PREDICTING IRON REDOX RATIOS IN CLINOAMPHIBOLES WITH X-RAY ABSORPTION SPECTROSCOPY

A set of 11 clinoamphibole standards with known Fe³⁺/ΣFe were analyzed for X-ray absorption spectra at the Advanced Photon Source at Argonne National Laboratory. Fe³⁺/ΣFe of each sample is known from Mössbauer data, and span from 0% to 100% Fe³⁺/ΣFe. Due to the extreme absorption anisotropy of crystals, we determined the orientation of clinoamphiboles crystals using single-crystal X-ray diffraction and spindle stage techniques. Clinoamphiboles crystal were then analyzed with the beam propagation and polarization axes along known direction relative to the crystallographic basis. Absorption anisotropy was then interpolated to predict the range of absorption magnitudes a clinoamphibole may exhibit at each energy. Partial least square fits for Fe³⁺/ΣFe indicate predictions are closest to the known Fe³⁺/ΣFe when comparing the same orientation of each sample in the dataset. Predictions including all orientations, while less accurate, still yield strong results with an R-squared of 0.9 and RMSE of 4.7. The results from this dataset demonstrate the efficacy of XAS as a microanalytical technique for crystalline materials, and functions as a reference dataset to research in oxybarometry.