A NEW UNQUENCHABLE HIGH-PRESSURE POLYMORPH OF CA3AL2SI3O12

A perovskite structured new high-pressure polymorph of Ca3Al2Si3O12 grossular garnet was synthesized using a diamond anvil cell combined with laser heating at 30.2 GPa and about 1000-1500 degrees C. This high-pressure phase is unquenchable on release of pressure, and the in situ X-ray diffraction analysis indicates that the new phase is slightly distorted from a cubic symmetry. It can be indexed by a Pbnm orthorhombic structure which is isostructural with MgSiO3 perovskite. The bulk modulus calculated from the compression curve using the Birch-Murnaghan equation of state is 283 +/- 7 GPa, when its pressure derivative is fixed at four. The estimated zero-pressure density is 11% larger than that of grossular garnet. The volume difference between this phase and CaSiO3 perovskite is less than 1%, which indicates that, in perovskite structure, the substitution of two Al3+ for Ca2+ and Si4+ does not change the volume significantly.