Synthesis and crystal chemistry of Fe3+-bearing (Mg,Fe3+)(Si,Fe3+)O3 perovskite

We have synthesized magnesium-iron silicate perovskites with the general formula Mg1-xFex+y3+Si1-y O-3, in which the iron cation is exclusively trivalent. To investigate the crystal chemistry of Fe3+-bearing perovskite, six samples (both with and without Al) were analyzed using scanning electron microscopy, electron microprobe, X-ray diffraction, and Mossbauer spectroscopy. Results indicate that Fe3+ substitutes significantly into both the octahedral and dodecahedral sites in the orthorhombic perovskite structure, but prefers the octahedral site at Fe3+ concentrations between 0.04 and 0.05 Fe per formula unit, and the dodecahedral site at higher Fe3+ concentrations. We propose a model in which Fe3+ in the A/B site (in excess of that produced by charge coupled substitution) is accommodated by Mg/O vacancies. Hyperfine parameters refined from the Mossbauer spectra also indicate that a portion of dodecahedral sites undergo significant structural distortion. The presence of Fe3+ in the perovskite structure increases the unit-cell volume substantially compared to either the Mg end-member, or Fe2+-bearing perovskite, and the addition of Al did not significantly alter the volume. Implications for increased compressibility and a partially suppressed spin transition of Fe3+ in lower mantle perovskite are also discussed.