Effect of Chemistry on the Physical Properties of Perovskite and Post‐Perovskite

Abstract

We use density functional theory to study the effect of Fe and Al on properties of MgSiO 3 perovskite and post-perovskite. The addition of Fe increases the compressibility and density of MgSiO 3 and considerably decreases the transition pressure between the two phases. MgSiO 3 perovskite transforms to post-perovskite at about 112 GPa. FeSiO 3 is stable as post-perovskite at all pressures relative to perovskite. We find ferrous iron to be in a high spin state over the whole mantle pressure range, and it partitions preferentially into the post-perovskite structure. Ferrous iron in MgSiO 3 decreases the seismic wave velocities and slightly decreases the seismic anisotropy. At 120 GPa, FeSiO 3 post-perovskite has Vp= 12.4 km/s and Vs = 6.3 km/s and MgSiO 3 post-perovskite has Vp = 14.2 km/s and Vs = 7.9 km/s. The seismic anisotropy of post-perovskite MgSiO 3 is 15% for Vp and 26% for Vs. Aluminum slightly decreases the density and increases the transition pressure. Pure alumina transforms from perovskite to post-perovskite at 120 GPa. Al 2 O 3 also increases the compressibility of perovskite and decreases that of post-perovskite. Al decreases the seismic wave velocities and considerably increases the seismic anisotropy of post-perovskite. At 120 GPa, post-perovskite Al 2 O 3 has V p = 13.8 km/s and Vs = 7.4 km/s and seismic anisotropy of 18% for Vp and 43% for Vs. For proposed mantle compositions such as pyrolite the changes in seismic wave velocities due to the transition from perovskite to post-perovskite, that is positive jumps in both Vp and Vs, can explain those observed at the top of the D layer.