Chromium speciation in oxide-type compounds: application to minerals,gems, aqueous solutions and silicate glasses |
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| Authors: | François Farges |
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| Institution: | (1) USM 201, CNRS UMR 7202, Muséum National d’Histoire Naturelle, Paris, France;(2) Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305-2115, USA;(3) USM 201, Case Postale 52, Muséum National d’Histoire Naturelle, 61 rue Buffon, 75005 Paris, France |
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| Abstract: | Cr K-edge XANES spectra were obtained for a variety of Cr-bearing model compounds containing Cr(II), Cr(III), Cr(IV), Cr(V) and Cr(VI), in which the Cr-site symmetry is D4h, Oh and Td. The centroid position of the pre-edge feature is a better indicator of the Cr valence than the edge position. In Cr-rich oxides, higher-energy transitions must be excluded in order to refine a robust valence for Cr. The pre-edge for chromates is not unique and varies as a function of the CrO4 2? moiety distortion, which is often related to Cr-polymerization (monochromate vs. dichromate). Both the analogy with the Mn K-pre-edge information and ab initio FEFF calculations of the pre-edge feature for Cr(III) and Cr(VI) confirm the experimental trends. This methodology is applied to the Cr K-edge pre-edge feature collected in gems (emerald, spinel and ruby), the layered minerals fuchsite and kämmererite, two Cr-bearing aqueous solutions and a set of sodo-calcic silicate glasses used for bottling sparkling white wine. In emerald and fuchsite, the Cr-site is differently distorted than its ruby or spinel counterpart. In a Cr(III)-bearing aqueous solution and sodo-calcic glass, no evidence for Cr(III) with Td and C3v symmetry is detected. However, minor amounts of chromate moieties (most likely monomeric) are detected in a glass synthesized in air. Preliminary spectra for the wine bottle glass suggest that only trace amounts of chromates might possibly be present in these glasses. |
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| Keywords: | Cr XANES Emerald Ruby Fuchsite Spinel K?mmererite Solution Glass |
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