More infoAbstract: The laboratory spectrum published in Fig. 3 of A. L. Sprague and T. L. Roush (1998, Icarus133, 174-183) is a mixture of magnetite and hematite (~34-45% magnetite, ~55-66% hematite) rather than magnetite or hematite (M. D. Lane 2000, Icarus143, 000-000) alone. Because of the unusual nature of this sample (it is uniformly magnetic) we present results of X-ray diffraction, petrographic microscopy, and additional microprobe analyses. We compare this spectrum with laboratory spectra of hematite and magnetite. We can draw no conclusions regarding the presence or absence of magnetite on Mercury based solely on data in the spectral region 7.5-12μm. © 2000 Academic Press.
- Downs, R. T., Yang, H., Hazen, R. M., Finger, L. W., & Prewitt, C. T. (1999). Compressibility mechanisms of alkali feldspars: New data from reedmergnerite. American Mineralogist, 84(3), 333-340.More infoAbstract: Structural and volume compressibility data for reedmergnerite, NaBSi3O8, were obtained by single-crystal X-ray diffraction at pressures up to 4.7 GPa. The bulk modulus was determined to be 69.8(5) GPa with the pressure derivative constrained to 4. Unit-cell compression is anisotropic, as indicated by unit strain tensors. Tetrahedral bond lengths and angles remained relatively constant over the pressure interval, whereas Na-O bonds decreased systematically. T-O-T angles underwent a variety of behaviors, remaining constant or decreasing with pressure. The compression for reedmergnerite is similar to that of low-albite, wherein bending of the (Al,B)-Oco-Si angle compresses the Na-bearing zigzag channels. In contrast, microcline compresses by shearing the four-membered rings, which in turn compresses the K-bearing channels. At about 4 GPa, a new bond between K and Obm appears that alters the compression mechanism and explains the discontinuity in the pressure variation of crystallographic parameters observed by Allan and Angel (1997). Thus, the compression mechanism of the alkali feldspars is dominated by the compression of alkali containing channels. However, because of low symmetry, this can be accomplished in several ways. The observed variety of compression pathways resulted from T-O-T angle bending energetics that were coupled with the effects of alkali cation bonding.
- Cohen, R. E., Fei, Y., Downs, R., Mazin, I. I., & Isaak, D. G. (1998). Magnetic collapse and the behavior of transition metal oxides: FeO at high pressures. Materials Research Society Symposium - Proceedings, 499, 27-37.More infoAbstract: Linearized augmented plane wave (LAPW) results are presented for FeO at high pressures using the Generalized Gradient Approximation (GGA) to study the high-spin low-spin transition previously predicted by LAPW with the Local Density Approximation (LDA) and Linear Muffin Tin Orbital (LMTO-ASA) methods within the GGA. We find a first-order transition at a pressure of about 105 GPa for the cubic lattice, consistent with earlier LAPW results, but much lower than obtained with the LMTO. The results are generally consistent with recent Moessbauer experiments that show a transition at about 100 GPa. We also discuss the origin of the transition, and show that it is not due to electrostatic crystal-field effects, but is rather due to hybridization and band widening with pressure. Examination of experimental data and computations suggest that the high pressure hexagonal phase of FeO is likely a polytype between the B8 NiAs and anti-B8 AsNi structures. The former is predicted to be an antiferromagnetic metal, and the latter an antiferromagnetic insulator. Implications for geophysics are discussed.
- Downs, R. T., & Somayazulu, M. S. (1998). Carbon dioxide at 1.0 GPa. Acta Crystallographica Section C: Crystal Structure Communications, 54(7), 897-898.More infoAbstract: An X-ray diffraction study of single-crystal CO2 has been undertaken at 1.00 (5) GPa pressure. The crystal exhibits Pa3 symmetry with a cell edge of 5.4942 (2) Å and a C-O bond length of 1.168 (1) Å (corrected for thermal motion effects). The study demonstrates that an earlier claim of a new dry ice II phase at this pressure is unfounded.
- Gibbs, G. V., Boisen, M. B., Hill, F. C., Tamada, O., & Downs, R. T. (1998). SiO and GeO bonded interactions as inferred from the bond critical point properties of electron density distributions. Physics and Chemistry of Minerals, 25(8), 574-584.More infoAbstract: The topological properties of the electron density distributions for more than 20 hydroxyacid, geometry optimized molecules with SiO and GeO bonds with 3-, 4-, 6- and 8-coordinate Si and Ge cations were calculated. Electronegativities calculated with the bond critical point (bcp) properties of the distributions indicate, for a given coordination number, that the electronegativity of Ge (~1.85) is slightly larger than that of Si (~1.80) with the electronegativities of both atoms increasing with decreasing bond length. With an increase in the electron density, the curvatures and the Laplacian of the electron density at the critical point of each bond increase with decreasing bond length. The covalent character of the bonds are assessed, using bond critical point properties and electronegativity values calculated from the electron density distributions. A mapping of the (3, -3) critical points of the valence shell concentrations of the oxide anions for bridging SiOSi and GeOGe dimers reveals a location and disposition of localized nonbonding electron pairs that is consistent with the bridging angles observed for silicates and germanates. The bcp properties of electron density distributions of the SiO bonds calculated for representative molecular models of the coesite structure agree with average values obtained in X-ray diffraction studies of coesite and danburite to within ~5%.
- Gibbs, G. V., Hill, F. C., Boisen, M. B., & Downs, R. T. (1998). Power law relationships between bond length, bond strength and electron density distributions. Physics and Chemistry of Minerals, 25(8), 585-590.More infoAbstract: The strength of a bond, defined as p=s/r, where s is the Pauling bond strength and r is the row number of an M cation bonded to an oxide anion, is related to a build-up of electron density along the MO bonds in a relatively large number of oxide and hydroxyacid molecules, three oxide minerals and three molecular crystals. As p increases, the value of the electron density is observed to increase at the bond critical points with the lengths of the bonds shortening and the electronegativities of the M cations bonded to the oxide anion increasing. The assertion that the covalency of a bond is intrinsically connected to its bond strength is supported by the electron density distribution and its bond critical point properties. A connection also exists between the properties of the electron density distributions and the connectivity of the bond strength network formed by the bonded atoms of a structure.
- Jacobsen, S. D., Smyth, J. R., Swope, R. J., & Downs, R. T. (1998). Rigid-body character of the SO4 groups in celestine, anglesite and barite. Canadian Mineralogist, 36(4), 1053-1060.More infoAbstract: The crystal structures of natural celestine (Sr1.00)SO4, anglesite (Pb0.99Sr0.01)SO4, and barite (Ba0.99Sr0.01)SO4 have been refined in space group Pbnm utilizing rotating anode, Mo X-ray diffraction data from single crystals. Unit-cell parameters for celestine are a 6.8671(7), b 8.3545(8), and c 5.3458(6) Å, for anglesite, a 6.9549(9), b 8.472(1), and c 5.3973(8) Å, and for barite, a 7.154(1), b 8.879(2), and c 5.454(1) Å. Structural data are presented for these sulfates with greatly improved precision over previous studies owing to high peak-to-background intensity ratios and precise analytical absorption corrections. The final model R(F) values are 0.025, 0.041, and 0.019, for celestine, anglesite, and barite, respectively. The average bond-distance from divalent cation to the nearest twelve oxygen atoms is 2.827(1) Å in celestine, 2.864(5) Å in anglesite, and 2.951(2) Å in barite. The average sulfur-to-oxygen bond distance is 1.475(2) Å in celestine, 1.476(6) Å in anglesite, and 1.476(2) Å in barite. The sulfate tetrahedra in each structure show very similar distortions that are attributed to the bonding of the various oxygen atoms to the divalent cations, which is similar in each structure. Thus, the different metal cations do not seem to affect the size or shape of the sulfate tetrahedra. An analysis of the displacement parameters suggests that the SO4 groups behave as rigid molecular units, with an apparent shortening of the S-O bonds of 0.008-0.010 Å.
- Mazin, I. I., Fei, Y., Downs, R., & Cohen, R. (1998). Possible polytypism in FeO at high presures. American Mineralogist, 83(5-6), 451-457.More infoAbstract: Examination of X-ray diffraction intensities for FeO collected in situ at high pressure and high temperature reveals that the atomic arrangements of FeO in the hexagonal structure are not the same as those in the simple NiAs-type structure (B8) where Fe takes the place of Ni. The observed diffraction intensity can be explained by adding an anti-B8 component (where Fe takes the place of As). Substitution of Fe and O atoms for each other is crystallochemically unique. The exchange of Fe and O positions provides a critical measure of the change in chemical bonding. Our conclusion is consistent with the observed transition of FeO from an ionically bonded structure to a strongly covalent and metallic one. First-principles electronic structure computations using the linearized augmented plane wave (LAPW) method with the generalized gradient approximation (GGA) indicate that both B8 and anti-B8 should be antiferomagnetic, but only anti-B8 should be an insulator. The GGA and local density approximation (LDA) incorrectly compute anti-B8 as the ground state structure.
- Zha, C., Duffy, T. S., Downs, R. T., Mao, H., & Hemley, R. J. (1998). Brillouin scattering and X-ray diffraction of San Carlos olivine: direct pressure determination to 32 GPa. Earth and Planetary Science Letters, 159(1-2), 25-33.More infoAbstract: The single-crystal elastic moduli of San Carlos olivine, (Mg0.9Fe0.1)2SiO4, were determined at seven pressures between 2.5 and 32.5 GPa by Brillouin spectroscopy in a diamond anvil cell. The unit cell volume was also determined at each pressure (and ambient pressure) by single-crystal X-ray diffraction. The shear elastic moduli exhibit non-linear pressure dependencies. In particular, a non-linear dependence is required for the elastic modulus, C55, above 10 GPa. Aggregate bulk and shear moduli were determined from Hashin-Shtrikman averages of the individual constants. Pressures were determined by fitting the bulk modulus (Reuss bound) and volume data to a third-order Eulerian finite strain equation and integrating. The results are compared to independent pressure measurements made using the ruby fluorescence scale. The ruby measurements yield pressures that are in good agreement with the Brillouin results. On average, the ruby fluorescence measurements produce pressures that are larger than the Brillouin determinations by 1.6%.
- Badro, J., Teter, D. M., Downs, R. T., Gillet, P., Hemley, R. J., & Barrat, J. (1997). Theoretical study of a five-coordinated silica polymorph. Physical Review B - Condensed Matter and Materials Physics, 56(10), 5797-5806.More infoAbstract: Theoretical calculations are performed to study transformations in silica as a function of nonhydrostatic stress. Molecular-dynamics calculations reveal a crystalline-to-crystalline transition from α-quartz to a phase with five-coordinated silicon (vSi) at high pressure in the presence of deviatoric stress. The phase, which appears for specific orientations of the stress tensor relative to the crystallographic axes of quartz, is a crystalline polymorph of silica with five-coordinated silicon. The structure possesses P3221 space-group symmetry. First-principles calculations within the local-density approximation, as well as molecular dynamics and energy minimization with interatomic potentials, find this phase to be mechanically and energetically stable with respect to quartz at high pressure. The calculated x-ray diffraction pattern and vibrational properties of the phase are reported. Upon decompression, the vSi phase reverts to α-quartz through an intermediate four-coordinated phase and an unusual isosymmetrical phase transformation. The results suggest the importance of application of nonhydrostatic stress conditions in the design and synthesis of novel materials.
- Yang, H., Downs, R. T., Finger, L. W., Hazen, R. M., & Prewitt, C. T. (1997). Compressibility and crystal structure of kyanite, Al2SiO5, at high pressure. American Mineralogist, 82(5-6), 467-474.More infoAbstract: The unit-cell dimensions and crystal structure of kyanite at various pressures up to 4.56 GPa were refined from single-crystal X-ray diffraction data. The bulk modulus is 193(1) GPa, assuming K′ = 4.0. Calculated unit-strain tensors show that kyanite exhibits more isotropic compressibility than andalusite or sillimanite. The most and least compressible directions in the kyanite structure correspond approximately to the most and the least thermally expandable directions. The analysis of the distortion of the closest packing in kyanite indicates that the most compressible direction of the structure (along [012]) corresponds to the direction along which the closest-packed O monolayers are sucked. The bulk moduli for the Al1, Al2, Al3, and Al4 octahedra are 274(43), 207(14), 224(26), and 281(24) GPa, respectively, and those for the Si1 and Si2 tetrahedra are 322(80) and 400(95) GPa, respectively. Four AlO6 octahedra that all become less distorted at higher pressures do not display clearly dominant compression directions. The average unshared O-O distance for each octahedron is considerably more compressible than the shared O-O distance. The high-pressure behaviors of the Al1 and Al4 octahedra are very similar but different from those of the Al2 and Al3 octahedra. Bulk moduli for the three Al2SiO5 polymorphs (kyanite, sillimanite, and andalusite), as well as those for the AlO6 octahedra in their structures, appear to decrease linearly as their volumes increase. The significantly larger bulk modulus and more isotropic compressibility for kyanite than for andalusile or sillimanite are a consequence of the nearly cubic close-packed arrangement of O atoms and the complex edge-sharing among four distinct AlO6 octahedra in the kyanite structure.
- Yang, H., Hazen, R. M., Downs, R. T., & Finger, L. W. (1997). Structural change associated with the incommensurate-normal phase transition in akermanite, Ca2MgSi2O7, at high pressure. Physics and Chemistry of Minerals, 24(7), 510-519.More infoAbstract: The structural changes associated with the incommensurate (IC)-normal (N) phase transition in akermanite have been studied with high-pressure single-crystal X-ray diffraction up to 3.79 GPa. The IC phase, stable at room pressure, transforms to the N phase at ∼ 1.33 GPa. The structural transformation is marked by a small but discernable change in the slopes of all unitcell parameters as a function of pressure. It is reversible with an apparent hysteresis and is classified as a tricritical phase transition. The linear compressibility of the a and c axes are 0.00280(10) and 0.00418(6) GPa-1 for the IC phase, and 0.00299(11) and 0.00367(8) GPa-1 for the N phase, respectively. Weighted volume and pressure data, fitted to a second-order Birch-Murnaghan equation of state (K′=4.0), yield V0-307.4(1) Å3 and K0= 100(3) GPa for the IC phase and V0=307.6(2) Å3 and K0=90 (2) GPa for the N phase. No significant discontinuities in Si-O, Mg-O and Ca-O distances were observed across the transition, except for the Ca-O1 distance, which is more compressible in the IC phase than in the N phase. From room pressure to 3.79 GP the volume of the [SiO4] tetrahedron is unchanged (2.16 Å3), whereas the volumes of the [MgO4] and [CaO8] polyhedra decrease from 3.61 to 3.55(1) Å3 and 32.8 to 30.9 (2) Å3, respectively. Intensities of satellite reflections are found to vary linearly with the Isotropic displacement parametr of Ca and the librational amplitude of the [SiO4] tetrahedron. At room pressure, there is a mismatch between the size of the Ca cations and the configuration of tetrahedral sheets, which appears to be responsible for the formation of the modulated structure; as pressure increases, the misfit is diminished through the relative rotation and distortion of [MgO4] and [SiO4] tetrahedra and the differential compression of individual Ca-O distances, concurrent with a displacement of Ca along the (110) mirror plane toward the O1 atom. We regard the high-pressure normal structure as a result of the elimination of microdomains in the modulated structure.
- Yang, H., Hazen, R. M., Finger, L. W., Prewitt, C. T., & Downs, R. T. (1997). Compressibility and crystal structure of sillimanite, Al2SiO5, at high pressure. Physics and Chemistry of Minerals, 25(1), 39-47.More infoAbstract: The unit-cell dimensions and crystal structure of sillimanite at various pressures up to 5.29 GPa have been refined from single-crystal X-ray diffraction data. As pressure increases, a and b decrease linearly, whereas c decreases nonlinearly with a slightly positive curvature. The axial compression ratios at room pressure are βa:βb:βc=1-22:1.63:1.00. Sillimanite exhibits the least compressibility along c. but the least thermal expansivity along a (Skinner et al. 1961; Winter and Ghose 1979). The bulk modulus of sillimanite is 171(1) GPa with K'-4 (3), larger than that of andalusite (151 GPa), but smaller than that of kyanite (193 GPa). The bulk moduli of the [Al1O6], [Al2O4], and [SiO4] polyhedra are 162(8), 269(33), and 367(89) GPa, respectively. Comparison of high-pressure data for Al2SiO5 polymorphs reveals that the [SiO4] tetrahedra are the most rigid units in all these polymorphic structures, whereas the [AlO6] octahedra are most compressible. Furthermore, [AlO6] octahedral compressibilities decrease from kyanitc to sillimanite, to andalusite, the same order as their bulk moduli, suggesting that [AlO6] octahedra control the compression of the Al2SiO5 polymorphs. The compression of the [Al1O6] octahedron in sillimanite is anisotropic with the longest Al1-OD bond shortening by ∼1.9% between room pressure and 5.29 GPa and the shortest Al1-OB bond by only 0.3%. The compression anisotropy of sillimanite is primarily a consequence of its topological anisotropy, coupled with the compression anisotropy of the Al-O bonds within the [Al1O6] octahedron.
- Zha, C., Duffy, T. S., Mao, H., Downs, R. T., Hemley, R. J., & Weidner, D. J. (1997). Single-crystal elasticity of β-Mg2SiO4 to the pressure of the 410 km seismic discontinuity in the Earth's mantle. Earth and Planetary Science Letters, 147(1-4), E9-E15.More infoAbstract: The complete set of elastic stiffness coefficients of a single crystal of Mg2SiO4 in the modified spinel (wadsleyite) structure are reported at eight pressures from ambient to 14 Gigapascals (GPa). All moduli vary linearly with pressure within the resolution of the data. Third-order Birch-Murnaghan equation fits to aggregate elastic moduli yield values of 170 (± 2) GPa and 4.3 (± 0.2) for the adiabatic bulk modulus and its first pressure derivative, and 115 (± 2) GPa and 1.4 (± 0.2) for the shear modulus and its first pressure derivative. These data, together with previous results for α-Mg2SiO4, enable us to make the first in situ determination of the velocity increase due to the α-β transition at the pressure (∼ 13.8 GPa) of the 410 km discontinuity. The room-temperature velocity increase at the transition pressure is 9.8% for compressional waves and 12.4% for shear waves.
- Downs, R. T., Andalman, A., & Hudacsko, M. (1996). The coordination numbers of Na and K atoms in low albite and microcline as determined from a procrystal electron-density distribution. American Mineralogist, 81(11-12), 1344-1349.More infoAbstract: Procrystal models for the electron-density distributions of low albite and microcline were constructed by placing spherically averaged Clementi-Roetti atomic wave functions at experimentally observed atomic positions. The topography of the model electron-density distribution was analyzed to locate (3,-1) critical points between the Na or K positions and the positions of the surrounding framework O atoms. Because the number of (3,-1) critical points associated with a given atom corresponds with its coordination number, the analysis indicated that Na is fivefold and K is sevenfold coordinated. In particular, the results indicate that the Oco atom in low albite is not coordinated with Na, whereas in microcline it is coordinated with K. This suggests that in low albite the Oco atom is underbonded and thus a possible hydrophilic site, whereas in microcline the bonding requirements of Oco are satisfied. Details of the local bonding explain the effects of pressure, H, and H2O on ordering of Al and Si, as well as compressibility systematics.
- Downs, R. T., Zha, C., Duffy, T. S., & Finger, L. W. (1996). The equation of state of forsterite to 17.2 GPa and effects of pressure media. American Mineralogist, 81(1-2), 51-55.More infoAbstract: Pressure-volume X-ray diffraction data were collected at eight pressures to 17.2 GPa for synthetic single-crystal forsterite, Mg2SiO4. Birch-Murnaghan equation-of-state parameters were determined to be K0 = 125(2) GPa and K′0 = 4.0(4). To ensure hydrostatic conditions, we monitored the diffraction-peak shapes while the sample was in different pressure media: 4:1 methanol-ethanol, He, and Ar. The diffraction peaks for forsterite in 4:1 methanol-ethanol at 12.2 GPa were broadened to such an extent that we believe that any data collected at similar pressures and in this medium may be suspect and should be examined critically. The diffraction-peak profiles for the sample in He were sharp and well-formed, suggesting He provides an excellent medium at pressures ≤ 17.2 GPa. However, upon decompression, the diffraction peaks of the sample in He also showed significant broadening. This broadening suggests that with increasing pressure He diffused into the crystal, but with the release of pressure the He was unable to diffuse out quickly. The diffraction peaks regained their sharpness after several months.
- Hazen, R. M., Downs, R. T., & Finger, L. W. (1996). High-pressure crystal chemistry of LiScSiO4: An olivine with nearly Isotropic compression. American Mineralogist, 81(3-4), 327-334.More infoAbstract: Single-crystal X-ray diffraction data have been obtained at several pressures to 5.6 GPa for synthetic LiScSiO4 olivine. The bulk modulus is 118 ± 1 GPa, assuming K′ = 4. This value is smaller than that of forsterite because compressibility of Li-O bonds in the M1 octahedral site is approximately twice that of the M12+-O bonds in other isomorphs. Compressibilities of a, b, and c orthorhombic axes are 2.70, 2.80, and 2.61 (all × 10-3 GPa-1), respectively. This nearly isotropic compression (axial compression ratios = 1.00: 1.04:0.97) contrasts with that of forsterite (1.00:1.99:1.55), fayalite (1.00:2.83:1.22), monticellite (1.00:1.85:1.10), and chrysoberyl (1.00:1.30:1.17). These differences arise from the distinctive distribution of cations of different valences and consequent differences in M1-O and M2-O bond compressibilities. The Li M1 octahedron displays a significant decrease in polyhedral distortions with pressure, a behavior not observed in other olivines.
- Hazen, R. M., Downs, R. T., & Finger, L. W. (1996). High-pressure framework silicates. Science, 272(5269), 1769-1771.More infoAbstract: Recent syntheses of high-pressure alkali and alkaline earth silicates reveal a class of framework structures with corner-linked 4- and 6- coordinated silicon. These compounds possess the structural formula (A(4- 2x)/1+B(x)/2+)Si(m)/(VI)(Si(n)/(IV)O(2(m+n)+2)) where x, m, and n specify the amounts of alkaline earth, 6-coordinated silicon, and 4-coordinated silicon, respectively. Appropriate values of m and n yield a range of high- pressure structures, from fully 4-coordinated to fully 6-coordinated silicate frameworks. Recognition of this class of framework silicates leads to predictions of high-pressure structures as weft as room-pressure isomorphs of high-pressure silicates.
- Zha, C., Duffy, T. S., Downs, R. T., Mao, H., & Hemley, R. J. (1996). Sound velocity and elasticity of single-crystal forsterite to 16 GPa. Journal of Geophysical Research B: Solid Earth, 101(8), 17535-17545.More infoAbstract: As an abundant component of Earth's upper mantle, forsterite (Mg2SiO4) has been the subject of extensive investigation for many decades. The pressure dependences of sound velocity and elasticity are of great importance for modeling the composition and evolution of Earth. For this reason, room temperature Brillouin scattering experiments were performed on single-crystal forsterite at pressures up to 16 GPa in a diamond anvil cell. The nine elastic moduli increase linearly over the pressure range studied. Together with previous ambient pressure data, the pressure variation of the bulk modulus is KS(GPa) = 128.8(5) + 4.2(2)P, and the shear modulus is G(GPa) = 81.6(2) + 1.4(1)P, where the numbers in parentheses are one standard deviation uncertainties in the last digit. Combining with previous ultrasonic data for polycrystalline β-Mg2SiO4 to 3 GPa, the sound velocity contrasts between α- and β-Mg2SiO4 were found to be largely pressure independent at room temperature. If, on the other hand, the pressure dependences of the bulk and shear modulus of the β phase of Mg2SiO4 are assumed to be identical to those for the α phase, the room temperature velocity contrasts between the two phases decrease by 20% and 12% from 1 bar to 13.8 GPa for compressional and shear waves, respectively.
- Bartelmehs, K. L., Downs, R. T., Gibbs, G. V., Boisen Jr, M. B., & Birch, J. B. (1995). Tetrahedral rigid-body motion in silicates. American Mineralogist, 80(7-8), 680-690.More infoAbstract: The atomic displacement parameters (ADPs) obtained from published refinements of silicate crystals measured at or below room temperature are examined to determine if TO4 (T = Al,Si) tetrahedra display rigid-body thermal motion. In many cases, the correlation found among the ADPs is consistent with the TLS model of rigid-body motion. The libration angle for rigid tetrahedra is quadratically dependent on the difference between the isotropic equivalent displacement parameter of the T and O atoms, Beq(T) and Beq(O), respectively. The value of Beq(O) is on average twice that of Beq(T), with an observed maximum value of ~2.0 Å3. -from Authors
- Downs, R. T., Hazen, R. M., Finger, L. W., & Gaspasik, T. (1995). Crystal chemistry of lead aluminosilicate hollandite: a new high- pressure synthetic phase with octahedral Si. American Mineralogist, 80(9-10), 937-940.More infoAbstract: Single crystals of lead aluminosilicate hollandite, with composition Pb0.8Al1.6Si2.4O8, have been synthesized at 16.5 GPa and 1450°C. Crystals are tetragonal [space group I4, Z = 2, a = 9.414(3), c = 2.750(3) Å, V = 243.7(3) Å3]. Si and Al are disordered on the octahedral site. Pb is best modeled by two atom sites, one on the fourfold axis and the other split, lying off the fourfold axis. -from Authors
- Duffy, T. S., Zha, C., Downs, R. T., Mao, H., & Hemley, R. J. (1995). Elasticity of forsterite to 16 GPa and the composition of the upper mantle. Nature, 378(6554), 170-173.More infoAbstract: NEARLY 60 years ago, Bernai1 proposed that a polymorphic phase transformation in olivine might be responsible for the seismic velocity discontinuity near 410 km depth in the mantle. Phase equilibria experiments2,3 have since shown that the olivine (α) to wadsleyite (β) transition in (Mg,Fe)2SiO4 occurs at the appropriate pressure (13.8GPa) under mantle conditions. Comparison of laboratory measurements of the acoustic velocity contrast in the α-β system to the magnitude of the seismically observed discontinuity at 410 km provides a way to constrain the olivine content of the mantle at this depth. Here we report measurements of the full set of elastic moduli of single-crystal forsterite (α-Mg2SiO4) at pressures between 3 and 16 GPa, using Brillouin scattering in a diamond anvil cell. At 13.8 GPa, the aggregate compressionaland shear-wave velocities of α-Mg2SiO4 are 2.7 ±0.7% lower than predicted from earlier low-pressure data4,5. From our data, and assuming a homogeneous mantle composition, the seismic velocity contrast at 410 km depth can be satisfied only by a mantle containing less than ∼40% olivine. This is well below the olivine abundance assumed in peridotite-based upper-mantle models.
- Downs, R. T., & Palmer, D. C. (1994). The pressure behavior of α cristobalite. American Mineralogist, 79(1-2), 9-14.More infoAbstract: Structural and volume compressibility data for two α cristobalite samples were determined by single-crystal X-ray diffraction methods at pressures up to ~1.6 GPa. As with the other silica polymorphs that have been studied at high pressure, the change of the Si-O-Si angle is correlated with the volume compressibility. The Si-O bond lengths and the O-Si-O angles remain essentially unchanged. The bulk modulus was determined to be 11.5(7) GPa with a pressure derivative of 9(2). Both crystals underwent reversible structural phase transitions in the pressure interval 1.18-1.60 GPa. -Authors
- Downs, R. T., Hazen, R. M., & Finger, L. W. (1994). The high-pressure crystal chemistry of low albite and the origin of the pressure dependency of Al-Si ordering. American Mineralogist, 79(11-12), 1042-1052.More infoAbstract: Structural and volume compressibility data for low albite were obtained by single-crystal X-ray diffraction methods at pressures up to ~4 GPa. The bulk modulus was determined to be 54(1) GPa, with a pressure derivative of 6(1). Intensity data were collected, and structures were refined at 0.00, 0.44, 1.22, 2.68, and 3.78 GPa. With increasing pressure, (1) the volumes of the TO4 tetrahedra do not vary, (2) the volume of the NaO7 polyhedron varies linearly with the volume of the unit cell and (3) Si-O-Si angles increase or remain constant, but only Al-O-Si angles decrease, which is consistent with the smaller force constant of the Al-O-Si vs. Si-O-Si angle. It is concluded that compression is accomplished through the bending of Al-O-Si angles, which squeezes together the chains of four-membered rings that run parallel to [001] and that are separated by zigzag channels containing Na atoms. -from Authors
- Finger, L., Hazen, R., Downs, R., Meng, R., & Chu, C. (1994). Crystal chemistry of HgBa2CaCu2O8+δ and HgBa2Ca2Cu3O8+δ single-crystal x-ray diffraction results. Physica C: Superconductivity and its applications, 226(3-4), 216-221.More infoAbstract: Crystal structures of HgBa2CaCu2O6+δ (Hg-1212) and HgBa2Ca2Cu3O8+δ (Hg-1223) have been determined on single crystals at room temperature conditions by X-ray diffraction techniques. Both tetragonal specimens are isomorphous with compounds in the Tl-Ba-Ca-Cu-O system. We observe excess oxygen in both samples on two sites near the z=0 plane: at ( 1 200) and close to ( 1 2, 0.4, 0) - but no oxygen at ( 1 2 1 20) as reported in earlier studies. Refined Hg occupancies for both samples indicate a substitution of Cu for Hg of more than ten percent. If the substitution of Cu for Hg and excess O at ( 1 200) are coupled, then the effect is to insert a perovskite-like six-coordinated copper site at (000). No evidence was found for a split atom at the barium sites in these compounds. © 1994.
- Hazen, R. M., Downs, R. T., Conrad, P. G., Finger, L. W., & Gasparik, T. (1994). Comparative compressibilities of majorite-type garnets. Physics & Chemistry of Minerals, 21(5), 344-349.More infoAbstract: Relative compressibilities of five silicate garnets were determined by single-crystal X-ray diffraction on crystals grouped in the same high-pressure mount. Bulk-moduli of the garnets range from 164.8 ± 2.3 GPa for calcium majorite to 191.5 ± 2.5 GPa for sodium majorite, assuming K′ = 4. Two factors, molar volume and octahedral cation valence, appear to control garnet compression. -from Authors
- Hazen, R. M., Downs, R. T., Finger, L. W., Conrad, P. G., & Gasparik, T. (1994). Crystal chemistry of Ca-bearing majorite. American Mineralogist, 79(5-6), 581-584.More infoAbstract: Single crystals of Ca-bearing majorite have been synthesized at 18.2 GPa and 2050°C. This sample is the first silicate garnet to display ordering on both octahedral and dodecahedral sites - behavior that may increase the compositional flexibility of garnet, affect element partitioning at high pressure, and stabilize the garnet structure in the transition zone and upper portion of the lower mantle. The garnet is tetragonal, and like MgSiO3 majorite, displays twinning by twofold rotation about [110]. -from Authors
- Nicoll, J. S., Gibbs, G. V., Boisen Jr., M. B., Downs, R. T., & Bartelmehs, K. L. (1994). Bond length and radii variations in fluoride and oxide molecules and crystals. Physics and Chemistry of Minerals, 20(8), 617-624.More infoAbstract: Molecular orbital calculations completed on fluoride molecules containing first and second row cations have generated bond lengths, R, that match those observed for coordinated polyhedra in crystals to within ∼0.04 Å, on average. The calculated bond lengths and those observed for fluoride crystals can be ranked with the expression R=Kp-0.22, where p=s/r, s is the Pauling strength of the bond, r is the row number of the cation and K=1.34. The exponent -0.22 (≈ -2/9) is the same as that observed for oxide, nitride and sulfide molecules and crystals. Bonded radii for the fluoride anion, obtained from theoretical electron density maps, increase linearly with bond length. Those calculated for the cations as well as for the fluoride anion match calculated promolecule radii to within ∼0.03 Å, on average, suggesting that the electron density distributions in the vicinity of the minima along the bond paths possess a significant atomic component despite bond type. Bonded radii for Si and O ions provided by experimental electron density maps measured for the oxides coesite, danburite and stishovite match those calculated for a series of monosilicic acid molecules. The resulting radii increase with bond length and coordination number with the radius of the oxide ion increasing at a faster rate than that of the Si cation. The oxide ion within danburite exhibits several distinct radii, ranging between 0.9 and 1.2 Å, rather than a single radius with each exhibiting a different radius along each of the nonequivalent bonds with B, Si and Ca. Promolecule radii calculated for the coordinated polyhedra in danburite match procrystal radii obtained in a structure analysis to within 0.002 Å. The close agreement between these two sets of radii and experimentally determined bonded radii lends credence to Slater's statement that the difference between the electron density distribution observed for a crystal and that calculated for a procrystal (IAM) model of the crystal "would be small and subtle, and very hard to determine by examination of the total charge density." © 1994 Springer-Verlag.
- Downs, R. T., Bartelmehs, K. L., Gibbs, G. V., & Boisen Jr, M. B. (1993). Interactive software for calculating and displaying X-ray or neutron powder diffractometer patterns of crystalline materials. American Mineralogist, 78(9-10), 1104-1107.More infoAbstract: Two computer programs, XPOW and XPOWPLOT, that generate and graph X-ray or neutron powder diffractometer patterns of crystalline materials are described. The input for XPOW requires only the radiation wavelength, cell dimensions, space group, and positional parameters for the atoms in the asymmetric unit. The output includes a listing of the d values, 2θ values, and the relative intensities for the nonequivalent Bragg reflections within a given 2θ interval. Using the XPOW output, the XPOWPLOT program creates menu-aided interactive color displays of up to five powder diffractometer patterns simultaneously on the PC monitor. -Authors
- Hazen, R. M., Downs, R. T., Finger, L. W., & Jaidong, K. o. (1993). Crystal chemistry of ferromagnesian silicate spinels: evidence for Mg-Si disorder. American Mineralogist, 78(11-12), 1320-1323.More infoAbstract: Single-crystal X-ray diffraction data have been obtained for synthetic Fe-Mg silicate spinels. The measurements on γ-Mg2SiO4 synthesized at 20 GPa and 1400°C, compared with previous data on a specimen synthesized at 22 GPa and 1000°C, provide evidence that approximately 4% of Sitot enters octahedral coordination in the sample synthesized at higher temperature. Fe-bearing silicate spinels synthesized at lower pressures display no evidence for Fe-Si disorder. -Authors
- Downs, R. T., Gibbs, G. V., Bartelmehs, K. L., & Boisen Jr, M. B. (1992). Variations of bond lengths and volumes of silicate tetrahedra with temperature. American Mineralogist, 77(7-8), 751-757.More infoAbstract: The apparent SiO bond lengths recorded for silicates over a range of temperatures either typically are invariant or exhibit a contraction with increasing temperature. A rigid-body thermal analysis was completed for the tetrahedra in nine silicates whose structures have been determined over a range of temperatures from 15 to 1250 K and whose tetrahedra seem to behave as rigid units. The bond lengths and volumes estimated for tetrahedra with four bridging O atoms seem to increase with temperature at a faster rate than those with four nonbridging O atoms. Those for tetrahedra with two or three nonbridging O atoms tend to increase an an intermediate rate. -from Authors
- Sterner, S., Chou, I., Downs, R. T., & Pitzer, K. S. (1992). Phase relations in the system NaCl-KCl-H2O: V. Thermodynamic-PTX analysis of solid-liquid equilibria at high temperatures and pressures. Geochimica et Cosmochimica Acta, 56(6), 2295-2309.More infoAbstract: The Gibbs energies of mixing for NaCl-KCl binary solids and liquids and solid-saturated NaCl-KCl-H2O ternary liquids were modeled using asymmetric Margules treatments. The coefficients of the expressions were calibrated using an extensive array of binary solvus and solidus data, and both binary and ternary liquidus data. Over the PTX range considered, the system exhibits complete liquid miscibility among all three components and extensive solid solution along the anhydrous binary. Solid-liquid and solid-solid phase equilibria were calculated by using the resulting equations and invoking the equality of chemical potentials of NaCl and KCl between appropriate phases at equilibrium. The equations reproduce the ternary liquidus and predict activity coefficients for NaCl and KCl components in the aqueous liquid under solid-saturation conditions between 673 and 1200 K from vapor saturation up to 5 kbar. In the NaCl-KCl anhydrous binary system, the equations describe phase equilibria and predict activity coefficients of the salt components for all stable compositions of solid and liquid phases between room temperature and 1200 K and from 1 bar to 5 kbar. © 1992.
- Gunter, M. E., & Downs, R. T. (1991). Drill: a computer program to aid in building ball and spoke crystal models. American Mineralogist, 76(1-2), 293-294.More infoAbstract: Drill is a Fortran computer program that, based upon crystal structure data, performs the required calculations to build ball and spoke models of crystals. Ball and spoke representations of minerals provide the best representation of the crystal structure of a mineral and are invaluable in teaching and research. Unfortunately, models of complex silicate minerals may contain 500 or more atoms (balls) and cost several thousand dollars. With the aid of Drill, an orienting ball drill, and a supply of plastic balls, these same models can be made at one-tenth, or less, the commercial price. Drill also generates all the atomic positions, bond lengths, and bond angles in the unit cell of a mineral, if given the cell parameters, the general positions of the atoms, and the space group. -Authors
- Boisen Jr, M. B., Gibbs, G. V., Downs, R. T., & D'Arco, P. (1990). The dependence of the SiO bond length on structural parameters in coesite, the silica polymorphs, and the clathrasils. American Mineralogist, 75(7-8), 748-754.More infoAbstract: Stepwise multiple regression analyses of the apparent R(SiO) bond lengths were completed for coesite and for the silica polymorphs together with the clathrasils as a function of the variables fs(O), P(pressure), fs(Si), B(O), and B(Si). The analysis of 94 bond-length data recorded for coesite indicates that all five of these variables make a significant contribution to the regression sum of squares with an R2 value of 0.84. A similar analysis of 245 R(SiO) data recorded for silica polymorphs and clathrasils indicate that only three of the variables (B(O), fs(O), and P) make a significant contribution to the regression with an R2 value of 0.90. -from Authors
- Downs, R. T., Gibbs, G. V., & Boisen Jr, M. B. (1990). A study of the mean-square displacement amplitudes of Si, Al and O atoms in framework structures: evidence for rigid bonds, order, twinning, and stacking faults. American Mineralogist, 75(11-12), 1253-1267.More infoAbstract: A study of the mean-square displacement amplitudes (MSDA) of the T(Al,Si) and O atoms in ordered framework silicates and aluminosilicates indicates that the sizes, shapes, and orientations of the displacement ellipsoids in these crystals are consistent with a rigid bond model. In particular, the MSDAs of the T and O atoms in the direction of the TO bonds tend to be equal and to increase with temperature. When this relationship does not hold, it appears to be related to the presence of static disorder. This observation leads to criteria that may be used to detect disorder in crystals. A simple rigid bond model is presented that, to a reasonable degree, describes the ellipticity and the orientation of the displacement ellipsoids of the bridging O atoms. -Authors
Poster Presentations
- Downs, R. T. (2013, April). Curiosity: Results from the Mars Science Laboratory. Arizona Senior Academy.
- Downs, R. T. (2013, February). Curiosity: Results from the Mars Science Laboratory. Tucson Gem and Mineral Show.
- Downs, R. T. (2013, November). Future directions in mineralogy and crystallography: Computational methods for crystal structure and education. National Science Foundation EarthCube End-User Domain Workshop for Rock Deformation and Mineral Physics Research.
- Morris, R., Ming, D., Blake, D., Vaniman, D., Bish, D., Chipera, S., Downs, R., Gellert, R., Treiman, A., Yen, A., Achilles, C., Anderson, R., Bristow, T., Crisp, J., Des, M. D., Farmer, J., Grotzinger, J., Leshin, L., McAdam, A., , Morookian, J., et al. (2013, Fall). The amorphous component in Martian basaltic soil in global perspective from MSL and MER missions. 44th Lunar and Planetary Science conference.More infoAbstract #1653
- Morrison, S., Downs, R., Blake, D., Bish, D., Ming, D., Morris, R., Yen, A., Chipera, S., Treiman, A., Vaniman, D., Gellert, R., Achilles, C., Rampe, E., Bristow, T., Crisp, J., Sarrazin, P., Farmer, J., Des, M. D., Stolper, E., , Morookian, J., et al. (2013, Fall). Crystal-chemical analysis of soil at Rocknest, Gale Crater. 44th Lunar and Planetary Science conference.More infoAbstract #1831
- Rampe, E., Bish, D., Chipera, S., Morris, R., Achilles, C., Ming, D., Blake, D., Anderson, R., Bristow, T., Crisp, J., Des, M. D., Downs, R., Farmer, J., Morookian, J., Morrison, S., Sarrazin, P., Spanovich, N., Stolper, E., Treiman, A., , Vaniman, D., et al. (2013, Fall). Detecting nanophase weathering products with CheMin: Reference intensity ratios of allophane, aluminosilicate Gel, and ferrihydrite. 44th Lunar and Planetary Science conference.
Others
- Downs, R. T., & Saposnik, A. (2013, July). UA Mineral Museum Receives Its Largest Donation of Minerals Ever. http://www.geo.arizona.edu/xtal/group/pdf/UA%20Mineral%20Museum%20Receives%20Its%20Largest%20Donation%20of%20Minerals%20Ever%20%20UANews.mht
- Downs, R. T., & Stolte, D. (2013, October). UA Student Finds 'Hawaiian Beach' Sand on Mars. http://www.geo.arizona.edu/xtal/group/pdf/UA%20Student%20Finds%20'Hawaiian%20Beach'%20Sand%20on%20Mars%20%20UANews.mht
- Downs, R. T. (2012, June). Mercury Mineral Evolution Tied to Supercontinent Assembly Over Last 3 Billion Year. Science Daily. http://www.geo.arizona.edu/xtal/group/pdf/Mercury%20mineral%20evolution%20tied%20to%20Supercontinent%20assembly%20over%20last%203%20billion%20years.mht
- Downs, R. T., & Stolte, D. (2012, August). Taking a robotic geologist to Mars. http://www.geo.arizona.edu/xtal/group/pdf/Taking%20a%20Robotic%20Geologist%20to%20Mars%20%20UANews.mht