ENSTATITE  , a

rock-forming mineral belonging to the group of orthorhombic pyroxenes . It is a magnesium metasilicate, MgSiO3, 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 ivgTaTi S, " an opponent," because the mineral is almost in-fusible before the blowpipe: the material he described consisted of imperfect prismatic crystals, previously thought to be scapolite, from the serpentine of Mount Zdjar near Schonberg in Moravia . Crystals suitable for goniometric measurement were later found in the meteorite which fell at Breitenhach 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 Kjorrestad, near Brevig in southern Norway . Isolated crystals are of rare occurrence, the mineral 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 S8° 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, and sp. gr . 3.2—3.3 .

(L . J .