The spin state of iron in Fe³⁺-bearing Mg-perovskite and its crystal chemistry at high pressure^†

Valence, spin states, and crystallographic sites of Fe in (Mg,Fe)SiO₃ perovskite were investigated using energy-domain ⁵⁷Fe-synchrotron Mössbauer spectroscopy and powder X-ray diffraction up to 86 GPa. The volumes of Fe³⁺-bearing perovskite in this study are slightly smaller than those of Mg end-member perovskite. Our Mössbauer data suggest that Fe³⁺ prefers A sites coupled with Mg vacancies, which is consistent with previous data at ambient conditions. Fe³⁺ in the A site remains in a high-spin state up to 86 GPa, and some fraction of the A site is occupied by Fe²⁺ at pressures above 30 GPa. Fe²⁺ in the A sites is also in a high-spin state up to 86 GPa. The coupled substitution from Mg²⁺ to a high-spin state of Fe³⁺ and Mg²⁺ vacancy would make the volume of perovskite smaller than that of Mg end-member perovskite. If the lower mantle is saturated in silica, perovskite containing high-spin Fe³⁺ in A site has a higher density. Such silica oversaturated regions could sink by the density difference.