X-ray absorption near edge structure (XANES) study of the speciation of uranium and thorium in Al-rich CaSiO3 perovskite

Abstract

X-ray absorption spectroscopy was used to investigate the oxidation state of uranium in various U- and Th-bearing Al-rich CaSiO 3 perovskite samples synthesized at high-pressure and high-temperature using a multi-anvil press apparatus. X-ray absorption near edge spectroscopy (XANES) spectra collected at the U L III - and Th L III -edges using both micro- and macro-focused beams show U 4+ in the Al-rich CaSiO 3 perovskite. The structure of the U- and Th-bearing Al-rich CaSiO 3 perovskite samples have been cross-checked by XANES spectra collected at the Ca K -, Al K -, and Si K -edges. Al K and Si K spectra suggest that Al incorporates exclusively on the Si site of the CaSiO 3 perovskite. Ca K spectra of the (U,Th)-bearing Al-rich CaSiO 3 perovskite samples were succesfully compared to FEFF8.2 ab initio models of a tetragonal CaSiO 3 perovskite with space group P 4/ mmm . Our results confirm previous assumptions of the coupled substitution of CaSi 2 by UAl 2 in CaSiO 3 perovskite and that U and Th can be incorporated separately or together in CaSiO 3 perovskite by means of this mechanism. The possible occurrence of the U- and Th-bearing Al-rich CaSiO 3 perovskite are discussed as a potential candidate to locally host a large amount of actinides in the Earth’s deep mantle. The study of a phase that can act as a storage mineral for heat-producing actinide elements such as uranium and thorium is fundamental to the understanding of the geodynamics and thermal behavior of Earth.