Small shear modulus of cubic CaSiO3 perovskite

Abstract

Ca-perovskite (CaPv) is considered to be one of the most abundant minerals in the Earth's lower mantle (LM). Furthermore, previous static calculations and mean-field theory suggest that it has a much larger shear modulus than bridgmanite (MgPv). In this study, the elasticity of cubic CaPv was reinvestigated using the density functional constant-temperature first principles molecular dynamics method under the correct conditions to simulate its elasticity. Our new results clearly demonstrate that cubic CaPv has comparable bulk and slightly smaller shear moduli than Fe-bearing MgPv. This is because the boundary condition for the supercell used in this study allows for the rotational phonon motion of SiO6 octahedra under strain, which predominantly affects the decrease in C11 and C44. Acoustic wave velocities determined from the elastic moduli indicate that cubic CaPv has slower velocities and larger densities than Fe-bearing MgPv and preliminary reference Earth model in the LM. This suggests that if CaPv-rich material exists, it can accumulate in the lowermost LM and produce a seismically low-velocity anomaly.