The Polymorphism of MgSiO3and Solid-State Equilibria in the System MgsiO3-CaMgSi2O6

The investigation is divided into two parts: the first is concerned primarily with stability relations among the various crystalline phases of MgSiO₃. Contrary to results given in previous published investigations, experimental evidence favors the stability of rhombic enstatite and protoenstatite. Clinoenstatite is found to be a metastable low-temperature phase formed during rapid cooling of protoenstatite. The latter modification is the stable form above 990° C; rhombic enstatite is stable below that temperature. A preliminary structure analysis of protoenstatite and clinoenstatite is presented. X-ray data lead to the conclusion that protoenstatite is also orthorhombic. Phase transitions appear to be attended by a one-dimensional stacking disorder in which intermediate states are unstable. The second part of the investigation is concerned with subsolidus phase equilibria in the system MgsiO₃-CaMgSiO₃. Evidence is presented which corroborates Hess's findings that two monoclinic pyroxene phases may coexist in equilibrium. At lower temperatures a monoclinic diopside solid solution is in equilibrium successively with protoenstatite and rhombic enstatite solid solutions.