Synthesis, Elasticity, and Spin State of an Intermediate MgSiO3‐FeAlO3 Bridgmanite: Implications for Iron in Earth’s Lower Mantle

Abstract

Fe‐Al‐bearing bridgmanite may be the dominant host for ferric iron in Earth’s lower mantle. Here we report the synthesis of (Mg0.5Fe3+0.5)(Al0.5Si0.5)O3 bridgmanite (FA50) with the highest Fe3+‐Al3+ coupled substitution known to date. X‐ray diffraction measurements showed that at ambient conditions, the FA50 adopted the LiNbO3 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 V0 = 172.1(4) Å3, K0 = 229(4) GPa with K0′ = 4(fixed). The calculated bulk sound velocity of the FA50 bridgmanite is ~7.7% lower than MgSiO3 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 Fe3+‐Al3+ substitution. X‐ray emission and synchrotron Mössbauer spectroscopy measurements showed that after laser annealing, ~6% of Fe3+ cations exchanged with Al3+ and underwent the high‐ to low‐spin transition at 59 GPa. The low‐spin proportion of Fe3+ increased gradually with pressure and reached 17–31% at 80 GPa. Since the cation exchange and spin transition in this Fe3+‐Al3+‐enriched bridgmanite do not cause resolvable unit‐cell volume reduction, and the increase of low‐spin Fe3+ 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.Plain Language SummaryFe‐Al‐bearing bridgmanite may be the dominant mineral in the lower mantle, which occupies more than half of Earth’s volume. A subject of much debate is whether spin transition of Fe in bridgmanite produces an observable influence on the physics and chemistry of the lower mantle. In this study, we synthesized a new (Mg0.5Fe3+0.5)(Al0.5Si0.5)O3 bridgmanite with the highest Fe3+‐Al3+ coupled substitution known to date. We studied its structure, elasticity, and spin state by multiple experimental and theoretical methods. The high Fe content allowed us to better resolve a pressure‐induced spin transition of Fe3+ caused by Fe‐Al cation exchange at high temperature. Our results suggest that the spin transition is enabled by cation exchange but has a minor effect on the seismic velocity, although it may introduce chemical heterogeneity in the lower mantle. Our study helps resolve existing discrepancies on the nature of spin transition of Fe‐Al bridgmanite and its influence on the physics and chemistry of the lower mantle.Key PointsBridgmanite may contain 50% trivalent cations through Fe3+‐Al3+ coupled substitutionThe bulk sound velocity of (Mg0.5Fe3+0.5)(Al0.5Si0.5)O3 bridgmanite is 7.7% lower than MgSiO3Through Fe‐Al cation exchange, Fe3+ in (Mg0.5Fe3+0.5)(Al0.5Si0.5)O3 bridgmanite undergoes gradual spin transition at lower mantle conditions