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Electron energy-loss spectroscopy of silicate perovskite-magnesiowüstite high-pressure assemblages

¹ Laboratoire de Physique des Solides, Université Paris Sud, Bâtiment 510, Orsay, France
² Laboratoire de Mineralogie-Cristallographie and Institut de Physique du Globe de Paris, 4, place Jussieu, 75252 Paris cedex 05, France
³ Laboratoire de Géochimie des Isotopes Stables, Institut de Physique du Globe de Paris, 4, place Jussieu, 75252 Paris cedex 05, France

Correspondence: ^* E-mail: guyot{at}lmcp.jussieu.fr

Silicate perovskite-magnesiowüstite 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 magnesiowüstite were collected at the Fe and Si L_2,3 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 magnesiowüstite. Spectra at the Si L_2,3 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 L_2,3 edge show that Fe²⁺ is the major form of Fe in olivine, ringwoodite, and magnesiowüstite, whereas Fe³⁺ is dominant in crystalline silicate perovskite and its amorphization products. In magnesiowüstite 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 magnesiowüstite. In magnesiowüstite, 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.

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