{{Short description|Member of the clinopyroxene group of silicate minerals}} {{infobox mineral | name = Omphacite | category = Pyroxene | boxwidth = | boxbgcolor = #7a5e59 | boxtextcolor = #FFFFFF | image = Eclogite Norway.jpg | imagesize = | alt = | 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 | dana = 65.01.03b.01 <br/>(clinopyroxene) | system = Monoclinic | class = Prismatic (2/m) <br/><small>(same H-M symbol)</small> | symmetry = ''P2/n'' or ''C2/c'' | unit cell = a = 9.66, b = 8.81, <br/>c = 5.22&nbsp;[Å]; β = 106.56°; Z = 4 | color = Green to dark green; colorless to pale green in thin section | colour = | habit = Rarely in rough crystals; anhedral, granular to massive | twinning = Single and polysynthetic twinning common on {100} | cleavage = Good on {110}, {110} ^ {1{{overline|1}}0} ≈87°; parting on {100} | fracture = Uneven to conchoidal | tenacity = Brittle | mohs = 5–6 | luster = Vitreous to silky | streak = Greenish white | diaphaneity = Translucent | gravity = 3.16–3.43 | 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 | 2V = Measured: 58° to 83°, Calculated: 74° to 88° | dispersion = | extinction = | length fast/slow = | fluorescence= | absorption = | melt = | fusibility = | diagnostic = | solubility = | other = | alteration = | 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 clinopyroxene group of silicate minerals with formula: (Ca, Na)(Mg, Fe<sup>2+</sup>, Al)Si<sub>2</sub>O<sub>6</sub>. It is a variably deep to pale green or nearly colorless variety of clinopyroxene. It normally appears in eclogite, which is the high-pressure metamorphic rock of basalt. Omphacite is the solid solution of Fe-bearing 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|url-access=subscription}}</ref> It crystallizes in the monoclinic system with prismatic, typically twinned forms, though usually anhedral. Its space group can be P2/n or C2/c depending on the thermal history.<ref name=":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. It is brittle with specific gravity of 3.29 to 3.39 and a Mohs hardness of 5 to 6.

== Formation and occurrence == [[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|url-access=subscription}}</ref> Omphacite general dissolves into garnet as depth increases. Omphacite can stable up to ~500 km depth.]] Omphacite is the dominant phase in the subducted oceanic crust in the Earth's upper mantle. The Mid-Ocean Ridge Basalt, which makes up oceanic crust, goes through ultrahigh-pressure metamorphic process and transforms to eclogite at depth ~60 km in the 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 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|url-access=subscription}}</ref> Considering the cold geotherm of 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|url-access=subscription}}</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|url-access=subscription}}</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.

== 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 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 and shear modulus linearly increases as jadeite component increases.<ref name=":1" />

A typical omphacite from eclogite has the following major elemental proportions expressed as percentages of oxides: {| | SiO<sub>2</sub> || {{bartable|56.02|% (silica)|5}} |- | Al<sub>2</sub>O<sub>3</sub> || {{bartable|12.74|% (alumina)|5}} |- | CaO || {{bartable|12.45|%|5}} |- | MgO || {{bartable|8.01|%|5}} |- | Na<sub>2</sub>O || {{bartable|7.05|%|5}} |- | FeO || {{bartable|1.64|%|5}} |- | Fe<sub>2</sub>O<sub>3</sub> || {{bartable|0.88|%|5}} |- | K<sub>2</sub>O || {{bartable|0.40|%|5}} |- | TiO<sub>2</sub> || {{bartable|0.38|%|5}} |- | MnO || {{bartable|0.00|%|5}} |} <ref>Deer, Howie and Zussman (1992). An Introduction to the Rock-Forming Minerals, 2nd Edition, p. 153 ISBN 0-582-30094-0</ref>

== 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|url-access=subscription}}</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|url-access=subscription}}</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 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|url-access=subscription}}</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 ''omphax'' or ''unripe grape'' for the typical green color.

==References== {{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