Enstatite
The pyroxene silicate minerals enstatite (MgSiO3) and ferrosilite (FeSiO3) form a complete solid solution series and are common rock-forming minerals found in igneous and metamorphic rocks and meteorites.
The intermediate composition, (Mg,Fe)SiO3, is sometimes known as hypersthene although this name has been formally abandoned and replaced by enstatite or ferrosilite.
Weathered enstatite with a small amount of iron takes on a submetallic luster and a bronze-like color. This material is temed bronzite although it is more correctly called altered enstatite.
Most natural crystals are orthorhombic (space group Pbca) although three polymorphs are known. The high temperature, low pressure polymorphs are protoenstatite and proroferrosilite (also orthorhombic, space group Pbcn) while the low temperature forms, clinoenstatite and clinoferrosilite, are monoclinic (space group P21/c). Bronzite and hypersthene were known long before enstatite, which was first described by G. A. Kenngott in 1855. 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. In addition, bronzite is also sometimes used as a gemstone.
Identification
Enstatite and the other orthorhombic pyroxenes are distinguished from those of the monoclinic series by their optical characteristics, such as straight extinction, much weaker double refraction and stronger pleochroism. They also have a prismatic cleavage that is perfect in two directions at nearly 90 degrees. Enstatite is white, gray, greenish or brown in color; its hardness is 5 - 6, and its specific gravity is 3.2 - 3.3.
Occurrence
Crystals have been found in found in stony and iron meteorites, including one that 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 rare, the mineral being usually found as an essential constituent of igneous rocks, either as irregular masses in plutonic rocks (norite, peridotite, pyroxenite, etc.) and the serpentines that have resulted by their alteration, or as small idiomorphic crystals in volcanic rocks (trachyte, andesite).
It is also a common constituent of meteorites, forming with olivine the bulk of the material; here it often forms small spherical masses, or chondrules, with an internal radiated structure. References Deer, W. A., Howie, R. A., and Zussman, J. (1992). An introduction to the rock-forming minerals (2nd ed.). Harlow: Longman ISBN 0-582-30094-0
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