Stability Relations of Magnesium Metasilicate Polymorphs

The observed stability relations of enstatite, protoenstatite, and clinoenstatite were in good agreement with the findings of earlier workers. Polycrystalline preparations of MgSiO 3 polymorphs were synthesized with the aid of small additions of MgF 2 or LiF as mineralizers. At 1042°± 3°C, enstatite, the stable low‐temperature modification, inverts to protoenstatite, the stable high‐temperature modification. The metastable transition between protoenstatite and clinoenstatite occurs at 865°± 5°C, the rate of transformation of polycrystalline protoenstatite to clinoenstatite being dependent on type and amount of mineralizer used, degree of compaction of sample, firing temperature, rate of cooling, degree of grinding at room temperature, and time at room temperature. Under certain conditions, a 1 to 2 mole % substitution of Mn 2+ for Mg 2+ inhibits the inversion of protoenstatite to either stable enstatite or metastable clinoenstatite below about 1042°C. Metastable clinoenstatite was never found to invert to the stable enstatite modification below the metastable inversion temperature of clinoenstatite to protoenstatite at 865°C. Above this temperature but below 1042°C, it inverts to protoenstatite which in turn inverts to enstatite. The aggregate linear thermal expansion coefficients of enstatite, protoenstatite, and clinoenstatite between 300° and 700° C were respectively 120 × 10 −7 , 98 × 10 −7 , and 135 × 10 −7 °C −1 .