Synthesis and equation of state of post-perovskites in the (Mg,Fe)₃Al₂Si₃O₁₂ system

The formation and properties of the post-perovskite (CaIrO₃-type) phase were studied in Fe-rich compositions along the pyrope-almandine ((Mg,Fe)₃Al₂Si₃O₁₂) join. Natural and synthetic garnet starting materials with almandine fractions from 38 to 90mol% were studied using synchrotron X-ray diffraction in the laser-heated diamond anvil cell. Single-phase post-perovskite could be successfully synthesized from garnet compositions at pressures above 148GPa and temperatures higher than 1600K. In some cases, evidence for a minor amount of Al₂O₃ post-perovskite was observed for Alm38 and Alm54 compositions in the perovskite+post-perovskite two-phase region. Pressure-volume data for the post-perovskite phases collected during decompression show that incorporation of Fe leads to a systematic increase of unit cell volume broadly similar to the variation observed in the (Mg,Fe)SiO₃ system. The presence of Al₂O₃ increases the stability of perovskite relative to post-perovskite, requiring higher pressures (>148GPa) for synthesis of pure post-perovskites. Our data together with those of Tateno et al. (2005) also suggest that in the Al-rich system the presence of Fe has no strong effect on the pressure required to synthesize the pure post-perovskite phase, but the two-phase perovskite and post-perovskite region may be broad and its width dependent on Fe content. Our results suggest that any regions highly enriched in Al₂O₃ may consist of either the perovskite phase or a mixture of perovskite and post-perovskite phases throughout the entire thickness of the D″ region. The observed synthesis pressures (>148GPa) for a pure post-perovskite phase are beyond that at the Earth's core-mantle boundary (~135GPa).