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, (Mg,Fe)SiO
₃, 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).

Enstatite
General
Category	Inosilicate
Formula (repeating unit)	MgSiO3
Strunz classification	9.DA.05
Crystal system	Orthorhombic
Crystal class	Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m)
Space group	Pbca
Unit cell	a = 18.23, b = 8.84 c = 5.19 [Å]; Z = 8
Identification
Formula mass	100.387 g·mol^−1
Color	White, grey, green, yellow or brown - colorless in thin section.
Crystal habit	Prismatic crystals, commonly lamellar, fibrous, or massive
Twinning	Simple and lamellar on [100]
Cleavage	Good/distinct on [210]
Fracture	Uneven
Tenacity	Brittle
Mohs scale hardness	5 to 6
Luster	Vitreous, pearly on cleavage
Streak	Gray
Diaphaneity	Translucent to opaque
Specific gravity	3.2–3.3
Optical properties	Biaxial (+)
Refractive index	nα = 1.650–1.668; nβ = 1.652–1.673; nγ = 1.659–1.679
Birefringence	δ = 0.009–0.011
Pleochroism	Pale green to pale orange
2V angle	55–90°
References	[1][2][3]

Polymorphs and varieties

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.[4]

Bronzite variety from 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, 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.[4] They also have a prismatic cleavage that is perfect in two directions at 90 degrees. Enstatite is white, gray, greenish, or brown in color; its hardness is 5–6 on the Mohs scale, and its specific gravity is 3.2–3.3. This prismatic form is used in gemstones, and for academic purposes.

Occurrence

Isolated crystals are rare, but orthopyroxene is an essential constituent of various types of igneous rocks and metamorphic rocks. Magnesian orthopyroxene occurs in plutonic rocks such as gabbro (norite) and diorite. It may form small idiomorphic phenocrysts and also groundmass grains in volcanic rocks such as basalt, andesite, and dacite.

Enstatite, close to En₉₀Fs₁₀ in composition, is an essential mineral in typical peridotite and pyroxenite of the Earth's mantle. Xenoliths of peridotite are common in kimberlite and in some basalt. Measurements of the calcium, aluminum, and chromium contents of enstatite in these xenoliths have been crucial in reconstructing the depths from which the xenoliths were plucked by the ascending magmas.

Orthopyroxene is an important constituent of some metamorphic rocks such as granulite. Orthopyroxene near pure enstatite in composition occurs in some metamorphosed serpentines. 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 Kjørstad, near Brevik in southern Norway.[4]

Enstatite is a common mineral in meteorites. Crystals have been found in stony and iron meteorites, including one that fell at Breitenbach in the Ore Mountains, Bohemia. In some meteorites, together with olivine it forms the bulk of the material; it can occur in small spherical masses, or chondrules, with an internal radiated structure.[4]

Enstatite in Space

Enstatite is one of the few silicate minerals that have been observed in crystalline form outside the Solar System, particularly around evolved stars and planetary nebulae such as NGC 6302. Enstatite is thought to be one of the early stages for the formation of crystalline silicates in space and many correlations have been noted between the occurrence of the mineral and the structure of the object around which it has been observed.

Enstatite is thought to be a main component of the E-type asteroids;[5] in the Solar System, the main example are Hungaria asteroids.

See also

• Wollastonite – Single chain calcium inosilicate (CaSiO₃)
• Rhodonite – Single chain manganese inosilicate (MnSiO₃)

References

1. Handbook of Mineralogy
2. Mindat
3. Webmineral data
4.  One or more of the preceding sentences incorporates text from a publication now in the public domain: Spencer, Leonard James (1911). "Enstatite". In Chisholm, Hugh (ed.). Encyclopædia Britannica. 9 (11th ed.). Cambridge University Press. p. 654.
5. H. U. Keller, et all - E-Type Asteroid (2867) Steins as Imaged by OSIRIS on Board Rosetta - Science 8 January 2010: Vol. 327. no. 5962, pp. 190 - 193 doi:10.1126/science.1179559

• 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