Enstatite is a relatively common mineral and is found in metamorphic and some igneous rocks as well as in stony and iron meteorites. It forms a solid solution series with the minerals hypersthene and ferrosilite. A solid solution series occurs when two or more elements can substitute for each other in a crystal structure without much alteration of the structure. In this case, the elements are iron and magnesium and enstatite is the magnesium end member of the series. Hypersthene is the intermediate member with around 50% iron and ferrosilite is the iron rich end member of the series. Hypersthene is fairly common but ferrosilite is extremely rare. The iron deeply colors the minerals and therefore any deeply colored specimens of this series are usually called hypersthene. In fact the two most common members of the series are often considered together as enstatite-hypersthene in many mineral guides and texts.
Enstatite is an orthopyroxene or a pyroxene with an orthorhombic symmetry. At high temperatures, enstatite’s structure changes to a structure with a monoclinic symmetry, a clinopyroxene or more specifically, clinoenstatite. Clinoenstatite is a polymorph of enstatite, meaning that it has the same chemistry but a different structure.
Enstatite has a couple of ornamental and gemstone varieties. A weathered variety that has a submetallic luster and a bronze like color is called “bronzite”. It is sometimes used as an ornamental stone. Both enstatite and hypersthene contribute to the bronzite variety. An emerald green variety of enstatite is called chrome-enstatite and is cut as a gemstone. The green color is caused by traces of chromium, hence the varietal name.
Enstatite would form more commonly in igneous rocks if it were not for the abundance of iron in most magmas. As a result, enstatite is more commonly seen in highly metamorphosed rocks called granulites. It commonly forms from the alteration and de-watering of the amphibole mineral anthophyllite, (Mg, Fe)7Si8O22(OH)2.
Color is typically white, colorless, gray, light brown or pale green.
Luster is vitreous to pearly. Weathered specimens can have a submetallic luster (“bronzite”).
Transparency: Crystals are generally translucent and rarely transparent.
Crystal System is orthorhombic; 2/m 2/m 2/m
Crystal Habits include rare individual crystals that have a stubby prismatic habit. More typically massive or in coarse lamellar or fibrous aggregates.
Cleavage is perfect in two directions at nearly 90 degrees.
Fracture is conchoidal.
Hardness is 5 – 6.
Specific Gravity is approximately 3.2+ (slightly above average for translucent minerals)
Streak is white.
Other Characteristics: Index of refraction is approximately 1.65.
Associated Minerals include iron and stony meteorites, augite, feldspars and certain types of garnets.
Notable Occurrences include the Tilly Foster Mine, Brewster, New York; Boulder, Colorado; Jackson Co., North Carolina; Lancaster Co., Pennsylvania and Bare Hills, Maryland, USA as well as many localities from around the world especially in India, Burma and Tanzania.
Best Field Indicators are color, crystal habit, hardness, cleavage, index of refraction and luster.