View source for Enstatite

{{short description|Pyroxene: magnesium-iron silicate with MgSiO<sub>3</sub> and FeSiO<sub>3</sub> end-members}}
{{for|E-type chondrites|Enstatite chondrite}}
{{Infobox mineral
| name        = Enstatite
| category    = [[Inosilicate]]
| image       = Enstatite-202036.jpg
| imagesize   = 260px
| caption     = 
| formula     = MgSiO<sub>3</sub>
| IMAsymbol   = En<ref>{{Cite journal|last=Warr|first=L.N.|date=2021|title=IMA–CNMNC approved mineral symbols|journal=Mineralogical Magazine|volume=85|issue=3|pages=291–320|doi=10.1180/mgm.2021.43|bibcode=2021MinM...85..291W|s2cid=235729616|doi-access=free}}</ref>
| molweight   = {{chem molar mass|Mg=1|Si=1|O=3}}
| strunz      = 9.DA.05
| system      = [[Orthorhombic]]
| class       = Dipyramidal (mmm) <br/>[[H-M symbol]]: (2/m 2/m 2/m)
| symmetry    = ''Pbca''
| unit cell   = a = 18.23, b = 8.84 <br/>c = 5.19&nbsp;[Å]; Z&nbsp;=&nbsp;8
| color       = White, grey, green, yellow or brown - colorless in thin section.
| habit       = Prismatic crystals, commonly lamellar, fibrous, or massive
| twinning    = Simple and lamellar on [100]
| cleavage    = Good/distinct on [210]
| fracture    = Uneven
| tenacity    = Brittle
| mohs        = 5 to 6
| luster      = Vitreous, pearly on cleavage
| refractive  = n<sub>α</sub> = 1.650–1.668; n<sub>β</sub> = 1.652–1.673; n<sub>γ</sub> = 1.659–1.679
| opticalprop = Biaxial (+) 
| 2V = 55–90°
| birefringence = δ = 0.009–0.011
| pleochroism = Pink to green pleochroism diagnostic for enstatite, pale green to pale orange perpendicular to pink-green axis 
| streak      = Gray
| gravity     = 3.2–3.3
| density     = 
| melt        = 
| fusibility  = 
| diagnostic  = 
| solubility  = 
| diaphaneity = Translucent to opaque
| other       = 
| references  = <ref name=HBM>[http://rruff.geo.arizona.edu/doclib/hom/enstatite.pdf Handbook of Mineralogy]</ref><ref name=Mindat>[http://www.mindat.org/min-1384.html Mindat]</ref><ref name=Webmin>[http://www.webmineral.com/data/Enstatite.shtml Webmineral data]</ref>
}}

'''Enstatite''' is a mineral; the magnesium endmember of the [[pyroxene]] [[silicate mineral]] series enstatite (MgSiO<sub>3</sub>) – ferrosilite  (FeSiO<sub>3</sub>). The magnesium rich members of the [[solid solution]] series are common rock-forming [[mineral]]s found in [[igneous]] and [[metamorphic rock|metamorphic]] rocks. The intermediate composition, {{chem|(Mg|,Fe)|Si|O|3}}, has historically been known as [[hypersthene]], although this name has been formally abandoned and replaced by orthopyroxene. When determined [[Petrography|petrographically]] or chemically the composition is given as relative proportions of enstatite (En) and ferrosilite (Fs) (e.g., En<sub>80</sub>Fs<sub>20</sub>).

==Polymorphs and varieties==
[[File:Enstatite-83152.jpg|thumb|left|Gem quality enstatite from Myanmar (size: 2.4×1.0×0.8 cm)]]
Most natural [[crystal]]s are [[orthorhombic]] ([[space group]] P''bca'') although three [[polymorphism (materials science)|polymorphs]] are known.  The high temperature, low pressure polymorphs are protoenstatite and protoferrosilite (also orthorhombic, space group P''bcn'') while the low temperature forms, clinoenstatite and clinoferrosilite, are [[monoclinic]] (space group P2''<sub>1</sub>/c'').

[[Weathering|Weathered]] enstatite with a small amount of iron takes on a submetallic [[Lustre (mineralogy)|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 [[Gustav Adolph Kenngott|G. A. Kenngott]] in 1855.<ref name="EB1911">{{EB1911|inline=y|wstitle=Enstatite|volume=9|page=654|first=Leonard James|last=Spencer|authorlink=Leonard James Spencer}}</ref>
[[File:Enstatite-pas-146b.jpg|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]].<ref name="EB1911"/> They also have a prismatic [[cleavage (crystal)|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 rock]]s and [[metamorphic rock]]s. Magnesian orthopyroxene occurs in [[Intrusion (geology)|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 En<sub>90</sub>Fs<sub>10</sub> in composition, is an essential mineral in typical [[peridotite]] and [[pyroxenite]] of the [[Earth's mantle]]. [[Xenolith]]s 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 [[serpentine group|serpentine]]s. 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, Norway|Brevik]] in southern [[Norway]].<ref name="EB1911"/>

Enstatite is a common mineral in meteorites. [[Crystal]]s have been found in stony and iron [[meteorite]]s, including one that fell at [[Potůčky|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 [[chondrule]]s, with an internal radiated structure.<ref name="EB1911"/>

==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. 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|date=September 2008}}

Enstatite is thought to be a main component of the [[E-type asteroid]]s.<ref name=keller>[http://www.sciencemag.org/cgi/content/abstract/327/5962/190 H. U. Keller, et al. - '''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}}</ref> The [[Hungaria asteroid]]s 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]].<ref>{{Cite journal |last1=Burningham |first1=Ben |last2=Faherty |first2=Jacqueline K. |last3=Gonzales |first3=Eileen C. |last4=Marley |first4=Mark S. |last5=Visscher |first5=Channon |last6=Lupu |first6=Roxana |last7=Gaarn |first7=Josefine |last8=Fabienne Bieger |first8=Michelle |last9=Freedman |first9=Richard |last10=Saumon |first10=Didier |date=2021-09-01 |title=Cloud busting: enstatite and quartz clouds in the atmosphere of 2M2224-0158 |journal=Monthly Notices of the Royal Astronomical Society |volume=506 |issue=2 |pages=1944–1961 |doi=10.1093/mnras/stab1361 |doi-access=free |arxiv=2105.04268 |bibcode=2021MNRAS.506.1944B |issn=0035-8711}}</ref>

==See also==
* {{Annotated link|Rhodonite}}
* {{Annotated link|Wollastonite}}

==References==
{{More footnotes|date=November 2019}}
{{reflist}}
*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}}

{{Commons category}}

[[Category:Inosilicates]]
[[Category:Magnesium minerals]]
[[Category:Iron(II) minerals]]
[[Category:Pyroxene group]]
[[Category:Orthorhombic minerals]]
[[Category:Minerals in space group 61]]
[[Category:Blendes]]