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Analysis uncovers local and average Fe distribution in micas, indicating relationship between structure and absorption features.
In Fe-bearing trioctahedral micas Fe2+ occurs essentially in the octahedral O sheet whereas Fe3+ can either coexist with Fe2+ in the octahedral M sites or occur in tetrahedral T sites. Fe K-edge absorption spectra of twelve micas were recorded in an effort to relate their absorption features to the results of single-crystal X-ray structure refinements obtained on crystals from the same rock sample. This study involves the three end members phlogopite, annite and tetra-ferriphlogopite, used as reference materials, and nine micas intermediate in composition along the joins phlogopite-tetra-ferriphlogopite and phlogopite-annite. A detailed analysis of the XAS spectral features singles out the independent effects of coordination and valence shows and distinctly different position vs. intensity trends to be present that depend upon the local environment of Fe2+ and Fe3+. Bond distances evaluated from XANES spectra at the edge, or FMS, region agree well with those obtained by X-ray diffraction refinement. Distinct variations in the local to intermediate ranges of order of certain micas are determined by the analysis of XANES, or IMS, regions; they supplement well the information on their long range orders best obtained by X-ray refinement.
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Franca et al. (2002) studied this question.
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