Precession x-ray photographs showed that single crystals of MgSiO 3 obtained from the Norton meteorite converted to proto-enstatite between 1050 ± 50°C and 1300°C (the upper limit of the apparatus). Diffractometer study of synthetic MgSiO 3 showed that proto-enstatite forms up to about 1440°C. Proto-enstatite, upon cooling from high temperature, converted into a variety of products ranging from twinned clino-enstatite through disordered structures to a product similar to ortho-enstatite, but with stacking faults. Rapid quenching produced clino-enstatite or disordered material closely related to it. Slow cooling through the range 800–1000°C sometimes produced partly disordered material similar to ortho-enstatite. From consideration of these and earlier results by many investigators on both natural and synthetic enstatites, it is concluded that ortho-enstatite is the form stable below 1000°C while proto-enstatite is stable above this temperature, and that clino-enstatite and the disordered enstatites are metastable forms, occurring because they grow faster than ortho-enstatite. Disordered enstatites similar to those produced by the cooling of protoenstatite have been found in the Norton County and Cumberland Falls meteorites. Twinned clino-enstatite was found in the Juvinas meteorite. The occurrence of such enstatites may give information on the temperature history of meteorites, though it is possible that shearing stress may cause similar structural disorder.