An opposed anvil shear press was used to determine the mechanical properties and the polymorphism of MgSiO3 at normal pressures to 50 kb, at temperatures to 920°C, and at strain rates ranging from 10/sec to l0−4/sec. Experiments were made on natural single crystals of enstatite from Bamle, Norway, synthetic MgSiO3, and stoichiometric mixtures of MgO and SiO2. The natural enstatites have a shear strength of 14.9 kb at 50-kb normal pressure and 27°C. They decrease in strength to 5.5 kb at 40-kb normal pressure and 920°C. All specimens yielded clinoenstatite from orthorhombic-starting materials when they were sheared for longer than 1 sec. This suggests that shear stress greatly enlarges the stability field of clinoenstatite. Compression tests held for 24 hours at 80-kb normal pressure and 27°C yielded only orthoenstatite.

Specimens deformed at high pressures and temperatures exhibit kink bands with the slip system identified as T = (100), t = [001]. Although the volume change from orthorhombic to monoclinic MgSiO3 is small, it appears that the inversion is sufficiently large to produce seismic energy.

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