Fe XANES spectra of iron-rich micas

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+/Sigma 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 mum 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 (similar to +/- 10 %) in determinations of Fe3+/Sigma 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+/Sigma Fe on comparable scales to electron microprobe analyses.