Phase relations in Mg 3 Al 2 Si 3 O 12 to 180 GPa: Effect of Al on post‐perovskite phase transition

Phase relations in Mg 3 Al 2 Si 3 O 12 (MgSiO 3 + 25 mol% Al 2 O 3 ; 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 CaIrO 3 ‐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 MgSiO 3 perovskite transforms to post‐perovskite phase above 125 GPa and 2500 K based on the same pressure standard. These results indicate that addition of Al 2 O 3 expands the stability of MgSiO 3 perovskite relative to post‐perovskite.