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{{Short description|Member of the clinopyroxene group of silicate minerals}}
{{infobox mineral
| name = Omphacite
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| caption = Picture of pieces of [[eclogite]] (type of rock) from the Western Gneiss Region in Norway. The rock contains the minerals omphacite (green), [[pyrope]]-garnet (red), [[quartz]] (milky), [[kyanite]] (blue) and some [[phengite]] (golden white).
| formula = (Ca,Na)(Mg,Fe<sup>2+</sup>,Al)Si<sub>2</sub>O<sub>6</sub>
| IMAsymbol = Omp<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 =
| strunz = 9.DA.20
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| fracture = Uneven to conchoidal
| tenacity = Brittle
| mohs = 5-65–6
| luster = Vitreous to silky
| streak = Greenish white
| diaphaneity = Translucent
| gravity = 3.16-316–3.43
| density =
| polish =
| opticalprop = Biaxial (+)
| refractive = n<sub>α</sub> = 1.662 -– 1.701 n<sub>β</sub> = 1.670 -– 1.712 n<sub>γ</sub> = 1.685 -– 1.723
| birefringence = δ = 0.023
| pleochroism = Weak; X = colorless; Y = very pale green; Z = very pale green, blue-green
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| references = <ref name=Hurlbut>Hurlbut, Cornelius S.; Klein, Cornelis, 1985, ''Manual of Mineralogy'', 20th ed., pp. 398 - 405, John Wiley and Sons, New York {{ISBN|0-471-80580-7}}</ref><ref name=Handbook>[http://rruff.geo.arizona.edu/doclib/hom/omphacite.pdf Handbook of Mineralogy]</ref><ref name=Mindat>[http://www.mindat.org/show.php?id=2991&ld=1&pho= Mindat.org]</ref><ref name=Webmin>[http://webmineral.com/data/Omphacite.shtml Webmineral data]</ref>
}}
'''Omphacite''' is a member of the [[pyroxenePyroxene|clinopyroxene]] group of [[silicate minerals]] with formula: ([[calciumCalcium|Ca]], [[sodiumSodium|Na]])([[magnesiumMagnesium|Mg]], [[ironIron|Fe]]<sup>2+</sup>, [[aluminiumAluminium|Al]])[[siliconSilicon|Si]]<sub>2</sub>[[oxygenOxygen|O]]<sub>6</sub>. It is a variably deep to pale green or nearly colorless variety of [[pyroxenePyroxene|clinopyroxene]]. OmphaciteIt compositionsnormally areappears intermediatein between[[eclogite]], calciumwhich is the high-richpressure [[augitemetamorphic rock]] andof [[basalt]]. sodiumOmphacite is the [[solid solution]] of Fe-richbearing [[diopside]] and [[jadeite]].<ref name=":1">{{Cite journal|last1=Hao|first1=Ming|last2=Pierotti|first2=Caroline E.|last3=Tkachev|first3=Sergey|last4=Prakapenka|first4=Vitali|last5=Zhang|first5=Jin S.|date=2019|title=The single-crystal elastic properties of the jadeite-diopside solid solution and their implications for the composition-dependent seismic properties of eclogite|url=https://pubs.geoscienceworld.org/msa/ammin/article-abstract/104/7/1016/571784/The-single-crystal-elastic-properties-of-the|journal=American Mineralogist|language=en|volume=104|issue=7|pages=1016–1021|doi=10.2138/am-2019-6990|bibcode=2019AmMin.104.1016H|s2cid=195790171|issn=0003-004X}}</ref> It crystallizes in the [[monoclinic]] [[Crystal system|system]] with prismatic, typically [[Crystal twinning|twinned]] forms, though usually anhedral. Its [[space group]] (can be P2/n) isor distinctC2/c fromdepending thaton ofthe augitethermal andhistory.<ref jadeitename=":0">{{Cite (journal|last1=Fleet|first1=M. E.|last2=Herzberg|first2=C. T.|last3=Bancroft|first3=G. M.|last4=Aldridge|first4=L. P.|date=1978|title=Omphacite studies; I, The P2/n-->C2/c) transformation.|url=https://pubs.geoscienceworld.org/msa/ammin/article/63/11-12/1100/40818|journal=[[American Mineralogist]]|volume=63|pages=1100–1106}}</ref> It exhibits the typical near 90° pyroxene [[Cleavage (crystal)|cleavage]]. It is brittle with [[specific gravity]] of 3.29 to 3.39 and a [[Mohs hardness]] of 5 to 6.
 
== Formation and occurrence ==
It is a major mineral component of [[eclogite]] along with [[pyrope]] garnet and also occurs in [[blueschist facies]] and UHP (ultrahigh-pressure) [[metamorphic rocks]]. It also occurs in eclogite xenoliths from [[kimberlite]] as well as in crustal rocks metamorphosed at high pressures. Associated minerals in eclogites include [[garnet]], [[quartz]] or [[coesite]], [[rutile]], [[kyanite]], [[phengite]], and [[lawsonite]]. Minerals such as [[glaucophane]], [[lawsonite]], [[titanite]], and [[epidote]] occur with omphacite in [[blueschist]] facies metamorphic rocks. The name "jade," usually referring to rocks made of [[jadeite]], is sometimes also applied to rocks consisting entirely of omphacite.
[[File:Phase_diagram_of_eclogite.jpg|alt=|left|thumb|330x330px|[[Phase diagram]] of slab crust in the Earth's upper mantle from 200 to 500 km depth.<ref name=":3">{{Cite journal|last1=Aoki|first1=Ichiro|last2=Takahashi|first2=Eiichi|date=2004|title=Density of MORB eclogite in the upper mantle|url=http://www.sciencedirect.com/science/article/pii/S0031920104000500|journal=Physics of the Earth and Planetary Interiors|series=New Developments in High-Pressure Mineral Physics and Applications to the Earth's Interior|language=en|volume=143-144|pages=129–143|doi=10.1016/j.pepi.2003.10.007|bibcode=2004PEPI..143..129A|issn=0031-9201}}</ref> Omphacite general dissolves into garnet as depth increases. Omphacite can stable up to ~500 km depth.]]
Omphacite is the dominated phase in the [[Subduction|subducted]] [[oceanic crust]] in the Earth's upper mantle. The [[MORB|Mid-Ocean Ridge Basalt]], which makes up oceanic crust, goes through ultrahigh-pressure [[Metamorphic rock|metamorphic process]] and transforms to [[eclogite]] at depth ~60 km in the [[Subduction|subduction zones]].<ref>{{Cite journal|last1=Ahrens|first1=Thomas J.|last2=Schubert|first2=Gerald|date=1975|title=Gabbro-eclogite reaction rate and its geophysical significance|url=https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/RG013i002p00383|journal=Reviews of Geophysics|language=en|volume=13|issue=2|pages=383–400|doi=10.1029/RG013i002p00383|bibcode=1975RvGSP..13..383A|issn=1944-9208}}</ref> The major mineral components of eclogite include omphacite, [[garnet]] and high-pressure silica phases ([[coesite]] and [[stishovite]]).<ref name=":3" /> As depth increases, the omphacite in eclogite gradually transforms to [[Majorite|majoritic garnet]]. Omphacite is stable up to 500 km depth in the Earth's interior.<ref name=":3" /><ref>{{Cite journal|last1=Irifune|first1=T.|last2=Sekine|first2=T.|last3=Ringwood|first3=A. E.|last4=Hibberson|first4=W. O.|date=1986|title=The eclogite-garnetite transformation at high pressure and some geophysical implications|url=https://dx.doi.org/10.1016/0012-821X%2886%2990165-2|journal=Earth and Planetary Science Letters|language=en|volume=77|issue=2|pages=245–256|doi=10.1016/0012-821X(86)90165-2|bibcode=1986E&PSL..77..245I|issn=0012-821X}}</ref> Considering the cold [[Geothermal gradient|geotherm]] of [[Subduction|subducted slabs]], omphacite can be stable even in deeper mantle.
 
It also occurs in [[blueschist facies]] and ultrahigh-pressure [[metamorphic rocks]].<ref>{{Cite journal|last1=Guillot|first1=S.|last2=Mahéo|first2=G.|last3=de Sigoyer|first3=J.|last4=Hattori|first4=K. H.|last5=Pêcher|first5=A.|date=2008|title=Tethyan and Indian subduction viewed from the Himalayan high- to ultrahigh-pressure metamorphic rocks|url=http://www.sciencedirect.com/science/article/pii/S0040195107004210|journal=Tectonophysics|series=Asia out of Tethys: Geochronologic, Tectonic and Sedimentary Records|language=en|volume=451|issue=1|pages=225–241|doi=10.1016/j.tecto.2007.11.059|bibcode=2008Tectp.451..225G|issn=0040-1951}}</ref> It is also found in eclogite xenoliths from [[kimberlite]] as well as in crustal rocks metamorphosed at high pressures.<ref>{{Cite journal|last=Jacob|first=D. E.|date=2004|title=Nature and origin of eclogite xenoliths from kimberlites|url=http://www.sciencedirect.com/science/article/pii/S0024493704001045|journal=Lithos|series=Selected Papers from the Eighth International Kimberlite Conference. Volume 2: The J. Barry Hawthorne Volume|language=en|volume=77|issue=1|pages=295–316|doi=10.1016/j.lithos.2004.03.038|bibcode=2004Litho..77..295J|issn=0024-4937}}</ref> Associated minerals in eclogites except the major minerals include [[rutile]], [[kyanite]], [[phengite]], and [[lawsonite]]. Minerals such as [[glaucophane]], lawsonite, [[titanite]], and [[epidote]] occur with omphacite in [[blueschist]] facies metamorphic rocks. The name "jade", usually referring to rocks made of [[jadeite]], is sometimes also applied to rocks consisting entirely of omphacite.
The name "omphacite" has been applied to compositions that contain between 20% and 80% jadeite. The stability of intermediate compositions between [[augite]] and [[jadeite]] is not well understood, but [[miscibility gap]]s appear to be present at temperatures below 300&nbsp;°C to 400&nbsp;°C, and perhaps at higher temperatures. Pairs of pyroxenes -- both augite plus omphacite and omphacite plus jadeite -- appear to have existed in equilibrium at low temperatures.<ref>Tatsuki Tsujimori, Juhn G. Liou, and Robert G. Coleman, 2005, ''Coexisting retrograde jadeite and omphacite in a jadeite-bearing lawsonite eclogite from the Motagua Fault Zone, Guatemala.'' American Mineralogist, v. 90, pp. 836-842</ref>
 
== Chemical composition ==
Omphacite is the solid solution of Fe-bearing diopside (CaMgSi<sub>2</sub>O<sub>6</sub>) and jadeite (NaAlSi<sub>2</sub>O<sub>6</sub>). Depending on how much the coupled substitution of (Na, Al)-(Mg-Fe, Ca) happens, the [[chemical composition]] of omphacite varies continuously from pure diopside to pure jadeite.<ref name=":1" /> Due to the relatively small radius of (Na, Al) atoms, the [[Unit Cell|unit cell]] volume linearly decreases as jadeite component increases.<ref name=":2">{{Cite journal|last1=Pandolfo|first1=Francesco|last2=Cámara|first2=Fernando|last3=Domeneghetti|first3=M. Chiara|last4=Alvaro|first4=Matteo|last5=Nestola|first5=Fabrizio|last6=Karato|first6=Shun-Ichiro|last7=Amulele|first7=George|date=2015|title=Volume thermal expansion along the jadeite–diopside join|url=https://doi.org/10.1007/s00269-014-0694-9|journal=Physics and Chemistry of Minerals|language=en|volume=42|issue=1|pages=1–14|doi=10.1007/s00269-014-0694-9|bibcode=2015PCM....42....1P|hdl=2318/153763 |s2cid=96677363|issn=1432-2021|hdl-access=free}}</ref> In addition, the coupled substitution also stiffens the crystals. The [[Bulk modulus|bulk]] and [[Shear modulus|shear]] modulus linearly increases as jadeite component increases.<ref name=":1" />
 
== Space group ==
Although omphacite is the solid solution of [[diopside]] and [[jadeite]], its [[space group]] may be different with them. The space group of diopside and jadeite is C2/c. However, omphacite can show both P2/n and C2/c space group. At low temperature, the partial coupled substitution of (Na, Al)-(Mg-Fe, Ca) in omphacite orders the atoms in the unit cell and makes omphacite shows a relatively low symmetry space group P2/n.<ref>{{Cite journal|last1=Skelton|first1=Richard|last2=Walker|first2=Andrew M.|date=2015|title=The effect of cation order on the elasticity of omphacite from atomistic calculations|url=https://doi.org/10.1007/s00269-015-0754-9|journal=Physics and Chemistry of Minerals|language=en|volume=42|issue=8|pages=677–691|doi=10.1007/s00269-015-0754-9|bibcode=2015PCM....42..677S|s2cid=92245503|issn=1432-2021}}</ref> As temperature increases, the movements of the atoms increase and finally the coupled substitution will not influence the order of the structure. When temperature reaches ~700–750 °C, the structure of omphacite becomes totally disordered and the space group will transform to C2/c.<ref name=":0" /> Natural omphacite may show C2/c structure even at room temperature if the omphacite crystal went through fast temperature decreasing.<ref>{{Cite journal|last1=Bhagat|first1=Snehal S.|last2=Bass|first2=Jay D.|last3=Smyth|first3=Joseph R.|date=1992|title=Single-crystal elastic properties of omphacite-C2/c by Brillouin spectroscopy|url=https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/92JB00030|journal=Journal of Geophysical Research: Solid Earth|language=en|volume=97|issue=B5|pages=6843–6848|doi=10.1029/92JB00030|bibcode=1992JGR....97.6843B|issn=2156-2202}}</ref>
 
Although the atomic positions in the two space groups have a subtle difference, it does not clearly change the physical properties of omphacite.<ref name=":1" /> The absolute [[Unit Cell|unit cell volumes]] are a little different for the two different space group, the compressibility and [[thermal expansion]] does not show obviously different within experimental uncertainties.<ref name=":2" /><ref>{{Cite journal|last1=Hao|first1=Ming|last2=Zhang|first2=Jin S.|last3=Pierotti|first3=Caroline E.|last4=Ren|first4=Zhiyuan|last5=Zhang|first5=D.|date=2019|title=High-Pressure Single-Crystal Elasticity and Thermal Equation of State of Omphacite and Their Implications for the Seismic Properties of Eclogite in the Earth's Interior|journal=Journal of Geophysical Research: Solid Earth|language=en|volume=124|issue=3|pages=2368–2377|doi=10.1029/2018JB016964|bibcode=2019JGRB..124.2368H|issn=2169-9356|doi-access=free}}</ref><ref>{{Cite journal|last1=Nishihara|first1=Yu|last2=Takahashi|first2=Eiichi|last3=Matsukage|first3=Kyoko|last4=Kikegawa|first4=Takumi|date=2003|title=Thermal equation of state of omphacite|url=https://pubs.geoscienceworld.org/msa/ammin/article-abstract/88/1/80/43805/Thermal-equation-of-state-of-omphacite|journal=American Mineralogist|language=en|volume=88|issue=1|pages=80–86|doi=10.2138/am-2003-0110|bibcode=2003AmMin..88...80N|s2cid=101319641|issn=0003-004X}}</ref>
 
== Etymology and history ==
It was first described in 1815 in the Münchberg Metamorphic complex, Franconia, [[Bavaria]], [[Germany]]. The name ''omphacite'' derives from the [[Greek language|Greek]] ''omphax'' or ''unripe grape'' for the typical green color.
 
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{{Reflist}}
 
{{commonscat|Omphacite}}
[[Category:Inosilicates]]
[[Category:Calcium minerals]]
[[Category:Sodium minerals]]
[[Category:Magnesium minerals]]
[[Category:Iron(II) minerals]]
[[Category:Aluminium minerals]]
[[Category:Monoclinic minerals]]
[[Category:Minerals in space group 13]]
[[Category:Minerals in space group 15]]
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