Synthesis and equation of state of perovskites in the (Mg, Fe)₃Al₂Si₃O₁₂ system to 177GPa

Natural and synthetic pyrope-almandine compositions from 38 to 100 mol% almandine (Alm38-Alm100) werestudied by synchrotron X-ray diffraction in the laser-heate ddiamond anvil cell to 177 GPa. Single-phase orthorhombic GdFeO₃-type perovskites were synthesized across the entire examined compositional range at deep lower mantle pressures, with higher Fe-contents requiring higher synthesis pressures. The formation of perovskite with Alm100 (Fe₃Al₂Si₃O₁₂) composition at 80 GPa marks the first observation of asilicateperovskiteinaFe end-member.Fe- enrichment broadens and lowers the pressurerange of the post-perovskite transition for intermediate compositions suchas Alm54, but the more Fe-richAlm100-composition perovskite remains stable to pressures as highas 149 GPa. Volume compression data for the Alm54 and Alm100 compositions were fit to the Birch-Murnaghan equation of state. The compressibility of perovskites synthesized from compositions long the pyrope-almandine join is not strongly sensitive to Fecontent. The compression curves were smooth over the entire measured range, and no evidence for a volume anomaly associated with a spin transition was observed.