View source for Peridotite

{{Short description|Coarse-grained ultramafic igneous rock type}}
{{For|the yellow-green transparent variety of [[olivine]]|Peridot}}
{{Infobox rock
|name = Peridotite
|type = Igneous
|image = Peridotite mantle xenoliths in phonotephrite (Peridot Mesa Flow, Middle Pleistocene, 580 ka; Peridot Mesa, San Carlos Volcanic Field, Arizona, USA) 28 (cropped)).jpg
|image_size = 300px
|caption = 
|composition = [[olivine]], [[pyroxene]]
}}
'''Peridotite''' ({{IPAc-en|US|ˈ|p|ɛ|r|ɪ|d|oʊ|ˌ|t|aɪ|t|,_|p|ə|ˈ|r|ɪ|d|ə|-}} {{respell|PERR|ih|doh|tyte|,_|pə|RID|ə|-}}) is a dense, [[Phanerite|phaneritic]] (coarse-grained) [[igneous rock]] consisting mostly of the silicate minerals [[olivine]] and [[pyroxene]]. Peridotite is [[ultramafic]], as the rock contains less than 45% [[silica]]. It is high in [[magnesium]] (Mg<sup>2+</sup>), reflecting the high proportions of magnesium-rich olivine, with appreciable [[iron]]. Peridotite is derived from [[Earth's mantle]], either as solid blocks and fragments, or as crystals accumulated from magmas that formed in the mantle. The compositions of peridotites from these layered [[igneous]] complexes vary widely, reflecting the relative proportions of [[pyroxene]]s, [[chromite]], [[plagioclase]], and [[amphibole]].

Peridotite is the dominant rock of the [[upper mantle (Earth)|upper part of Earth's mantle]]. The compositions of peridotite [[nodule (geology)|nodules]] found in certain basalts are of special interest along with [[kimberlite pipe|diamond pipes]] ([[kimberlite]]), because they provide samples of Earth's mantle brought up from depths ranging from about 30&nbsp;km to 200&nbsp;km or more. Some of the nodules preserve [[isotope]] ratios of [[osmium]] and other elements that record processes that occurred when Earth was formed, and so they are of special interest to [[historical geology|paleogeologists]] because they provide clues to the early composition of Earth's mantle and the complexities of the processes that occurred.

The word ''peridotite'' comes from the gemstone [[peridot]], which consists of pale green olivine.<ref>Collins Australian Dictionary, 7th edition</ref> Classic peridotite is bright green with some specks of black, although most hand samples tend to be darker green. Peridotitic outcrops typically range from earthy bright yellow to dark green; this is because olivine is easily weathered to [[iddingsite]]. While green and yellow are the most common colors, peridotitic rocks may exhibit a wide range of colors including blue, brown, and red.

==Classification==
[[File:Peridotite Olivine-Orthopyroxene-Clinopyroxene - Common Peridotites highlighted.png|thumb|upright=1.4|right|Classification diagram for peridotite and pyroxenite, based on proportions of olivine and pyroxene. The pale green area encompasses the most common compositions of peridotite in the upper part of the Earth's mantle (partly adapted from Bodinier and Godard (2004)).]]

Igneous rocks rich in [[magnesium]] and [[iron]] with a [[color index (geology)|color index]] greater than 90 are defined as [[ultramafic rocks|ultramafic]].<ref name=Downes>{{cite book |editor1-last=Downes |editor1-first=Hilary |title=Encyclopedia of Geology |date=2021 |publisher=American Geological Institute |location=Alexandria, Virginia |isbn=978-0-08-102909-1 |edition=Second }}</ref>  Ultramafic rocks may be further classified by their relative proportions of [[olivine]], [[orthopyroxene]], [[clinopyroxene]], and [[hornblende]], which are the most abundant families of mafic minerals in most ultramafic rocks. Peridotite is then defined as coarse-grained ultramafic rock in which olivine makes up 40% or more of the total volume of these four mineral families in the rock.<ref name=PhilpottsAgue2009ch6>{{cite book |last1=Philpotts |first1=Anthony R. |last2=Ague |first2=Jay J. |title=Principles of igneous and metamorphic petrology |date=2009 |publisher=Cambridge University Press |location=Cambridge, UK |isbn=978-0-521-88006-0 |edition=2nd |pages=137–142}}</ref><ref name="bgs">{{Cite journal|date=1999|title=Rock Classification Scheme - Vol 1 - Igneous|url=http://nora.nerc.ac.uk/id/eprint/3223/1/RR99006.pdf|journal=British Geological Survey: Rock Classification Scheme|volume=1|pages=1–52}}</ref>

Peridotites are further classified as follows:<ref name="bgs"/>{{sfn|Philpotts|Ague|2009|p=142}}

* [[Dunite]]: more than 90% olivine
: Dunite is found as prominent veins in the peridotite layer of [[ophiolites]], which are interpreted as slices of oceanic lithosphere (crust and upper mantle) thrust onto continents.{{sfn|Philpotts|Ague|2009|pp=43–44, 372–373}} Dunite also occurs as a [[cumulate]] in [[layered intrusion]]s, where olivine crystallized out of a slowly cooling body of [[magma]] and accumulated on the floor of the magma body to form the lowest layer of the intrusion.{{sfn|Philpotts|Ague|2009|p=385}} Dunite almost always contains accessory [[chromite]].{{sfn|Jackson|1997|loc="dunite"}}

*[[Kimberlite]]: formed in [[volcanic pipe]]s and at least 35% olivine<ref>{{Cite web |title=Peridotite: Igneous Rock - Pictures, Definition & More |url=https://geology.com/rocks/peridotite.shtml |access-date=2022-07-13 |website=geology.com}}</ref>
: Kimberlite is a highly [[breccia]]ted variant of peridotite formed in [[volcanic pipe]]s and is known for being the host rock to diamonds. Unlike other forms of peridotite, kimberlite is quite rare.<ref name=":0">{{Cite web |title=kimberlite {{!}} rock {{!}} Britannica |url=https://www.britannica.com/science/kimberlite |access-date=2022-07-13 |website=www.britannica.com |language=en}}</ref>

* Pyroxene peridotite: From 40% to 90% olivine and less than 5% hornblende
** [[Harzburgite]]: less than 5% clinopyroxene
:: Harzburgite makes up the bulk of the peridotite layer of [[ophiolites]]. It is interpreted as depleted mantle rock, from which [[basaltic]] magma has been extracted. It also forms as a cumulate in Type I layered intrusions, forming a layer just above the dunite layer.{{sfn|Philpotts|Ague|2009|pp=43–44, 385}} Harzburgite likely makes up most of the mantle [[lithosphere]] underneath continental [[craton]]s.<ref name="Herzberg2004">{{cite journal |last1=Herzberg |first1=Claude |title=Geodynamic Information in Peridotite Petrology |journal=Journal of Petrology |date=December 2004 |volume=45 |issue=12 |pages=2507–2530 |doi=10.1093/petrology/egh039|doi-access=free }}</ref>
:* [[Wehrlite]]: less than 5% orthopyroxene
:: Wehrlite makes up part of the transition zone between the peridotite layer and overlying [[gabbro]] layer of [[ophiolites]].{{sfn|Philpotts|Ague|2009|p=178}} In  Type II layered intrusions, it takes the place of harzburgite as the layer just above the dunite layer.{{sfn|Philpotts|Ague|2009|p=385}}

:* [[Lherzolite]]: intermediate content of clinopyroxene and orthopyroxene
:: Lherzolite is thought to make up much of the upper mantle.{{sfn|Philpotts|Ague|2009|p=590}} It has almost exactly the composition of a mixture of three parts harzburgite and one part [[tholeiitic basalt]] (''[[pyrolite]]'') and is the likely source rock for basaltic magma. It is found as rare [[xenolith]]s in basalt, such as those of [[Kilbourne Hole]] in southern New Mexico, US,{{sfn|Philpotts|Ague|2009|p=604}} and at [[Oahu]], Hawaii, US.<ref>{{cite journal |last1=Yang |first1=H.-J. |last2=Sen |first2=G. |last3=Shimizu |first3=N. |title=Mid-Ocean Ridge Melting: Constraints from Lithospheric Xenoliths at Oahu, Hawaii |journal=Journal of Petrology |date=1 February 1998 |volume=39 |issue=2 |pages=277–295 |doi=10.1093/petroj/39.2.277|doi-access=free }}</ref>

* Hornblende peridotite: From 40% to 90% olivine and less than 5% pyroxene
: Hornblende peridotite is found as rare xenoliths in [[andesite]]s above [[subduction zones]]. They are direct evidence of alteration of mantle rock by fluids released by the subducting [[Slab (geology)|slab]].<ref name=BlatterCarmichael1998>{{cite journal |last1=Blatter |first1=Dawnika L. |last2=Carmichael |first2=Ian S. E. |title=Hornblende peridotite xenoliths from central Mexico reveal the highly oxidized nature of subarc upper mantle |journal=Geology |date=1 November 1998 |volume=26 |issue=11 |pages=1035–1038 |doi=10.1130/0091-7613(1998)026<1035:HPXFCM>2.3.CO;2|bibcode=1998Geo....26.1035B }}</ref>

* Pyroxene hornblende peridotite: Intermediate between pyroxene peridotite and hornblende peridotite
: Pyroxene hornblende peridotite is found as rare xenoliths, such as those of [[Wilcza Góra, Silesian Voivodeship|Wilcza Góra]] in southwest Poland. Here it likely formed by alteration of mantle rock by carbonated hydrous silicic fluids associated with volcanism.<ref name=MatusiakMatekEtal2017>{{cite journal |last1=Matusiak-Małek |first1=Magdalena |last2=Puziewicz |first2=Jacek |last3=Ntaflos |first3=Theodoros |last4=Grégoire |first4=Michel |last5=Kukuła |first5=Anna |last6=Wojtulek |first6=Piotr Marian |title=Origin and evolution of rare amphibole-bearing mantle peridotites from Wilcza Góra (SW Poland), Central Europe |journal=Lithos |date=August 2017 |volume=286-287 |pages=302–323 |doi=10.1016/j.lithos.2017.06.017|bibcode=2017Litho.286..302M }}</ref>

==Composition==
[[File:Olivine-Dunit.jpg|thumb|Typical peridotite sample ([[dunite]], left) and large olivine crystal (right)]]
Mantle peridotite is highly enriched in magnesium, with a typical magnesium number{{explain|date=January 2025}} of 89.<ref>{{cite journal |last1=Palme |first1=H. |last2=O'Neill |first2=H.St.C. |title=Cosmochemical Estimates of Mantle Composition |journal=Treatise on Geochemistry |date=2007 |pages=1–38 |doi=10.1016/B0-08-043751-6/02177-0|isbn=978-0-08-043751-4 }}</ref> In other words{{whose|date=January 2025}}, of the total content of iron plus magnesium, 89 [[mol%]] is magnesium. This is reflected in the composition of the mafic minerals making up the peridotite.

Olivine is the essential mineral found in all peridotites. It is an iron-magnesium [[orthosilicate]] with the variable formula {{chem2|(Mg,Fe)2SiO4}}. The magnesium-rich olivine of peridotites is typically olive-green in color.<ref name=Nesse2000>{{cite book |last1=Nesse |first1=William D. |title=Introduction to mineralogy |date=2000 |publisher=Oxford University Press |location=New York |isbn=978-0-19-510691-6 |pages=306–310}}</ref>

Pyroxenes are chain silicates having the variable formula {{chem2|(Ca,Na,Fe^{II},Mg)(Cr,Al,Fe^{III},Mg,Mn,Ti,V)Si2O6}} comprising a large group of different minerals. These are divided into orthopyroxenes (with an [[orthorhombic]] crystal structure) and clinopyroxenes (with a [[monoclinic]] crystal structure).{{sfn|Nesse|2000|pp=261–74}} This distinction is important in the classification of pyroxene peridotites<ref name="bgs"/>{{sfn|Philpotts|Ague|2009|p=142}} since clinopyroxene melts more easily than orthopyroxene or olivine. The most common orthopyroxene is [[enstatite]], {{chem2|Mg2Si2O6}}, in which iron substitutes for some of the magnesium. The most important clinopyroxene is [[diopside]], {{chem2|CaMgSi2O6}}, again with some substitution of iron for magnesium ([[hedenbergite]], {{chem2|FeCaSi2O6}}).{{sfn|Nesse|2000|pp=261–74}} Ultramafic rock in which the fraction of pyroxenes exceeds 60% are classified as [[pyroxenite]]s rather than peridotites. Pyroxenes are typically dark in color.{{sfn|Nesse|2000|pp=261–74}}

Hornblende is an [[amphibole]], a group of minerals resembling pyroxenes but with a double chain structure incorporating water. Hornblende itself has a highly variable composition, ranging from [[tschermakite]] ({{chem2|Ca2(Mg,Fe)3Al2Si6Al2O22(OH)2}}) to [[pargasite]] ({{chem2|NaCa2(Mg,Fe)4AlSi6Al2O22(OH)2}}) with many other variations in composition.{{sfn|Nesse|2000|pp=277–289}} It is present in peridotite mostly as a consequence of alteration by hydrous fluids.<ref name=BlatterCarmichael1998/><ref name=MatusiakMatekEtal2017/>

Although peridotites are classified by their content of olivine, pyroxenes, and hornblende, a number of other mineral families are characteristically present in peridotites and may make up a significant fraction of their composition. For example, chromite is sometimes present in amounts of up to 50%. (A chromite composition above 50% reclassifies the rock as a ''peridotitic chromitite''.) Other common accessory minerals include [[spinel]], [[garnet]], [[biotite]], or [[magnetite]]. A peridotite containing significant amounts of one of these minerals may have its classification refined accordingly; for example, if a lhertzolite contains up to 5% spinel, it is a ''spinel-bearing lhertzolite'', while for amounts up to 50%, it would be classified as a ''spinel lhertzolite''.{{sfn|Philpotts|Ague|2009|p=138}} The accessory minerals can be useful for estimating the depth of formation of the peridotite. For example, the aluminium in lhertzolite is present as [[plagioclase]] at depths shallower than about {{convert|20|km||sp=us}}, while it is present as spinel between 20&nbsp;km and {{convert|60|km||sp=us}} and as garnet below 60&nbsp;km.<ref>{{cite book |last1=Blatt |first1=Harvey |last2=Tracy |first2=Robert J. |title=Petrology: igneous, sedimentary, and metamorphic. |date=1996 |publisher=W.H. Freeman |location=New York |isbn=0-7167-2438-3 |edition=2nd |page=145}}</ref>

==Distribution and location==
[[File:Iddingsite.JPG|thumb|[[Olivine]] in a peridotite [[weathering]] to [[iddingsite]] within a [[Mantle (geology)|mantle]] [[xenolith]]]]
[[File:Serpentinized and carbonated peridotite.jpg|thumb|Serpentinized and carbonated peridotite<ref name=DCOdecadal>{{cite book |last1=Deep Carbon Observatory |title=Deep Carbon Observatory: A Decade of Discovery |doi=10.17863/CAM.44064 |date=2019 |location=Washington, DC |url=https://deepcarbon.net/deep-carbon-observatory-decade-discovery |access-date=13 December 2019 |archive-date=17 December 2019 |archive-url=https://web.archive.org/web/20191217174901/https://deepcarbon.net/deep-carbon-observatory-decade-discovery }}</ref>]]
Peridotite is the dominant rock of the Earth's mantle above a depth of about 400&nbsp;km; below that depth, olivine is converted to the higher-pressure mineral [[wadsleyite]].<ref>{{cite journal |last1=Bercovici |first1=David |last2=Karato |first2=Shun-ichiro |title=Whole-mantle convection and the transition-zone water filter |journal=Nature |date=September 2003 |volume=425 |issue=6953 |pages=39–44 |doi=10.1038/nature01918|pmid=12955133 |bibcode=2003Natur.425...39B |s2cid=4428456 }}</ref>

Oceanic plates consist of up to about 100&nbsp;km of peridotite covered by a thin crust. The crust, commonly about 6&nbsp;km thick, consists of basalt, gabbro, and minor sediments. The peridotite below the ocean crust, "abyssal peridotite," is found on the walls of rifts in the deep sea floor.<ref name="refname" >{{cite journal |last1=Dick |first1=H. J. B. |title=Abyssal peridotites, very slow spreading ridges and ocean ridge magmatism |journal=Geological Society, London, Special Publications |date=1989 |volume=42 |issue=1 |pages=71–105 |doi=10.1144/GSL.SP.1989.042.01.06|bibcode=1989GSLSP..42...71D |s2cid=129660369 }}</ref> Oceanic plates are usually subducted back into the mantle in [[Subduction|subduction zones]]. However, pieces can be emplaced into or overthrust on [[continental crust]] by a process called [[obduction]], rather than carried down into the mantle. The emplacement may occur during [[orogeny|orogenies]], as during collisions of one continent with another or with an [[island arc]]. The pieces of oceanic plates emplaced within continental crust are referred to as [[ophiolites]]. Typical ophiolites consist mostly of peridotite plus associated rocks such as [[gabbro]], [[pillow basalt]], diabase sill-and-dike complexes, and red chert.{{sfn|Philpotts|Ague|2009|pp=370–374}}<ref>{{cite journal |last1=Dilek |first1=Y. |last2=Furnes |first2=H. |title=Ophiolites and Their Origins |journal=Elements |date=1 April 2014 |volume=10 |issue=2 |pages=93–100 |doi=10.2113/gselements.10.2.93|bibcode=2014Eleme..10...93D }}</ref> ''Alpine peridotite'' or ''orogenic peridotite massif'' is an older term for an ophiolite emplaced in a mountain belt during a continent-continent plate collision.{{sfn|Philpotts|Ague|2009|p=371}}<ref>{{cite journal |last1=Piccardo |first1=Giovanni B. |last2=Guarnieri |first2=Luisa |title=Alpine peridotites from the Ligurian Tethys: an updated critical review |journal=International Geology Review |date=July 2010 |volume=52 |issue=10–12 |pages=1138–1159 |doi=10.1080/00206810903557829|bibcode=2010IGRv...52.1138P |s2cid=128877324 }}</ref><ref>{{cite journal |last1=Spengler |first1=Dirk |last2=van Roermund |first2=Herman L. M. |last3=Drury |first3=Martyn R. |last4=Ottolini |first4=Luisa |last5=Mason |first5=Paul R. D. |last6=Davies |first6=Gareth R. |title=Deep origin and hot melting of an Archaean orogenic peridotite massif in Norway |journal=Nature |date=April 2006 |volume=440 |issue=7086 |pages=913–917 |doi=10.1038/nature04644|pmid=16612379 |bibcode=2006Natur.440..913S |s2cid=4419956 }}</ref>

Peridotites also occur as fragments ([[xenolith]]s) carried up by magmas from the mantle. Among the rocks that commonly include peridotite xenoliths are [[basalt]] and [[kimberlite]].<ref name="padovani-reid-1989">{{cite journal |last1=Padovani |first1=Elaine R. |last2=Reid |first2=Mary R. |title=Field guide to Kilbourne Hole maar, Dona Ana County, New Mexico |journal=New Mexico Bureau of Mines and Mineral Resources Memoir |date=1989 |volume=46 |pages=174–185}}</ref> Although [[kimberlite]] is a variant of peridotite, kimberlite is also considered as [[breccia]]ted volcanic material as well,<ref name=":0" /> which is why it is referred to as a source of peridotite xenoliths. Peridotite xenoliths contain osmium and other elements whose [[stable isotope ratio]]s provide clues on the formation and evolution of the Earth's mantle.<ref>{{cite journal |last1=Meisel |first1=Thomas |last2=Walker |first2=Richard J |last3=Irving |first3=Anthony J |last4=Lorand |first4=Jean-Pierre |title=Osmium isotopic compositions of mantle xenoliths: a global perspective |journal=Geochimica et Cosmochimica Acta |date=April 2001 |volume=65 |issue=8 |pages=1311–1323 |doi=10.1016/S0016-7037(00)00566-4|bibcode=2001GeCoA..65.1311M }}</ref><ref>{{cite journal |last1=Walker |first1=R.J |last2=Carlson |first2=R.W |last3=Shirey |first3=S.B |last4=F.R |first4=Boyd |title=Os, Sr, Nd, and Pb isotope systematics of southern African peridotite xenoliths: Implications for the chemical evolution of subcontinental mantle |journal=Geochimica et Cosmochimica Acta |date=July 1989 |volume=53 |issue=7 |pages=1583–1595 |doi=10.1016/0016-7037(89)90240-8|bibcode=1989GeCoA..53.1583W }}</ref> Such xenoliths originate from depths of up to nearly {{convert|200|km||sp=us}}<ref>{{cite journal |last1=Burgess |first1=S. R. |last2=Harte |first2=Ben |title=Tracing Lithosphere Evolution through the Analysis of Heterogeneous G9-G10 Garnets in Peridotite Xenoliths, II: REE Chemistry |journal=Journal of Petrology |date=1 March 2004 |volume=45 |issue=3 |pages=609–633 |doi=10.1093/petrology/egg095|doi-access=free }}</ref> or more.<ref>{{cite journal |last1=Ave Lallemant |first1=H.G. |last2=Mercier |first2=J-C.C. |last3=Carter |first3=N.L. |last4=Ross |first4=J.V. |title=Rheology of the upper mantle: Inferences from peridotite xenoliths |journal=Tectonophysics |date=December 1980 |volume=70 |issue=1–2 |pages=85–113 |doi=10.1016/0040-1951(80)90022-0|bibcode=1980Tectp..70...85A }}</ref>

The volcanic equivalent of peridotites are [[komatiite]]s, which were mostly erupted early in the Earth's history and are rare in rocks younger than [[Archean]] in age.<ref>{{cite journal |last1=Herzberg |first1=Claude |last2=Condie |first2=Kent |last3=Korenaga |first3=Jun |title=Thermal history of the Earth and its petrological expression |journal=Earth and Planetary Science Letters |date=15 March 2010 |volume=292 |issue=1–2 |pages=79–88 |doi=10.1016/j.epsl.2010.01.022|bibcode=2010E&PSL.292...79H |s2cid=12612486 }}</ref>

Small pieces of peridotite have been found in lunar breccias.<ref>{{cite journal |last1=Anderson |first1=A. T. |title=The Texture and Mineralogy of Lunar Peridotite, 15445,10 |journal=The Journal of Geology |date=March 1973 |volume=81 |issue=2 |pages=219–226 |doi=10.1086/627837|bibcode=1973JG.....81..219A |s2cid=128747551 }}</ref>

The rocks of the peridotite family are uncommon at the surface and are highly unstable, because olivine reacts quickly with water at typical temperatures of the upper crust and at the Earth's surface. Many, if not most, surface outcrops have been at least partly altered to [[serpentinite]], a process in which the pyroxenes and olivines are converted to green [[Serpentine group|serpentine]].<ref name=Nesse2000/> This hydration reaction involves considerable increase in volume with concurrent deformation of the original textures.<ref name=Mevel2003>{{cite journal |last1=Mével |first1=Catherine |title=Serpentinization of abyssal peridotites at mid-ocean ridges |journal=Comptes Rendus Geoscience |date=September 2003 |volume=335 |issue=10–11 |pages=825–852 |doi=10.1016/j.crte.2003.08.006|bibcode=2003CRGeo.335..825M |url=https://comptes-rendus.academie-sciences.fr/geoscience/articles/10.1016/j.crte.2003.08.006/ |url-access=subscription }}</ref> Serpentinites are mechanically weak and so flow readily within the earth.<ref>{{cite journal |last1=Vannucchi |first1=Paola |author1-link=Paola Vannucchi|last2=Morgan |first2=Jason |last3=Polonia |first3=Alina |last4=Molli |first4=Giancarlo |title=How serpentine peridotites can leak through subduction channels |date=23 March 2020 |doi=10.5194/egusphere-egu2020-10250 |journal=EGU General Assembly 2020|article-number=10250 |bibcode=2020EGUGA..2210250V |s2cid=225971151 |doi-access=free }}</ref> Distinctive plant communities grow in soils developed on serpentinite, because of the unusual composition of the underlying rock.<ref name="Presidio">{{cite web |title=Serpentinite |url=https://www.nps.gov/prsf/learn/nature/serpentinite.htm |website=Presidio of San Francisco |publisher=National Park Service |access-date=3 September 2021}}</ref> One mineral in the serpentine group, [[chrysotile]], is a type of asbestos.{{sfn|Nesse|2000|pp=241–242}}

==Color, morphology, and texture==
[[File:Dunite0001.jpg|thumb|Alpine peridotite from the [[Ivrea zone]] in the [[Alps]] of Italy ([[dunite]] from Finero)]]
Most peridotite is green in color due to its high olivine content.  However, peridotites can range in color from greenish-gray<ref>{{cite web |title=Spinel peridotite |url=https://geogallery.si.edu/10026173/spinel-peridotite |website=National Museum of Natural History |publisher=Smithsonian Institution |access-date=26 February 2022 |archive-date=26 February 2022 |archive-url=https://web.archive.org/web/20220226012207/https://geogallery.si.edu/10026173/spinel-peridotite }}</ref><ref>{{cite web |title=Peridotite (Dunite) |url=https://flexiblelearning.auckland.ac.nz/rocks_minerals/rocks/peridotite.html |website=Geology: Rocks and minerals |publisher=University of Auckland |access-date=26 February 2022}}</ref> to  nearly black<ref>{{cite web |last1=Sepp |first1=Siim |title=Peridotite - Igneous Rocks |url=https://www.sandatlas.org/peridotite/ |website=www.sandatlas.org |access-date=26 February 2022}}</ref> to pale yellowish-green.<ref>{{cite journal |last1=Arai |first1=S. |title=Petrology of Peridotite Xenoliths from Iraya Volcano, Philippines, and its Implication for Dynamic Mantle-Wedge Processes |journal=Journal of Petrology |date=1 February 2004 |volume=45 |issue=2 |pages=369–389 |doi=10.1093/petrology/egg100|doi-access=free }}</ref> Peridotite weathers to form a distinctive brown crust in subaerial exposures<ref>{{cite journal |last1=Bucher |first1=Kurt |last2=Stober |first2=Ingrid |last3=Müller-Sigmund |first3=Hiltrud |title=Weathering crusts on peridotite |journal=Contributions to Mineralogy and Petrology |date=May 2015 |volume=169 |issue=5 |page=52 |doi=10.1007/s00410-015-1146-3|bibcode=2015CoMP..169...52B |s2cid=129292161 }}</ref> and to a deep orange color in submarine exposures.<ref>{{cite journal |last1=Luguet |first1=Ambre |last2=Lorand |first2=Jean-Pierre |last3=Seyler |first3=Monique |title=Sulfide petrology and highly siderophile element geochemistry of abyssal peridotites: a coupled study of samples from the Kane Fracture Zone (45°W 23°20N, MARK area, Atlantic Ocean) |journal=Geochimica et Cosmochimica Acta |date=April 2003 |volume=67 |issue=8 |pages=1553–1570 |doi=10.1016/S0016-7037(02)01133-X|bibcode=2003GeCoA..67.1553L }}</ref>

Peridotites can take on a massive form or may be in layers on a variety of size scales.{{sfn|Blatt|Tracy|1996|p=53}} Layered peridotites may form the base layers of layered intrusions.{{sfn|Philpotts|Ague|2009|pp=384–386}} These are characterized by [[cumulate texture]]s, characterized by a [[Fabric (geology)|fabric]] of coarse (>5mm) interlocking euhedral (well-formed) crystals in a groundmass of finer crystals formed from liquid magma trapped in the cumulate. Many show [[poikilitic texture]] in which crystallization of this liquid has produced crystals that overgrow and enclose the original cumulus crystals (called ''chadrocrysts'').{{sfn|Blatt|Tracy|1996|pp=130–131}}

Another texture is a ''well-annealed'' texture of equal sized anhedral crystals with straight grain boundaries intersecting at 120°. This may result when slow cooling allowed recrystallization to minimize surface energy. Cataclastic texture, showing irregular fractures and [[deformation twinning]] of olivine grains, is common in peridotites because of the deformation associated with their tectonic mode of emplacement.{{sfn|Blatt|Tracy|1996|p=53}}

==Origin==
[[File:Peridotite mantle xenoliths in phonotephrite (Peridot Mesa Flow, Middle Pleistocene, 580 ka; Peridot Mesa, San Carlos Volcanic Field, Arizona, USA) 30.jpg|thumb|Peridotite [[xenolith]]s in [[phonotephrite]], from Arizona]]
Peridotites have two primary modes of origin: as mantle rocks formed during the accretion and differentiation of the Earth, or as cumulate rocks formed by precipitation of olivine ± pyroxenes from basaltic or ultramafic magmas. These magmas are ultimately derived from the [[upper mantle (Earth)|upper mantle]] by partial melting of mantle peridotites.{{sfn|Philpotts|Ague|2009|p=2, 370–374, 384–390}}

Mantle peridotites are sampled as ophiolites in collisional mountain ranges, as xenoliths in basalt or kimberlite, or as abyssal peridotites (sampled from ocean floor).<ref name="refname"/> These rocks represent either fertile mantle (lherzolite) or partially depleted mantle (harzburgite, dunite).{{sfn|Philpotts|Ague|2009|pp=43–44, 370–374, 385, 391, 590, 601–604}} Alpine peridotites may be either of the ophiolite association and representing the uppermost mantle below ocean basins, or masses of subcontinental mantle emplaced along thrust faults in mountain belts.<ref>{{cite journal |last1=Gueydan |first1=Frédéric |last2=Mazzotti |first2=Stephane |last3=Tiberi |first3=Christel |last4=Cavin |first4=Remy |last5=Villaseñor |first5=Antonio |title=Western Mediterranean Subcontinental Mantle Emplacement by Continental Margin Obduction |journal=Tectonics |date=June 2019 |volume=38 |issue=6 |pages=2142–2157 |doi=10.1029/2018TC005058|bibcode=2019Tecto..38.2142G |s2cid=182877329 |url=https://hal.umontpellier.fr/hal-02262198/file/Gueydan_et_al-2019-Tectonics.pdf }}</ref>

Layered peridotites are igneous sediments and form by mechanical accumulation of dense olivine crystals.<ref>{{Cite journal|last1=Emeleus|first1=C. H.|last2=Troll|first2=V. R.|date=2014-08-01|title=The Rum Igneous Centre, Scotland|journal=Mineralogical Magazine|language=en|volume=78|issue=4|pages=805–839|doi=10.1180/minmag.2014.078.4.04|bibcode=2014MinM...78..805E|issn=0026-461X|doi-access=free}}</ref> They form from mantle-derived magmas, such as those of basalt composition.{{sfn|Philpotts|Ague|2009|p=384}} Peridotites associated with Alaskan-type ultramafic complexes are cumulates that probably formed in the root zones of volcanoes.<ref>{{cite journal |last1=Himmelberg |first1=G.R. |last2=Loney |first2=R.A. |title=Characteristics and petrogenesis of Alaskan-type ultramafic-mafic intrusions, Southeastern Alaska |journal=U.S. Geological Survey Professional Paper |series=Professional Paper |date=1995 |volume=1564 |doi=10.3133/pp1564|hdl=2027/uc1.31210017370071 |doi-access=free |hdl-access=free }}</ref> Cumulate peridotites are also formed in [[komatiite]] lava flows.<ref>{{cite journal |last1=Szilas |first1=Kristoffer |last2=van Hinsberg |first2=Vincent |last3=McDonald |first3=Iain |last4=Næraa |first4=Tomas |last5=Rollinson |first5=Hugh |last6=Adetunji |first6=Jacob |last7=Bird |first7=Dennis |title=Highly refractory Archaean peridotite cumulates: Petrology and geochemistry of the Seqi Ultramafic Complex, SW Greenland |journal=Geoscience Frontiers |date=May 2018 |volume=9 |issue=3 |pages=689–714 |doi=10.1016/j.gsf.2017.05.003|s2cid=32485665 |doi-access=free |bibcode=2018GeoFr...9..689S }}</ref>

==Associated rocks==
[[Komatiite]]s are high temperature partial melts of peridotite characterized by a high degree of partial melting deep below the surface.{{sfn|Philpotts|Ague|2009|pp=399–400}}

[[Eclogite]] is a [[metamorphic rock]] composed primarily of [[omphacite]] (sodic clinopyroxene) and [[pyrope]]-rich garnet. Eclogite is associated with peridotite in some xenolith occurrences;{{sfn|Philpotts|Ague|2009|p=395, 602}} it also occurs with peridotite in rocks [[Eclogitization|metamorphosed]] at high pressures during processes related to subduction.{{sfn|Philpotts|Ague|2009|p=590, 598}}

==Economic geology==
Peridotite may potentially be used in a low-cost, safe and permanent method of capturing and storing atmospheric CO<sub>2</sub> as part of [[global warming|climate change]]-related [[Carbon capture and storage|greenhouse gas sequestration]].<ref>{{cite news|title=Rocks Could Be Harnessed To Sponge Vast Amounts Of Carbon Dioxide From Air|work=Science Daily|date=November 6, 2008|url=https://www.sciencedaily.com/releases/2008/11/081105180813.htm |access-date=24 February 2022}}</ref> It was already known that peridotite reacts with CO<sub>2</sub> to form a solid [[Carbonate rock|carbonate]]-like limestone or marble mineral; and this process can be sped up a million times or more by simple [[drilling]] and [[hydraulic fracturing]] to allow injection of the CO<sub>2</sub> into the subsurface peridotite formation.<ref>{{cite journal |last1=Kelemen |first1=P. B. |last2=Matter |first2=J. |title=In situ carbonation of peridotite for CO<sub>2</sub> storage |journal=Proceedings of the National Academy of Sciences |date=2008 |volume=105 |issue=45 |pages=17295–17300 | pmc=2582290 |doi=10.1073/pnas.0805794105|doi-access=free }}</ref>

Peridotite is named for the [[gemstone]] [[peridot]], a glassy green gem originally mined on [[Zabargad Island|St. John's Island in the Red Sea]]<ref>[http://www.mindat.org/loc-6423.html St. John's Island peridot information and history] at [[Mindat.org]]</ref> and now mined on the [[San Carlos Apache Indian Reservation]] in Arizona.<ref>{{cite book |quote="Although some good olive-colored crystals are found in a few other places, like Burma, China, Zambia, and Pakistan, ninety percent of all known peridots are found in just one place. It is a Native American reservation, and it is located in a little-visited corner of the United States. San Carlos" |last1=Finlay |first1=Victoria |title=Jewels: A Secret History |pages= 2543–2546 |publisher=Random House Publishing Group |edition=Kindle}}</ref>

Peridotite that has been hydrated at low temperatures is the protolith for [[serpentinite]], which may include chrysotile asbestos (a form of serpentine){{sfn|Nesse|2000|pp=241–242}} and [[talc]].{{sfn|Nesse|2000|pp=242–243}}

Layered intrusions with cumulate peridotite are typically associated with sulfide or chromite ores. Sulfides associated with peridotites form nickel ores and platinoid metals; most of the [[platinum]] used in the world today is mined from the [[Bushveld Igneous Complex]] in [[South Africa]] and the [[Great Dyke]] of [[Zimbabwe]].{{sfn|Nesse|2000|pp=387–388}} The chromite bands found in peridotites are the world's major source of [[chromium]].{{sfn|Nesse|2000|pp=361–362}}

==References==
{{Reflist}}

==Further reading==
* {{cite encyclopedia|last1=Anderson|first1=A. T. Jr. |date=2019|title=Peridotite|publisher=McGraw-Hill|encyclopedia=AccessScience|doi=10.1036/1097-8542.498300}}
* J.-L. Bodinier and M. Godard, 2004, ''Orogenic, Ophiolitic, and Abyssal Peridotites'', in ''The Mantle and Core'' (ed. R. W. Carlson), ''Treatise on Geochemistry'' v. 2, Elsevier-Pergamon, Oxford {{ISBN|0-08-043751-6}}

==External links==
{{Commons category-inline|Peridotite}}

{{Igneous rocks}}
{{Rock type}}
[[Category:Ultramafic rocks]]
[[Category:Plutonic rocks]]