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°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 disproportinates into a mixture of MgSiO3 (perovskite) plus Al2O3 (corundum). The orthorhombic perovskite lattice parameters for the natural pyrope are ao = 4.816 ± 0.004 Å, bo = 4.973 ± 0.004 Å, and co = 6.997 ± 0.006 Å with Z = 4. The orthorhombic cell dimensions for MgSiO3 (perovskite) are ao = 4.818 ± 0.005 Å, bo = 4.869 ± 0.005 Å, and co = 6.906 ± 0.007 Å with Z=4. The calculated density of MgSiO3 perovskite is thus 4.12 g/cm³, 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.