Systematics in the structure and XANES spectra of pyroxenes, amphiboles, and micas as derived from oriented single crystals

In order to assess the extent of X-ray anisotropy in biaxial minerals, Fe XANES spectra were acquired on single crystals of pyroxenes, amphiboles, and micas with the X-ray beam polarized along the X, Y, and Z optical orientations. Specifically, single crystals of enstatite (En), augite (Aug), aegirine (Ae), kaersutite (Krs; three different compositions), dioctahedral (Ms and Ill), and trioctahedral (Ann and Phl) micas were selected. Each crystal was oriented morphologically and with the aid of EXCALIBR using a spindle stage equipped polarized light microscope, and their XANES spectra were acquired at beamline X26a, NSLS, using a special geometry of the beam to accommodate the goniometer head and a 20 X 30 p,m X-ray beam. As would be expected from visible and IR spectra of these minerals, the intensity of both pre-edge and main edge peaks is variable as a function of orientation. Structural similarities among these mineral groups result in similarities among their XANES spectra. Spectra acquired along the length of the chains (c for pyroxene, c for amphibole, and a for sheet silicates) are similar, with corresponding changes in the optical directions, such that Y-dioct approximate to Z(trioct) approximate to Z(Krs) approximate to Z(En) approximate to X-Ae (clinopyroxene is intermediate). Spectra taken along the b crystallographic axes (along the layer of octahedra, across the I-beam) are similar for all groups (Z(dioct) approximate to Y-trioct approximate to Y-Krs approximate to X-En approximate to Y-Aug), and more similar for the amphiboles and micas. Lastly, spectra acquired along the stacking direction, which is the a crystallographic direction for pyroxene and amphiboles and the c crystallographic direction in sheet silicates, are analogous. From these relationships, assignment of specific transitions to individual features in XANES spectra are facilitated. Furthermore, errors on use of pre-edge positions for determination of Fe3+/SigmaFe in minerals can be better constrained.