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

Spin and valence states of iron in single-crystal bridgmanite (Mg_0.89Fe_0.12Al_0.11Si_0.89O₃) 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.