Dyar, M. Darby - Valence State Measurements of Minerals using X-ray Absorption Spectroscopy

The oxidation state of igneous materials on a planet reflects the degree of oxidation of the magma source region and in some cases the additional effects of magma interaction with the near surface environment and the solar wind. We are developing techniques for pico- and nano-scale interrogation of primitive planetary materials to determine valence state of multivalent elements including Fe, Ti, and V using synchrotron micro-XAS spectroscopy, which makes use of the pre-edge, main edge, and EXAFS regions of absorption edge spectra. Our current work focuses on creating calibration data and protocols for determining valence states in isotropic phases (glass, maskelynite) and anisotropic minerals including olivine, pyroxene, and feldspars. In the latter group of phases, the polarized X-ray beam interacts with each crystal differently as a function of orientation (X-ray pleochroism). Moreover, XAS spectra of geologic samples have been shown to be sensitive to both the abundance of the species in each valence state and the geometry of the coordination polyhedra surrounding them.  Thus our calibrations must take into account all three variables that cause changes in the intensities of XAS features: optical orientation, valence state, and polyhedral distortion of each multivalent element site.
The key to accurate predictions of valence state lies in the use of multivariate analysis techniques, which take advantage of valuable predictive information not only in the major spectral peaks/features, but in any channel of the entire XAS spectrum. Algorithms for multivariate analysis that "learn" the characteristics of a data set as a function of varying sample characteristics as orientation can then be applied to the spectrum of an unknown. We show here the impressive results of prediction models from one such technique, partial least-squares regression. The models and the resultant predictions improve with each addition of new spectral calibration data, overcoming drawbacks imposed by orientation effects, variable composition, and differences in crystal structures as long as there are adequate calibration standards.