In situ-X-ray study of perovskite (MgSiO (sub 3) ); phase transition and dissociation at mantle conditions

The dissociation reaction: MgSiO (sub 3) (perovskite) = MgO (periclase) + SiO (sub 2) (high pressure silica phase) was studied at 82 (3) GPa by in situ heating to temperatures between 300 and 1780 (50) K and X-ray diffraction. The orthorhombic perovskite changed to a pseudo-cubic phase between 1280 to 1485 K followed by a break down to the oxide mixture which was observed to grow at temperatures between 1600 to 1700 K; orthorhombic perovskite was recovered upon cooling at 1140 K. In yet another experiment, MgO and the silica phase grew from a sample of enstatite composition at 82 GPa and 1250 K. Compositionally, the instability of perovskite makes the oxide mixture as the most probable mantle material which can provide a variety of possible mantle compositions. Dynamically, the lighter SiO (sub 2) component would facilitate differentiation of the lower mantle.