Electron microscopy of (Mg, Fe)SiO3 Perovskite: Evidence for structural phase transitions and implications for the lower mantle

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Abstract
A transmission electron microscopy study has been carried out on twin‐domain structure in (Mg, Fe)SiO3 perovskite and in five analogue perovskites: CaTiO3, CaGeO3, MnGeO3, LaGaO3, and SmAlO3. Three crystallographically distinct twin laws are found in all of these GdFeO3‐type (space group Pbnm) perovskites: reflection twins across the {112} and {110} planes, and 90° rotation twins about the [001] axis. Twin‐domain morphology of MgSiO3 perovskite is examined as a function of the temperatures from which the specimens were quenched in high‐pressure synthesis experiments. Crystals quenched from temperatures exceeding 1600°C contain significantly higher twin densities than those quenched from below 1300°C, suggesting that structural phase transitions may have taken place in high‐temperature MgSiO3 perovskite during quenching. On the basis of theoretically predicted twin laws and of the twin‐domain morphology observations on analog perovskites that undergo a variety of phase transitions, it appears that the most likely phase transitions in the silicate perovskite are (with decreasing temperature) cubic‐tetragonal‐orthorhombic. These results support our earlier suggestion that under the experimental synthesis conditions, and perhaps in certain regions of the Earth's lower mantle, the stable phase of MgSiO3 may have the cubic perovskite structure and that structural phase transitions may occur in the lower mantle.