Protoenstatite in MgSiO3 samples prepared by conventional solid state reaction

Enstatite (MgSiO3) sample was prepared by conventional solid state reaction at 1500 degrees C, followed by cooling to ambient temperature with different cooling rates (0.04-500 degrees C/s). Quantitative analysis of enstatite phases in the samples by Rietveld refinement and Si-29 MAS NMR revealed that a large amount of metastable protoenstatite (30-40 mol%) along with clinoenstatite was preserved in all samples. The presence of protoenstatite was also confirmed by micro-Raman spectroscopy. The crystal structures of protoenstatite and clinoenstatite at room temperature were refined using the Rietveld method, and were essentially the same as those reported by previous studies. As the solid state reaction method is commonly used to prepare synthetic clinoenstatite, the present result implies that the 'clinoenstatite' starting materials used in previous phase equilibrium studies of MgSiO3 pyroxene might contain overlooked protoenstatite, which could affect the interpretation for phase transition and thus those results should be reexamined.