The Ti environment in natural hibonite: XANES spectroscopy and computer modelling

The local atomic structure around Ti in Ti-bearing hibonite (CaAl12O19) was studied using X-ray absorption near-edge structure (XANES) spectroscopy and computer modelling. Structural models of the direct substitution of Al by Ti3+, Al by Ti4+ charge balanced by the coupled substitution of Mg2+ for Al, and small Ti clusters were considered. The Ti K-XANES spectra of natural hibonite with different Ti concentration were recorded. Theoretical Ti K- XANES spectra for structural models of hibonite were calculated. It was shown that the theoretical Ti K-XANES spectra for a model with Ti at the five-coordinated M2 site are in agreement with the experimental XANES spectra of hibonite with low concentrations of Ti, while the theoretical spectra for a structural model of clustered Ti are in agreement with the experimental spectra of hibonite with higher Ti contents

Analysis of XANES spectra for tektites using machine learning algorithms

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The limitations of hibonite as a single-mineral oxybarometer for early solar system processes

The relationships between the composition of hibonite with the general formula CaAl12-2x-yMgxTi4 +xTi3 +yO19, the oxidation state of Ti (Ti3 +/ΣTi, where ΣTi = Ti3 + + Ti4 +), and oxygen fugacity (fO2) were investigated experimentally. It was found that hibonite can be synthesised with a range of Ti3 +/ΣTi values at constant fO2 and with a constant Ti3 +/ΣTi value for a range of fO2s. It was also found that if hibonite with the formula CaAl12-yTi3 +yO19 (Ti3 +/ΣTi = 1) is equilibrated with a melt of CAI composition at fO2s below the iron-wüstite buffer then the resulting hibonite contained Mg, with Mg per formula unit (pfu) ~ 0.8 Ti pfu, and Ti3 +/ΣTi ~ 0.2, irrespective of the fO2. These results suggest that the availability of Mg, rather than fO2, is the key factor that determines Ti3 +/ΣTi of hibonite. The structures of synthetic samples of hibonite with the general formula CaAl12-2xMgxTi4 +xO19, where 0 ≤ X < 1, were determined by Rietveld refinement of X-ray powder diffraction data. The predominant site occupied by Ti4 + was found to change from M2 to M4 with increasing Ti content. The range of Ti concentrations over which the site occupancy changed corresponds to that observed in meteoritic hibonite. This change in the Ti4 + site produces changes in the Ti K-edge XANES spectra, particularly in the intensity of the pre-edge feature, for constant Ti3 +/ΣTi. The observed dependence of the pre-edge on the Ti4 + site was reproduced by ab initio simulations of the XANES spectra. The XANES spectra of natural hibonite with variable Ti content from the Murchison carbonaceous chondrite closely match the spectra of the synthetic samples with similar Ti contents. These differences in the spectra of meteoritic hibonite could be misinterpreted as being due to changes in Ti3 +/ΣTi, but are instead due to differences in ΣTi, which relate to the petrogenetic history. Crystal chemistry exerts a first order control on the Ti site occupancy and Ti3 +/ΣTi value of hibonite. As a result, no simple relationship between Ti3 +/ΣTi and fO2 should be expected. It is unlikely that hibonite will be useful as an oxybarometer for solar processes without Ti3 +/ΣTi standards that are compositionally matched to the unknown

Understanding X-ray absorption spectra by means of descriptors and machine learning algorithms

X-ray absorption near-edge structure (XANES) spectra are the fingerprint of the local atomic and electronic structures around the absorbing atom. However, the quantitative analysis of these spectra is not straightforward. Even with the most recent advances in this area, for a given spectrum, it is not clear a priori which structural parameters can be refined and how uncertainties should be estimated. Here, we present an alternative concept for the analysis of XANES spectra, which is based on machine learning algorithms and establishes the relationship between intuitive descriptors of spectra, such as edge position, intensities, positions, and curvatures of minima and maxima on the one hand, and those related to the local atomic and electronic structure which are the coordination numbers, bond distances and angles and oxidation state on the other hand. This approach overcoms the problem of the systematic difference between theoretical and experimental spectra. Furthermore, the numerical relations can be expressed in analytical formulas providing a simple and fast tool to extract structural parameters based on the spectral shape. The methodology was successfully applied to experimental data for the multicomponent Fe:SiO2 system and reference iron compounds, demonstrating the high prediction quality for both the theoretical validation sets and experimental data.ISSN:2057-396