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 res
pect to the polarization of the synchrotron beam. Polished, thinned samples
were prepared with cleavages perpendicular to the plane of their thin sect
ions, 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 orientat
ion-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 routi
ne, in situ analyses of Fe3+/Sigma Fe on comparable scales to electron micr
oprobe analyses.