SILICATE PEROVSKITE FROM PHASE TRANSFORMATIONS OF PYROPE-GARNET AT HIGH PRESSURE AND TEMPERATURE

Both natural and synthetic pyrope have been compressed to pressures above 300 kbar in a diamond-anvil press and heated to temperatures above 800 degrees C by a continuous YAG laser. After quenching and release of pressure, X-ray diffraction shows that the natural pyrope transforms to a single phase with perovskite-like structure whereas the synthetic pyrope disproportionates into a mixture of MgSiO3 (perovskite) plus Al2O3 (corundum). The orthorhombic perovskite lattice parameters for the natural pyrope are a(o) = 4.816 +/- 0.004 angstrom, b(o) = 4.973 +/- 0.004 angstrom, and c(o) = 6.997 +/- 0.006 angstrom with Z = 4. The orthorhombic cell dimensions for MgSiO3 (perovskite) are ao a(o) = 4.818 +/- 0.005 angstrom, b(o) = 4.869 +/- 0.005 angstrom, and c(o) = 6.906 +/- 0.007 angstrom with Z = 4. The calculated density of MgSiO3 perovskite is thus 4.12 g/cm(3), or 3.7% denser than the isochemical mixed oxides. These experiments are the first demonstration of static high pressure phase transformations in silicate garnets and the first examples of a pure silicate existing in the perovskite structure.