| Enstatite | |
|---|---|
 | |
| General | |
| Category | Inosilicate |
| Formula | MgSiO3 |
| IMA symbol | En[1] |
| 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 | Pink to green pleochroism diagnostic for enstatite, pale green to pale orange perpendicular to pink-green axis |
| 2V angle | 55–90° |
| References | [2][3][4] |
Enstatite is a mineral; the magnesium endmember of thepyroxenesilicate mineral series enstatite (MgSiO3) – ferrosilite (FeSiO3). The magnesium rich members of thesolid solution series are common rock-formingminerals found inigneous andmetamorphic rocks. The intermediate composition,(Mg,Fe)SiO
3, has historically been known ashypersthene, although this name has been formally abandoned and replaced by orthopyroxene. When determinedpetrographically or chemically the composition is given as relative proportions of enstatite (En) and ferrosilite (Fs) (e.g., En80Fs20).
Most naturalcrystals areorthorhombic (space group Pbca) although threepolymorphs 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, aremonoclinic (space group P21/c).
Weathered enstatite with a small amount of iron takes on a submetallicluster and a bronze-like color. This material is termedbronzite, although it is more correctly called altered enstatite.
Bronzite and hypersthene were known long before enstatite, which was first described byG. A. Kenngott in 1855.[5]
An emerald-green variety of enstatite is called chrome-enstatite and is cut as agemstone. The green color is caused by traces ofchromium, hence the varietal name. In addition, black, chatoyant hypersthene and brownish bronzite are also used as semi-precious gemstones.
Enstatite and the other orthorhombic pyroxenes are distinguished from those of the monoclinic series by their optical characteristics, such as straight extinction, much weakerdouble refraction and strongerpleochroism.[5] They also have a prismaticcleavage that is perfect in two directions at 90 degrees. Enstatite is white, gray, greenish, or brown in color; itshardness is 5–6 on theMohs scale, and itsspecific gravity is 3.2–3.3. This prismatic form is used in gemstones, and for academic purposes.
Isolated crystals are rare, but orthopyroxene is an essential constituent of various types ofigneous rocks andmetamorphic rocks. Magnesian orthopyroxene occurs inplutonic rocks such asgabbro (norite) anddiorite. It may form small idiomorphic phenocrysts and also groundmass grains in volcanic rocks such asbasalt,andesite, anddacite.
Enstatite, close to En90Fs10 in composition, is an essential mineral in typicalperidotite andpyroxenite of theEarth's mantle.Xenoliths of peridotite are common inkimberlite and in some basalt. Measurements of thecalcium,aluminum, andchromium 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 asgranulite. Orthopyroxene near pure enstatite in composition occurs in some metamorphosedserpentines. Large crystals, a foot in length and mostly altered tosteatite, were found in 1874 in theapatite veins traversingmica-schist andhornblende-schist at the apatite mine of Kjørstad, nearBrevik in southernNorway.[5]
Enstatite is a common mineral in meteorites.Crystals have been found in stony and ironmeteorites, including one that fell atBreitenbach in theOre Mountains,Bohemia. In some meteorites, together witholivine it forms the bulk of the material; it can occur in small spherical masses, orchondrules, with an internal radiated structure.[5]
Enstatite is one of the few silicate minerals that have been observed in crystalline form outside theSolar System, particularly around evolved stars andplanetary nebulae such asNGC 6302. Enstatite is thought to be one of the early stages for the formation of crystalline silicates in space. Many correlations have been noted between the occurrence of the mineral and the structure of the object around which it has been observed.[citation needed]
Enstatite is thought to be a main component of theE-type asteroids.[6] TheHungaria asteroids are the main examples in the Solar System.
A layer ofquartz and enstatite clouds above aniron cloud deck are thought to exist in the atmosphere of the youngbrown dwarf2M2224-0158.[7]
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One or more of the preceding sentences incorporates text from a publication now in thepublic domain: Spencer, Leonard James (1911). "Enstatite". InChisholm, Hugh (ed.).Encyclopædia Britannica. Vol. 9 (11th ed.). Cambridge University Press. p. 654.