Hinako Uchida

An experimental study of the oxidation state of vanadium in spinel and basaltic melt with implications for the origin of planetary basalt

Abstract 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 DVspinel/melt. 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 DVspinel-melt, at constant temperature and oxygen fugacity. However, increased Ti content causes only a slight decrease in DVspinel-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 DVspinel/melt for basaltic systems. Application of these expressions to natural suites illustrate their utility and also the great range of DVspinel/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.

Single-crystal X-ray diffraction of spinels from the San Carlos Volcanic Field, Arizona: Spinel as a geothermometer

Abstract 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+ (Mg0.69Fe2+ 0.31 to Mg0.80Fe2+0.19). Although Group II spinels display consistently low Cr values, they also show a well-correlated substitution of Mg for Fe2+ (Mg0.63Fe2+ 0.37 to Mg0.69Fe2+0.31). 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.

Compressibility of synthetic potassium-rich clinopyroxene: In-situ high-pressure single-crystal X-ray study

Abstract The crystal structure of a synthetic potassium-rich clinopyroxene, (Ca0.88K0.12)(Mg0.83Al0.17)(Si1.98 Al0.02)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.

Closure temperatures of intracrystalline ordering in anatectic and metamorphic hercynite, Fe2+Al2O4

Abstract The closure temperature, TC, of the intracrystalline ordering of Mg-hercynite is estimated with a comparative crystal-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 TC 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 TC 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 TC, ~400 °C, was obtained for the slow cooled metamorphic hercynite from the oxygen fractional coordinates. In contrast, an unreasonably high value of TC, ~600 °C, was obtained from the inversion parameters. In the case of the fast cooled anatectic samples, TC calculated from the two structural parameters are comparable; the five crystals show a range in the calculated values for TC over ~250 °C, from ~700 to ~950 °C, which is reasonable considering the known diversity of cooling rates exhibited by their volcanic host-rocks.

Investigation of synthetic Mg1.3V1.7O4 spinel with MgO inclusions: Case study of a spinel with an apparently occupied interstitial site

Abstract 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(V3+0.711V4+0.109Mg0.180), 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(V3+1.368V4+0.316Mg0.316)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.

Building the American Mineralogist Crystal Structure Database: A recipe for construction of a small Internet database

Crystal structure data represent one of the most important resources for developing scientifi c 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.

Reinvestigation of eakerite, Ca2SnAl2Si6O18(OH)2·2H2O: H-atom positions by single-crystal X-ray diffraction and correlation with Raman spectroscopic data

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 Libowtzkys regression curve. The Sn atom has site symmetry \overline{1}.