Synthesis, Elasticity, and Spin State of an Intermediate MgSiO 3 ‐FeAlO 3 Bridgmanite: Implications for Iron in Earth's Lower Mantle

Fe‐Al‐bearing bridgmanite may be the dominant host for ferric iron in Earth's lower mantle. Here we report the synthesis of (Mg 0.5 Fe 3+ 0.5 )(Al 0.5 Si 0.5 )O 3 bridgmanite (FA50) with the highest Fe 3+ ‐Al 3+ coupled substitution known to date. X‐ray diffraction measurements showed that at ambient conditions, the FA50 adopted the LiNbO 3 structure. Upon compression at room temperature to 18 GPa, it transformed back into the bridgmanite structure, which remained stable up to 102 GPa and 2,600 K. Fitting Birch‐Murnaghan equation of state of FA50 bridgmanite yields V 0  = 172.1(4) Å 3 , K 0  = 229(4) GPa with K 0 ′ = 4(fixed). The calculated bulk sound velocity of the FA50 bridgmanite is ~7.7% lower than MgSiO 3 bridgmanite, mainly because the presence of ferric iron increases the unit‐cell mass by 15.5%. This difference likely represents the upper limit of sound velocity anomaly introduced by Fe 3+ ‐Al 3+ substitution. X‐ray emission and synchrotron Mössbauer spectroscopy measurements showed that after laser annealing, ~6% of Fe 3+ cations exchanged with Al 3+ and underwent the high‐ to low‐spin transition at 59 GPa. The low‐spin proportion of Fe 3+ increased gradually with pressure and reached 17–31% at 80 GPa. Since the cation exchange and spin transition in this Fe 3+ ‐Al 3+ ‐enriched bridgmanite do not cause resolvable unit‐cell volume reduction, and the increase of low‐spin Fe 3+ fraction with pressure occurs gradually, the spin transition would not produce a distinct seismic signature in the lower mantle. However, it may influence iron partitioning and isotopic fractionation, thus introducing chemical heterogeneity in the lower mantle.