Both forsterite (Mg2SiO4) and enstatite (MgSiO3) enter a post-oxide phase characterized by the orhtorhombic perovskite structure when subjected to high pressure and temperature in the diamond-anvil press coupled with laser heating. The lattice parameters for the perovskite phase of MgSiO3 are a0=4.790¿0.002, b0=4.943¿0.002, and c0=6.897¿0.003 A with Z=4. The calculated density of MgSiO3 (perovskite) is thus 4.083 g/cm3, or 2.8% denser than its isochemical mixed oxides with rocksalt and rutile structures. The density of a mixture of MgSiO3 (perovskite) plus MgO (periclase) is 1.9% greater than that of the mixed oxides with the forsterite stoichiometry.