Abstract
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.
Highlights
Hibonite (CaAl12O19) is a mineral found in calcium-aluminum-rich inclusions (CAIs) of chondritic meteorites
We present the experimental Ti K-X-ray absorption near-edge structure (XANES) spectra of natural hibonite with different Ti concentration and compare these to theoretical spectra simulated for the first time
Experimental Ti K-XANES spectra of natural hibonite samples with low (0.1 Ti per formula unit, pfu) and high Ti (0.55 Ti pfu) concentration are shown in Figures 1 and 2, respectively
Summary
Hibonite (CaAl12O19) is a mineral found in calcium-aluminum-rich inclusions (CAIs) of chondritic meteorites. CAIs are among the oldest materials in the Solar System [1], so, hibonite has the potential to record conditions that were present when the Solar nebula condensed to form the Solar System. Hibonite has potential to be used to study the oxygen fugacity (fO2) due to the incorporation of polyvalent elements such as Fe, V, Cr and Ti in its structure. The Fe2+/Fe3+ ratio can be used to determine the fO2 of terrestrial materials, but it is not suitable for CAIs because the highly reduced conditions of formation result in the presence of metallic iron. In the case of CAIs, the Ti3+/Ti4+ ratio may be an alternative means of determining fO2 The goal of the present study was to investigate the local atomic structure around Ti in Ti-bearing hibonite In the case of CAIs, the Ti3+/Ti4+ ratio may be an alternative means of determining fO2 (e.g. [2]).
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