HIGH PRESSURE DECOMPOSITION OF gamma -Fe₂SiO₄.

The high pressure and high temperature runs are made on gamma -Fe//2SiO//4 by means of MASS-318, and the interpretation of the run products reveals the phase change sequence - spinel yields rocksalt plus perovskite-like yields rocksalt plus rutile, at relatively low temperatures. The perovskite-like phase is a new hexagonal form of FeSiO//3 presumably isostructural with the high-temperature form of BaTiO//3 with a (abcacb) type stacking mode along a hexagonal c-axis. At high temperatures above 2000 degree C however, gamma -Fe//2SiO//4 may decompose directly to the isochemical oxide mixture. A geophysical implication of the present results is discussed in connection with the constitution of the earth's interior. The correlation of the seismological transition layers in the upper mantle to the phase changes in (Mg,Fe)//2SiO//4 is not so simple as having been interpreted.