ELECTRON ENERGY-LOSS SPECTROSCOPY OF SILICATE PEROVSKITE-MAGNESIOWUSTITE HIGH-PRESSURE ASSEMBLAGES

Abstract

Silicate perovskite-magnesiowustite assemblages synthesized from natural olivine in the multi-anvil press and diamond-anvil cell were studied by electron energy-loss spectroscopy (EELS). Spectra of crystalline silicate perovskite, and its post-amorphization phase, as well as magnesiowustite were collected at the Fe and Si L2,3 edge, and in the low loss (<50 eV) domain. The technique of line spectra ensuring very low beam doses allows good quality spectra to be collected from crystalline perovskite prior to amorphization and permits characterization of coexisting crystals of perovskite and magnesiowustite. Spectra at the Si L2,3 edge show that the beam-induced amorphization of silicate perovskite is accompanied by a change from sixfold to fourfold oxygen coordination of silicon atoms. Spectra at the Fe L2.3 edge show that Fe2+ is the major form of Fe in olivine, ringwoodite, and magnesiowustite, whereas Fe3+ is dominant in crystalline silicate perovskite and its amorphization products. In magnesiowustite and silicate perovskite observed in contact in these samples, Fe3+ is strongly partitioned into the silicate phase. Careful experimental substraction of zero-loss peak by off Bragg acquisition of electron energy-loss spectra allows good quality low loss spectra to be collected from crystalline silicate perovskite and magnesiowustite. In magnesiowustite, interband transitions are well characterized, leading to a measured gap of 7.8 eV, in agreement with previous theoretical calculations. Interband transitions at 10 eV and 12.5 eV are also well resolved in crystalline silicate perovskite, leading to a gap of about 9.5 eV.