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 SmA103. 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 ø rotation twins about the [001] axis. Twin-domain morphology of MgSiO 3 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 1600øC contain significantly higher twin densities than those quenched from below 1300øC, 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.
Fe)SiO3 perovskite, X ray diffraction studies indicate thatthe orthorhombic distortion increases with pressure and decreases with temperature [Kudoh et al., 1987; Ross and Hazen, 1989, 1990; Mao et al., 1991]. All of these measurements on MgSiO3 perovskite, however, were carried out under conditions where the silicate perovskite is out of its stability field and it has been found that single crystals were twinned and polycrystals degraded at moderately high temperatures [Knittle et al., 1986; Ross and Hazen, 1989; Parise et al., 1990]. In their study on elastic constants using Brillouin spectroscopy, Yeganeh-Haeri et al., [1989a, b] found that crystals were also twinned under high-energy laser beam and suggested that this twinning is evidence of the existence of a high symmetry phase for the silicate perovskite at elevated temperatures. Various theoretical approaches have been developed in order to predict the behavior of the silicate perovskite under lower mantle pressure and temperature conditions [e.g., Wolf and Bukowinski, 1985, 1987; Wall et al., 1986; Matsui 12,327 12,328 WANG ET AL.: ELECTRON MICROSCOPY
