The compositions MgO·SiO₂ (enstatite) and 2MgO·SiO₂ (forsterite) were studied over the pressure range 20 to 130 kilobars at temperatures between 500° and 1,200°C. This pressure range corresponds with depths of about 75-400 km. which include part of the 200-900-km. seismic discontinuity zone in the upper mantle.

Orthoenstatite is the high-pressure high-temperature polymorph, and clinoenstatite is the high-pressure low-temperature polymorph. The equilibrium boundary for the orthoenstatite-clinoenstatite transition intersects the temperature axis at 540°C.; at 100 kb the equilibrium temperature is 870°C. At 115 kb and 600°C. clinoenstatite breaks down to forsterite plus stishovite. Forsterite is stable to at least 130 kb.

The experimental results: (1) support Birch's (1952) hypothesis that the inhomogeneous region in the upper mantle is due to pressure-dependent phase transformations; (2) confirm Ringwood's predictions that (a) enstatite breaks down to forsterite plus stishovite at about 120 kb, and (b) higher pressures are required for the forsterite-spinel inversion; (3) explain, when coupled with the results of Bowen and Tuttle (1949) on the system MgO--SiO₂--H₂O, the absence of clinoenstatite in terrestrial rocks and its occurrence in meteorites; and (4) explain the experimental results of Turner, Heard, and Griggs (1960) which show that orthoenstatite may be transformed to clinoenstatite by deformation; the latter exemplifies the Becke concept of shear-induced diaphthoresis.

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