1911 Encyclopædia Britannica/Enstatite
ENSTATITE, a rock-forming mineral belonging to the group of orthorhombic pyroxenes. It is a magnesium metasilicate, MgSiO2, 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.). Bronzite and hypersthene were known long before enstatite, which was first described by G. A. Kenngott in 1855, and named from ἐνστάτης, "an opponent," because the mineral is almost infusible before the blowpipe: the material he described consisted of imperfect prismatic crystals, previously thought to be scapolite, from the serpentine of Mount Zdjar near Schönberg in Moravia. Crystals suitable for goniometric measurement were later to be found in the meteorite which fell at Breitenbach in the Erzgebirge, Bohemia. Large crystals, a foot in length and mostly altered to steatite, were found in 1874 in the apatite veins traversing mica-schist and hornblende-schist at the apatite mine of Kjörrestad, near Brevig in southern Norway. Isolated crystals are of rare occurrence, the minerable being usually found as an essential constituent of igneous rocks; either as irregular masses in plutonic rocks (norite, peridotite, pyroxenite, &c.) and the serpentines which have resulted by their alteration, or as small idiomorphic crystals in volcanic rocks (trachyte, andesite). It is also a common constituent of meteoric stones, forming with olivine the bulk of the material: here it often forms small spherical masses, or chondrules, with an internal radiated structure..
Enstatite and the other orthorhombic pyroxenes are distinguished from those of the monoclinic series by their optical characters, viz. straight extinction, much weaker double refraction and stronger pleochroism: they have prismatic cleavages (with an angle of 88º 16') as well as planes of parting parallel to the planes of symmetry in the prism-zone. Enstatite is white, greenish or brown in colour; its hardness is 51⁄2, and sp. gr. 3.2-3.3. (L. J. S.)