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Abstract Anisotropic absorption in crystals is routinely observed in many spectroscopic methods and is recognized in visible light optics as pleochroism in crystalline materials. As with other spectrosco-pies, anisotropy in Fe K-edge X-ray absorption spectroscopy (XAS) can serve both as an indicator of the general structural arrangement and as a conundrum in quantifying the proportions of absorbers in crystals. In materials containing multiple absorbers, observed anisotropies can typically be represented by a linear relationship between measured spectroscopic peak intensities and relative absorber concentrations. In this study, oriented XAS analysis of pyroxenes demonstrates that the macroscopic theory that describes visible light absorption anisotropy of triaxially anisotropic materials can also be applied to X-ray absorption in pyroxenes, as long as the orientation and magnitude of the characteristic absorption vectors are known for each energy. Oriented single-crystal XAS analysis of pyroxenes also shows that the measured magnitude of characteristic absorption axes at a given orientation is energy-dependent and cannot be reproduced by linear combination of intermediate spectra. Although the macroscopic model describes a majority of the anisotropy, there is distinct discordance between the observed and interpolated spectra in the pre-edge between 7109 and 7115 eV, which is marked by spikes in RMSE/mean intensity ratio. Absorption indicatrices for samples analyzed in the visible and at X-ray wavelengths are modeled with a three-dimensional (3D) pedal surface, which functions as an empirical way of interpolating between the observed absorption data. This surface only requires a maximum of three coefficients, and results from the summation of 3D lemniscates. An absorption indicatrix model can be used to characterize anisotropic absorption in crystals and provides a way of comparing XAS spectra from randomly oriented crystals, such as those from polished sections, to a database of characterized crystals.
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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.
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- Award ID(s):
- 2042421
- PAR ID:
- 10507428
- Publisher / Repository:
- Geological Society of America
- Date Published:
- Format(s):
- Medium: X
- Location:
- Denver, CO
- Sponsoring Org:
- National Science Foundation
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