Direct EXAFS evidence for incorporation of As5+ in the tetrahedral site of natural andraditic garnet

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doi: 10.2138/am.2007.2541
Authors:Charnock, J. M.; Polya, D. A.; Gault, A. G.; Wogelius, R. A.
Author Affiliations:Primary:
CCLRC Daresbury Laboratory, Warrington, United Kingdom
Other:
University of Manchester, United Kingdom
Volume Title:American Mineralogist
Source:American Mineralogist, 92(11-12), p.1856-1861. Publisher: Mineralogical Society of America, Washington, DC, United States. ISSN: 0003-004X
Publication Date:2007
Note:In English. 36 refs.; illus., incl. 4 tables
Summary:Exceptionally high-quality X-ray absorption spectroscopy (XAS) spectra of the As K-edge measured on a garnet from the Central Oslo Rift (Jamtveit et al. 1993) are presented and interpreted. The best fit to the extended X-ray absorption fine structure (EXAFS) spectrum indicates that arsenic occurs predominantly as As5+ in the tetrahedral Si4+ site. The first shell As-O bond lengths of 1.69 Å are identical to those observed for As5+-O in sodium arsenate and the edge position of 11872 eV is also characteristic of As5+. Determination of oxidized As within the structure places an important constraint on fluid chemistry, but perhaps even more importantly, EXAFS fitting of outer shells (As-Ca 3.07, 3.75, 5.71, 6.11 Å; As-Fe/Al 3.47, 5.35 Å; As-Si 3.88, 5.60, 5.91 Å) in this high-quality data set gives unequivocal evidence that AsO4-3 substitutes for SiO4-4 in the garnet tetrahedral site. This is, to the best of our knowledge, the first such XAS study yielding direct evidence, through outer shell fitting, for both the coordination environment and oxidation state of arsenic within (as opposed to sorbed onto) a silicate mineral. [R.A.H.]
Sections:Crystal structure; Geochemistry; Mineral data
Mineral Groups:Ortho-and ring silicates
Subjects:Andradite; Arsenic; Cell dimensions; Coordination; Crystal chemistry; Crystal structure; EXAFS data; Garnet group; Lattice parameters; Metals; Nesosilicates; Orthosilicates; Oxidation; Polyhedra; Silicates; Spectra; Tetrahedra; Valency; X-ray spectra; XAFS spectra; Europe; Norway; Oslo Graben; Scandinavia; Western Europe
Abstract Numbers:08M/0020
Record ID:2008014022
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Abstract, copyright, Mineralogical Society of America, Reference includes data from GeoScienceWorld, Alexandria, VA, United States
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