Phase relations in Mg₃Al₂Si₃ O₁₂ to 180 GPa: Effect of Al on post-perovskite phase transition

Phase relations in Mg₃Al₂Si₃ O₁₂ (MgSiO₃ + 25 mol% Al₂O₃; pyrope garnet composition) were investigated on the basis of in-situ synchrotron X-ray diffraction measurements at high-pressure and -temperature in a laser-heated diamond anvil cell (LHDAC). Results demonstrate that perovskite is solely stable up to 140 GPa and 2200 K, and perovskite and CaIrO₃-type post-perovskite phase (space group: Cmcm) coexist above 140 GPa and 2200 K. Post-perovskite is formed as a single phase above 150-170 GPa. Previous study has shown that pure MgSiO₃ perovskite transforms to post-perovskite phase above 125 GPa and 2500 K based on the same pressure standard. These results indicate that addition of Al₂O₃ expands the stability of MgSiO₃ perovskite relative to post-perovskite.