Crystal chemistry and compressibility of Fe0.5Mg0.5Al0.5Si0.5O3 and FeMg0.5Si0.5O3 silicate perovskites at pressures up to 95 GPa
Résumé
Silicate perovskite, with the mineral name bridgmanite, is the most abundant mineral in the Earth’s lower mantle. We investigated crystal structures and equations of state of two perovskite-type Fe 3+ -rich phases, FeMg 0.5 Si 0.5 O 3 and Fe 0.5 Mg 0.5 Al 0.5 Si 0.5 O 3 , at high pressures, employing single-crystal X-ray diffraction and synchrotron Mössbauer spectroscopy. We solved their crystal structures at high pressures and found that the FeMg 0.5 Si 0.5 O 3 phase adopts a novel monoclinic double-perovskite structure with the space group of P21/n at pressures above 12 GPa, whereas the Fe 0.5 Mg 0.5 Al 0.5 Si 0.5 O 3 phase adopts an orthorhombic perovskite structure with the space group of Pnma at pressures above 8 GPa. The pressure induces an iron spin transition for Fe 3+ in a (Fe 0.7 ,Mg 0.3 )O 6 octahedral site of the FeMg 0.5 Si 0.5 O 3 phase at pressures higher than 40 GPa. No iron spin transition was observed for the Fe 0.5 Mg 0.5 Al 0.5 Si 0.5 O 3 phase as all Fe 3+ ions are located in bicapped prism sites, which have larger volumes than an octahedral site of (Al 0.5 ,Si 0.5 )O 6 .
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