Phase transformations of enstatite were in situ studied using high-temperature transmission electron microscopy (HTTEM). Both orthoenstatite and low-clinoenstatite did not transform to protoenstatite which is the stable high-tempera-ture phase. Instead, low-clinoenstatite transformed to high-clinoenstatite athermal-martensitically in the same manner as the high-low inversion in pigeonite. Orthoenstatite also partly transformed to high-clinoenstatite. The present HTTEM study is the first report of the latter type phase transformation with the MgSiO₃ composition. The reason why the high-clinoenstatite appears in place of the protoenstatite at high temperatures in the HTTEM study is discussed.

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