ENSTATITE, a rock-forming mineral belonging to the group of orthorhombic pyroxenes. It is a magnesium metasilicate, often with a little iron replacing the magnesium. As the iron increases in amount there is a transition to bronzite (q.v.), and with still more iron to hypersthene (q.v.). Enstatite was first described by G. A. Kenngott in 1855, and named from ivorarns "an opponent," because the mineral is almost infusible before the blowpipe; the material he described consisted of imperfect pris matic crystals, previously thought to be scapolite, from the serpen tine of Mt. Zdjar near Schonberg, Moravia. Crystals suitable for goniometric measurement were later found in the meteorite which fell at Breitenbach, Bohemia. Large crystals, a foot in length and almost altered to steatite, were found in 1874 in the apatite veins transversing mica-schist and hornblende-schist at the apatite mine of Kjorrestad, near Brevig, Norway.
The compound obtained from dry melts of the composition is not enstatite, but a monoclinic mineral clinoenstatite known only in meteorites. Clinoenstatite appears to be the stable modification of at all temperatures below its incongruent melting point. It melts with decomposition at 1,557°C., with formation of forsterite. Enstatite itself above 1,26o°C., is con verted into clinoenstatite. It has been crystallized from melts with albite and sodium silicate and obtained as an unstable phase from glasses at a temperature of I,000°–I,Ioo°C. The limited miscibility of enstatite with diopside indicates that enstatite and clinoenstatite are distinct minerals, for the latter forms a complete series of solid solutions with the monoclinic pyroxene. Inter growths of enstatite and clinoenstatite are known in meteorites. The rhombic pyroxene alters in nature to a lamellar serpentine known as bastite. The chief mode of occurrence of the mineral is as a constituent of basic igneous rocks—norites, pyroxenites, and some peridotites, and in dolerites and basalts. (C. E. T.)