MOSSBAUER AND ELNES SPECTROSCOPY OF (MG,FE)(SI,AL)O3 PEROVSKITE: A HIGHLY OXIDISED COMPONENT OF THE LOWER MANTLE

Abstract

(Mg,Fe)(Si,Al)O3 perovskite samples with varying Fe and Al concentration were synthesised at high pressure and temperature at varying conditions of oxygen fugacity using a multianvil press, and were characterised using ex situ X-ray diffraction, electron microprobe, Mossbauer spectroscopy and analytical transmission electron microscopy. The Fe3+/ΣFe ratio was determined from Mossbauer spectra recorded at 293 and 80 K, and shows a nearly linear dependence of Fe3+/ΣFe with Al composition of (Mg,Fe)(Si,Al)O3 perovskite. The Fe3+/ΣFe values were obtained for selected samples of (Mg,Fe)(Si,Al)O3 perovskite using electron energy-loss near-edge structure (ELNES) spectroscopy, and are in excellent agreement with Mossbauer data, demonstrating that Fe3+/ΣFe can be determined with a spatial resolution on the order of nm. Oxygen concentrations were determined by combining bulk chemical data with Fe3+/ΣFe data determined by Mossbauer spectroscopy, and show a significant concentration of oxygen vacancies in (Mg,Fe)(Si,Al)O3 perovskite.