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1 Dept. of Earth and Environment, Mount Holyoke College, 50 College St., South Hadley, MA 01075, U.S.A.
2 Department of Geological Sciences, Wright Geological Laboratory, Busch Campus, Rutgers University, Piscataway, NJ, 08854, U.S.A.
3 GSECARS and Department of Geophysics, University of Chicago, 5640 S. Ellis Ave., Chicago, IL 60637, U.S.A.
* E-mail: Mddyar{at}amherst.edu
Fe-rich mica crystals have been studied using microbeam XANES spectroscopy of the Fe K edges in order to evaluate the dependence of XANES features and resultant Fe3+/
Fe values on the orientation of the crystals with respect to the polarization of the synchrotron beam. Polished, thinned samples were prepared with cleavages perpendicular to the plane of their thin sections, and these sections were then rotated relative to the horizontal plane of the ring and the 10 x 15 µm beam. Results show that both pre-edge and main-edge features in these micas exhibit orientation-dependent changes in both peak intensity and energy. These shifts constitute the majority of the error (
± 10%) in determinations of Fe3+/Fe by pre-edge energy, a problem that will be overcome in future work through techniques for quantification of grain orientations. Results show distinctive orientation-dependent features in the main edge region, from which it will soon be possible to deconvolute contributions from multiple scattering interactions to obtain information about the electronic properties of micas. Data also demonstrate the exciting potential of XANES pre-edge spectroscopy for routine, in situ analyses of Fe3+/Fe on comparable scales to electron microprobe analyses.
Key-words: micas, XANES, iron, XAS, pre-edge..
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