USE OF THE SPINDLE STAGE FOR ORIENTATION OF SINGLE CRYSTALS FOR FE-XANES SPECTROSCOPY

Microfocused x-ray absorption near-edge spectroscopy (XANES) is a promising technology for measuring Fe³⁺at ca. 10 um scales on standard thin sections. It uses synchrotron radiation to probe the energy of the Fe Ka absorption edge and its pre-edge, known to be very sensitive to oxidation state. Widespread use of the micro-XANES technique for redox determinations has been hampered by the confirmation of the effects of x-ray “pleochroism.” Differential interactions of the plane-polarized x-ray beam with anisotropic crystals cause ±15-20% errors for Fe³⁺/ΣFe measurements based on pre-edge spectra of randomly-oriented crystals. These problems can be overcome by use of a spindle stage, as pioneered by Donald Bloss, to permit analysis of standards with the vibration direction of the x-ray beam oriented parallel to the optical orientation directions in single crystals.

To make these measurements, we use a modified spindle stage and an X-ray goniometer head to hold a crystal and the glass fiber on which it is mounted. The orientation of the crystallographic axes within the crystal can be determined by X-ray diffraction in all cases or with the use of the polarizing light microscope if the minerals are biaxial. Once an optical orientation direction has been located, it can be oriented parallel to the polarization direction of the synchrotron source for spectral acquisition.

XANES spectra have been acquired from single crystals oriented with the polarization direction of the synchrotron beam parallel to the X, Y, and Z optical orientation directions. These data provide end-members of spectra for samples with extremes of Fe³⁺/ΣFe and orientation. Data were analyzed using partial least-squares regression (PLS) to predict Fe³⁺/ΣFe using data from the pre-edge to the EXAFS regions. Results show that PLS analysis of the entire XANES spectral region yields the most accurate predictions of Fe³⁺ with both robustness and generalizability. Future use of XANES for evaluating oxidation state variations in anisotropic materials will depend on availability of oriented mineral group-specific standards with known redox states and on use of multivariate analyses to take advantage of all the information contained in the spectra.