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

Abstract

Natural and synthetic pyrope–almandine compositions from 38 to 100mol% almandine (Alm38–Alm100) were studied by synchrotron X-ray diffraction in the laser-heated diamond anvil cell to 177GPa. 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 80GPa marks the first observation of a silicate perovskite in a Fe end-member. Fe-enrichment broadens and lowers the pressure range of the post-perovskite transition for intermediate compositions such as Alm54, but the more Fe-rich Alm100-composition perovskite remains stable to pressures as high as 149GPa. 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 along the pyrope–almandine join is not strongly sensitive to Fe-content. The compression curves were smooth over the entire measured range, and no evidence for a volume anomaly associated with a spin transition was observed.