High‐spin Fe2+ and Fe3+ in single‐crystal aluminous bridgmanite in the lower mantle

Abstract

AbstractSpin and valence states of iron in single‐crystal bridgmanite (Mg0.89Fe0.12Al0.11Si0.89O3) are investigated using X‐ray emission and Mössbauer spectroscopies with laser annealing up to 115 GPa. The results show that Fe predominantly substitutes for Mg2+ in the pseudo‐dodecahedral A site, in which 80% of the iron is Fe3+ that enters the lattice via the charge‐coupled substitution with Al3+ in the octahedral B site. The total spin momentum and hyperfine parameters indicate that these ions remain in the high‐spin state with Fe2+ having extremely high quadrupole splitting due to lattice distortion. (Al,Fe)‐bearing bridgmanite is expected to contain mostly high‐spin, A‐site Fe3+, together with a smaller amount of A‐site Fe2+, that remains stable throughout the region. Even though the spin transition of B‐site Fe3+ in bridgmanite was reported to cause changes in its elasticity at high pressures, (Fe,Al)‐bearing bridgmanite with predominantly A‐site Fe will not exhibit elastic anomalies associated with the spin transition.