High-Pressure and High-Temperature in situ X-ray Diffraction Study of Natural Garnet (Mg_{0. 72}Fe_{0. l7}Ca_{0. 11}) ₃Al₂Si₃O₁₂ up to 32 GPa and 2000 K Using MA8-Type Apparatus

抄録

Breakdown reaction of natural garnet was studied by an in situ x-ray observation using an MA8-type multi-anvil apparatus up to about 32 GPa. Natural garnet with a composition (Mg_{0. 72}Fe_{0. l7}Ca_{0. 11}) ₃Al₂Si₃O₁₂ breaks down at a pressure between 24 and 27 GPa to a mixture of sub-aluminous orthorhombic silicate perovskite, stishovite, Al-rich phase with unknown structure, and possibly CaSiO₃ perovskite. The unreacted garnet was observed even after heating the sample at 31. 7 GPa and 2000 K for 6000 sec. This result implies that the transition rate of the garnet is lower than that of Al-free system such as olivine and pyroxene.