Enstatite

Enstatite is a mineral; the magnesium endmember of the pyroxene silicate mineral series enstatite (MgSiO₃) – ferrosilite (FeSiO₃). The magnesium rich members of the solid solution series are common rock-forming minerals found in igneous and metamorphic rocks. The intermediate composition, , has historically been known as hypersthene, although this name has been formally abandoned and replaced by orthopyroxene. When determined petrographically or chemically the composition is given as relative proportions of enstatite (En) and ferrosilite (Fs) (e.g., En₈₀Fs₂₀).

Polymorphs and varieties

thumb|left|Gem quality enstatite from Myanmar (size: 2.4×1.0×0.8 cm)

Most natural crystals are orthorhombic (space group Pbca) although three polymorphs are known. The high temperature, low pressure polymorphs are protoenstatite and protoferrosilite (also orthorhombic, space group Pbcn) while the low temperature forms, clinoenstatite and clinoferrosilite, are monoclinic (space group P2₁/c).

Weathered enstatite with a small amount of iron takes on a submetallic luster and a bronze-like color. This material is termed bronzite, although it is more correctly called altered enstatite.

Bronzite and hypersthene were known long before enstatite, which was first described by G. A. Kenngott in 1855.

thumb|Bronzite variety from [[Bare Hills, Maryland|Bare Hills, Baltimore County, Maryland, USA (size: 9.6×7.5×4.9 cm)]]

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, black, chatoyant hypersthene and brownish bronzite are also used as semi-precious gemstones.

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. The Hungaria asteroids are the main examples in the Solar System.

A layer of quartz and enstatite clouds above an iron cloud deck are thought to exist in the atmosphere of the young brown dwarf 2M2224-0158.

See also

References

• Deer, W. A., Howie, R. A., and Zussman, J. (1992). An introduction to the rock-forming minerals (2nd ed.). Harlow: Longman