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

Natural and synthetic pyrope-almandine compositions from 38 to 100 mol% almandine (Alm38-Alm100) were studied by synchrotron X-ray diffraction in the laser-heated diamond 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 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 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 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. (C) 2012 Elsevier B.V. All rights reserved.

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