Twinning in MgSiO3 Perovskite.

Crystals of MgSiO(3) perovskite synthesized at high pressures and temperatures have orthorhombic symmetry under ambient conditions. Examination by transmission electron microscopy shows that the microstructure of crystals synthesized at 26 gigapascals and 1600 degrees C is dominated by a large number of twin domains that are related by reflection operations with respect to {112} and {110} planes. These twins may be associated with the transformations of MgSiO(3) perovskite from the cubic to tetragonal and tetragonal to orthorhombic phases, respectively, upon decreasing pressure and temperature. These observations suggest that under the experimental synthesis conditions, and perhaps in the earth's lower mantle, the stable phase of MgSiO(3) might have the cubic perovskite structure.