Silicate perovskites: A review

Since the first discovery of silicate perovskites at high pressures and high temperatures in the laboratory in 1974, silicate perovskites have probably become the most studied materials in the geophysical community during the past decade or so and it is nearly established that these silicates are the most abundant materials making up the bulk of the Earth. There are basically two groups of silicate perovskites. Ferromagnesian silicates with or without Al₂O₃ crystallizing in a common orthorhombic perovskite structure at high pressures and temperatures (HPT) are preservable at ambient conditions. Silicates of large cations such as Ca and Na crystallizing in an ideal cubic perovskite structure at HPT cannot be preserved at ambient conditions. Thus, the lattice parameters, crystal structure, thermal expansion and compressional data have been studied, both experimentally and theoretically, mainly for orthorhombic silicate perovskites, and for MgSiO₃ in particular. For MgSiO₃ perovskite, the recommended lattice parameters area=4.777±0.003,b=4.931±0.003 andc=6.899±0.004 Å; bulk modulusB ₀=2.4±0.2 Mbar; and volume thermal expansivity =(3±1)×10^–5 deg^–1 at ambient conditions. Cubic CaSiO₃ perovskite is probably less compressible than orthorhombic MgSiO₃ perovskite. The lattice parameters of MgSiO₃ perovskite increase linearly with increasing contents of both FeSiO₃ and Al₂O₃, forming limited solid solutions. The degree of distortion of orthorhombic silicate perovskites does not appear to change at HPT.