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Many of the world's largest crystals are found within pegmatites. These include crystals of [[microcline]], [[quartz]], [[mica]], [[spodumene]], [[beryl]], and [[tourmaline]]. Some individual crystals are over {{cvt|10|m|ft}} long.<ref name="Schwartz 1928">{{cite journal |last=Schwartz |first=G. |title=The Black Hills Mineral Region |journal=American Mineralogist |date=1928 |volume=13 |pages=56–63 |url=http://www.minsocam.org/msa/collectors_corner/arc/black_hills.htm}}</ref>
Most pegmatites are thought to form from the last fluid fraction of a large crystallizing [[magma]] body. This residual fluid is highly enriched in [[Volatile (astrogeology)|volatiles]] and trace elements, and its very low viscosity allows components to migrate rapidly to join an existing crystal rather than coming together to form new crystals. This allows a few very large crystals to form. While most pegmatites have a simple composition of minerals common in ordinary igneous rock, a few pegmatites have a complex composition, with numerous unusual minerals of rare elements. These complex pegmatites are mined for [[lithium]], [[beryllium]], [[boron]], [[fluorine]], [[tin]], [[tantalum]], [[niobium]], [[Rare-earth element|rare earth elements]], [[uranium]], and other valuable commodities.
==Etymology==
The word ''pegmatite'' derives from [[Homeric Greek]], πήγνυμι (''pēgnymi''), which means “to bind together”, in reference to the intertwined crystals of [[quartz]] and [[feldspar]] in the [[texture (crystalline)|texture]] known as [[graphic texture|graphic granite]].<ref name=Morgan2012>{{cite journal |last1=London |first1=David |last2=Morgan |first2=George B. |date=2012-08-01 |title=The Pegmatite Puzzle |journal=Elements |language=en |volume=8 |issue=4 |pages=263–68 |doi=10.2113/gselements.8.4.263 |bibcode=2012Eleme...8..263L |issn=1811-5209}}</ref> The term was first used by [[René Just Haüy]] in 1822 as a synonym for [[graphic granite]]. [[Wilhelm Karl Ritter von Haidinger]] first used the term in its present meaning in 1845.<ref name=Jackson1997/>
==General description==
Pegmatites are exceptionally coarse-grained [[igneous rock]]s<ref name=Jackson1997>{{cite book |editor1-last=Jackson |editor1-first=Julia A. |title=Glossary of geology. |date=1997 |publisher=American Geological Institute |location=Alexandria, Virginia |isbn=0922152349 |edition=Fourth |chapter=pegmatite}}</ref> composed of interlocking [[crystal]]s, with individual crystals usually over {{convert|1|cm|sigfig=1|sp=us}} in size and sometimes exceeding {{convert|1|m|ft|sigfig=1|sp=us}}.<ref name=BlattTracy1980>{{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=0716724383 |edition=2nd |page=73}}</ref> Most pegmatites have a composition similar to [[granite]], so that their most common minerals are [[quartz]], [[feldspar]], and [[mica]].<ref name=BlattTracy1980/><ref name=KleinHurlbut1993>{{cite book |last1=Klein |first1=Cornelis |last2=Hurlbut | first2=Cornelius S. Jr. |title=Manual of mineralogy : (after James D. Dana) |date=1993 |publisher=Wiley |location=New York |isbn=047157452X |edition=21st |page=568}}</ref> However, other pegmatite compositions are known, including compositions similar to [[nepheline syenite]]<ref name=KleinHurlbut1993/> or [[gabbro]].<ref name=BlattTracy1980/> The term ''pegmatite'' is thus purely a [[Texture (geology)|textural]] description.<ref name=PhilpottsAgue2009_255>{{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=9780521880060 |edition=2nd |page=255}}</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=20–21}}</ref> Geologists typically prefix the term with a compositional description, so that ''granitic pegmatite'' is a pegmatite with the composition of granite while ''nepheline syenite pegmatite'' is a pegmatite with the composition of nepheline syenite.<ref name=PhilpottsAgue2009_255/> However, the [[British Geological Survey]] (BGS) discourages this usage, preferring terms like ''biotite-quartz-feldspar pegmatite'' for a pegmatite with a typical granitic composition, dominated by feldspar with lesser quartz and biotite. Under BGS terminology, a ''pegmatitic rock'' (for example, a ''pegmatitic gabbro'') is a coarse-grained rock containing patches of much coarser-grained rock of essentially the same composition.<ref name="bgs"/>
Individual crystals in pegmatites can be enormous in size. It is likely that the largest crystals ever found were feldspar crystals in pegmatites from [[Karelia]] with masses of thousands of tons. Quartz crystals with masses measured in thousands of pounds<ref name=KleinHurlbut1993/> and micas over {{convert|10|m|ft||sp=us}} across and {{convert|4|m|ft||sp=us}} thick have been found.<ref name="McBirney1984">{{cite book |last1=McBirney |first1=Alexander R. |title=Igneous petrology |date=1984 |publisher=Freeman, Cooper |location=San Francisco, Calif. |isbn=0198578105 |pages=349–350}}</ref> [[Spodumene]] crystals over {{convert|40|ft|m|order=flip|sp=us}} long have been found in the [[Black Hills]] of [[South Dakota]], and [[beryl]] crystals {{convert|27|ft|m|order=flip|sp=us}} long and {{convert|6|ft|m|order=flip|sp=us}} in diameter have been found at [[Albany, Maine]].<ref name=KleinHurlbut1993/> The largest beryl crystal ever found was from Malakialina on Madagascar, weighing about 380 tons, with a length of {{cvt|18|m|ft}} and a crosscut of {{cvt|3.5|m|ft}}.<ref>{{cite web |url=http://minsocam.org/MSA/collectors_corner/arc/large_crystals.htm |title=The largest crystals by Peter C. Rickwood |publisher=American Mineralogist}}</ref>
Pegmatite bodies are usually of minor size compared to typical [[intrusive rock]] bodies. Pegmatite body size is on the order of magnitude of one to a few hundred meters. Compared to typical igneous rocks they are rather [[Homogeneity and heterogeneity|inhomogeneous]] and may show zones with different mineral assemblages. Crystal size and mineral assemblages are usually oriented parallel to the wall rock or even concentric for pegmatite lenses.<ref name="London 2012">{{cite journal|last=London|first=D.|author2=Kontak, D. J.|title=Granitic Pegmatites: Scientific Wonders and Economic Bonanzas|journal=Elements|date=3 September 2012|volume=8|issue=4|pages=257–261|doi=10.2113/gselements.8.4.257|bibcode=2012Eleme...8..257L }}</ref>
== Classification ==
Modern pegmatite classification schemes are strongly influenced by the depth-zone classification of granitic rocks published by Buddington (1959), and the Ginsburg & Rodionov (1960) and Ginsburg et al. (1979) classification which categorized pegmatites according to their depth of emplacement and relationship to metamorphism and granitic plutons. Cerny’s (1991) revision of that classification scheme is widely used, Cerny’s (1991) pegmatite classification, which is a combination of emplacement depth, metamorphic grade and minor element content, has provided significant insight into the origin of pegmatitic melts and their relative degrees of fractionation.<ref>{{Cite journal |last1=Cerny |first1=P. |last2=Ercit |first2=T. S. |date=2005-12-01 |title=The Classification of Granitic Pegmatites Revisited |url=http://www.canmin.org/cgi/doi/10.2113/gscanmin.43.6.2005 |journal=The Canadian Mineralogist |language=en |volume=43 |issue=6 |pages=2005–2026 |doi=10.2113/gscanmin.43.6.2005 |bibcode=2005CaMin..43.2005C |s2cid=129967533 |issn=0008-4476}}</ref>
Granitic pegmatites are commonly ranked into three hierarchies (class – family – type – subtype) depending upon their mineralogical-geochemical characteristics and depth of emplacement according to Cerny (1991). Classes are Abyssal, Muscovite, Rare-Element and Miarolitic. The Rare-Element Class is subdivided based on composition into LCT and NYF families: LCT for Lithium, Cesium, and Tantalum enrichment and NYF for Niobium, Yttrium, and Fluorine enrichment. Most authors classify pegmatites according to LCT- and NYF-types and subtypes. Another important contribution of the classification is the petrogenetic component of the classification, which shows the association of LCT pegmatites with mainly orogenic plutons, and NYF pegmatites with mainly anorogenic plutons.<ref>{{Cite journal |last1=Simmons |first1=Wm. B. Skip |last2=Webber |first2=Karen L. |date=2008-08-29 |title=Pegmatite genesis: state of the art |url=http://www.schweizerbart.de/papers/ejm/detail/20/58158/Pegmatite_genesis_state_of_the_art?af=crossref |journal=European Journal of Mineralogy |language=en |volume=20 |issue=4 |pages=421–438 |doi=10.1127/0935-1221/2008/0020-1833 |bibcode=2008EJMin..20..421S |issn=0935-1221}}</ref>
Lately, there have been a few attempts to create a new classification for pegmatites less dependent on mineralogy and more reflective of their geological setting. On this issue, one of the most notable efforts on this matter is Wise's (2022) pegmatite classification, which focuses mostly on the source of the magma from which the pegmatite crystalizes.<ref>{{cite journal |last1=Wise |first1=Michael A. |title=A proposed new mineralogical classification system for granitic pegmatites |journal=The Canadian Mineralogist |date=2022 |volume=60 |issue=2 |pages=229–248 |doi=10.3749/canmin.1800006 |bibcode=2022CaMin..60..229W }}</ref>
==Petrology==
[[File:Muscovite-157168.jpg|thumb|Rose muscovite from the Harding pegmatite mine]]
[[File:Harding Mine apatite.jpg|thumb|Blue apatite crystals at the Harding pegmatite mine]]
Pegmatites form under conditions in which the rate of new crystal [[nucleation]] is much slower than the rate of [[crystal growth]]. Large crystals are favored. In normal igneous rocks, coarse texture is a result of slow cooling deep underground.{{sfn|Philpotts|Ague|2009|p=259}} It is not clear if pegmatite forms by slow or rapid cooling.{{sfn|Philpotts|Ague|2009|p=257}} In some studies, crystals in pegmatitic conditions have been recorded to grow at a rate ranging from 1 m to 10 m per day.<ref>{{cite journal |last1=Phelps |first1=Patrick R. |last2=Lee |first2=Cin-Ty A. |last3=Morton |first3=Douglas M. |title=Episodes of fast crystal growth in pegmatites |journal=Nature Communications |date=5 October 2020 |volume=11 |issue=1 |pages=4986 |doi=10.1038/s41467-020-18806-w|pmid=33020499 |pmc=7536386 |bibcode=2020NatCo..11.4986P }}</ref>
Pegmatites are the last part of a magma body to crystallize. This final fluid fraction is enriched in [[Volatile (astrogeology)|volatile]] and trace elements.<ref name=Allaby2013>{{cite book |last1=Allaby |first1=Michael |title=A dictionary of geology and earth sciences |date=2013 |publisher=Oxford University Press |location=Oxford |isbn=9780199653065 |edition=Fourth |chapter=pegmatite}}</ref><ref name=Jackson1997/> The residual magma undergoes [[phase separation]] into a melt phase and a hydrous fluid phase saturated with [[silica]], [[Alkali metal|alkalis]], and other elements.<ref name="McBirney1984"/>{{sfn|Philpotts|Ague|2009|p=256}} Such phase separation requires formation from a wet magma, rich enough in water to saturate before more than two-thirds of the magma is crystallized. Otherwise, the separation of the fluid phase is difficult to explain. Granite requires a water content of 4 [[wt%]] at a pressure of {{convert|0.5|GPa|psi|abbr=on|sigfig=3|lk=on}}, but only 1.5 wt% at {{convert|0.1|GPa|psi|abbr=on|sigfig=3}} for phase separation to take place.{{sfn|Philpotts|Ague|2009|p=259}}
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Large crystals nucleate on the margins of pegmatites, becoming larger as they grow inward. These include very large conical alkali feldspar crystals. [[Aplite]]s are commonly present. These may cut across the pegmatite, but also form zones or irregular patches around coarser material. The aplites are often layered, showing evidence of deformation.{{sfn|Philpotts|Ague|2009|p=255}} [[Xenoliths]] may be found in the body of the pegmatite, but their original mineral content is replaced by quartz and alkali feldspar, so that they are difficult to distinguish from the surrounding pegmatite. Pegmatite also commonly replaces part of the surrounding country rock.{{sfn|Philpotts|Ague|2009|p=255}}
Because pegmatites likely crystallize from a fluid-dominated phase, rather than a melt phase, they straddle the boundary between [[hydrothermal mineral deposit]]s and [[igneous intrusion]]s.<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=20–21}}</ref> Although there is broad agreement on the basic mechanisms by which they form, the details of pegmatite formation remain enigmatic.<ref name=Morgan2012>{{cite journal |last1=London |first1=David |last2=Morgan |first2=George B. |date=2012-08-01 |title=The Pegmatite Puzzle |journal=Elements |language=en |volume=8 |issue=4 |pages=263–68 |doi=10.2113/gselements.8.4.263 |bibcode=2012Eleme...8..263L |issn=1811-5209}}</ref> Pegmatites have characteristics inconsistent with other igneous intrusions. They are not [[porphyritic]], and show no [[chilled margin]]. On the contrary, the largest crystals are often found on the margins of the pegmatite body. While aplites are sometimes found on the margins, they are as likely to occur within the body of the pegmatite. The crystals are never aligned in a way that would indicate flow, but are perpendicular to the walls. This implies formation in a static environment. Some pegmatities take the form of isolated pods, with no obvious feeder conduit.{{sfn|Philpotts|Ague|2009|pp=255-256}} As a result, [[metamorphic]] or [[metasomatic]] origins have sometimes been suggested for pegmatites. A metamorphic pegmatite would be formed by removal of [[Volatile (astrogeology)|volatiles]] from metamorphic rocks, particularly felsic [[gneiss]], to liberate the right constituents and water, at the right temperature. A metasomatic pegmatite would be formed by [[hydrothermal circulation]] of hot alteration fluids upon a rock mass, with bulk chemical and textural change. Metasomatism is currently not favored as a mechanism for pegmatite formation and it is likely that metamorphism and magmatism are both contributors toward the conditions necessary for pegmatite genesis.<ref name=Morgan2012/>
==Mineralogy==
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Pegmatites are enriched in volatile and [[incompatible element]]s, consistent with their likely origin as the final melt fraction of a crystallizing body of magma.<ref name=KleinHurlbut1993/> However, it is difficult to get a representative composition of a pegmatite, due to the large size of the constituent mineral crystals. Hence, pegmatite is often characterised by sampling the individual minerals that compose the pegmatite, and comparisons are made according to mineral chemistry. A common error is to assume that the wall zone is a chilled margin whose composition is representative of the original melt.<ref name=Ercit2005>{{cite book |last1=Ercit |first1=T.S. |year=2005 |chapter=REE-enriched granitic pegmatites |editor-last1=Linnen |editor-first1=R.L. |editor-last2=Samson |editor-first2=I.M. |title=Rare-Element Geochemistry and Mineral Deposits (GAC Short Course Notes 17) |publisher=Geological Association of Canada |pages=175–199 |url=https://www.researchgate.net/publication/269701071 |access-date=23 December 2021}}</ref>
Pegmatites derived from batholiths can be divided into a family of NYF pegmatites, characterized by progressive enrichment in [[niobium]], [[yttrium]], and fluorine as well as enrichment in beryllium, rare earth elements, [[scandium]], titanium, zirconium, thorium, and uranium; and a family of LCT pegmatites, characterized by progressive accumulation of lithium, [[caesium]], and tantalum, as well as enrichment in [[rubidium]], beryllium, tin, barium, phosphorus, and fluorine.<ref name="CernyErcit2005">{{cite journal |last1=Cerny |first1=P. |last2=Ercit |first2=T. S. |title=The Classification of Granitic Pegmatites Revisited |journal=The Canadian Mineralogist |date=1 December 2005 |volume=43 |issue=6 |pages=2005–2026 |doi=10.2113/gscanmin.43.6.2005|bibcode=2005CaMin..43.2005C |s2cid=129967533 }}</ref>
The NYF pegmatites likely fractionated from A- to I-type granites that were relatively low in aluminium (subaluminous to metaluminous granites). These granites originated from depleted crust or mantle rock. LCT pegmatites most likely formed from S-type granites or possibly I-type granites, with a higher aluminium content (peraluminous granites).<ref name="CernyErcit2005"/>
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==Economic importance==
[[File:USGS-PP-1802k-K13.png|thumb|upright=2.5|Scatter plots of lithium grade and tonnage for selected world deposits, as of 2017]]
Pegmatites often contain rare elements and [[gemstone]]s.<ref>{{Cite journal|last1=Simmons|first1=W. B.|last2=Pezzotta|first2=F.|last3=Shigley|first3=J. E.|last4=Beurlen|first4=H.|date=2012-08-01|title=Granitic Pegmatites as Sources of Colored Gemstones|journal=Elements|language=en|volume=8|issue=4|pages=281–287|doi=10.2113/gselements.8.4.281|bibcode=2012Eleme...8..281S |issn=1811-5209}}</ref> Examples include [[Beryl#Aquamarine and maxixe|aquamarine]], tourmaline, topaz, fluorite, apatite, and [[corundum]], often along with [[tin]], rare earth, and [[tungsten]] minerals, among others.<ref name=Allaby2013/><ref name=Jackson1997/> Pegmatites have been mined for both quartz and feldspar.<ref name=Almq>{{Cite book|title=Nyttosten i Sverige|last1=Lundegårdh|first1=Per H.|publisher=[[Almqvist & Wiksell]]|year=1971|isbn=|location=Stockholm|language=Swedish|pages=16–17}}</ref> For quartz mining, pegmatites with central quartz masses have been of particular interest.<ref name=Almq/>
Pegmatites are the primary source of [[lithium]] either as spodumene, [[lithiophyllite]] or usually from lepidolite.<ref>{{Cite journal|last1=Linnen|first1=R. L.|last2=Lichtervelde|first2=M. Van|last3=Cerny|first3=P.|date=2012-08-01|title=Granitic Pegmatites as Sources of Strategic Metals|journal=Elements|language=en|volume=8|issue=4|pages=275–280|doi=10.2113/gselements.8.4.275|bibcode=2012Eleme...8..275L |issn=1811-5209}}</ref> The primary source for [[caesium]] is [[pollucite]], a mineral from a zoned pegmatite.<ref name="Cerny">{{cite journal|title=The Tanco Pegmatite at Bernic Lake, Manitoba: X. Pollucite|first1=Petr|last1=Černý|author-link1=Petr Černý|first2=F. M.|last2=Simpson|journal=Canadian Mineralogist|volume=16|pages=325–333|date=1978|url=http://rruff.geo.arizona.edu/doclib/cm/vol38/CM38_877.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://rruff.geo.arizona.edu/doclib/cm/vol38/CM38_877.pdf |archive-date=2022-10-09 |url-status=live|access-date=2010-09-26}}</ref> The majority of the world's beryllium is sourced from non-gem quality beryl within pegmatite.<ref name="deGruyter">{{cite book
| others=trans. rev. Eagleson, Mary
| editor1-first=Hans-Dieter | editor1-last=Jakubke
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| publisher=Walter de Gruyter
| location=Berlin
| date=1994}}</ref> Tantalum, niobium, and rare-earth elements are sourced from a few pegmatites worldwide, such as the [[Greenbushes mine|Greenbushes Pegmatite]],<ref name=":1">{{Cite journal|last1=Partington|first1=G. A.|last2=McNaughton|first2=N. J.|last3=Williams|first3=I. S.|date=1995-05-01|title=A review of the geology, mineralization, and geochronology of the Greenbushes Pegmatite, Western Australia|url=http://dx.doi.org/10.2113/gsecongeo.90.3.616|journal=Economic Geology|volume=90|issue=3|pages=616–635|doi=10.2113/gsecongeo.90.3.616|bibcode=1995EcGeo..90..616P |issn=1554-0774}}</ref> the Kibara Belt of [[Rwanda]] and [[Democratic Republic of the Congo]], the Kenticha mine of [[Ethiopia]] the Alto Ligonha Province of [[Mozambique]],<ref>{{cite journal |last1=Melcher |first1=F. |last2=Graupner |first2=T. |last3=Oberthür |first3=T. |last4=Schütte |first4=P. |title=Tantalum-(niobium-tin) mineralisation in pegmatites and rare-metal granites of Africa |journal=South African Journal of Geology |date=1 March 2017 |volume=120 |issue=1 |pages=77–100 |doi=10.25131/gssajg.120.1.77|bibcode=2017SAJG..120...77M }}</ref> and the Mibra (Volta) mine of [[Minas Gerais]], Brazil.<ref>{{cite book|last1=Linnen |first1=Robert |first2=David L. |last2=Trueman |first3=Richard |last3=Burt |chapter=Tantalum and niobium |title=Critical metals handbook |year=2014 |pages=361–384 |url=https://mmsallaboutmetallurgy.com/wp-content/uploads/2019/07/Critical-Metals-Handbook.pdf#page=373 |archive-url=https://ghostarchive.org/archive/20221009/https://mmsallaboutmetallurgy.com/wp-content/uploads/2019/07/Critical-Metals-Handbook.pdf#page=373 |archive-date=2022-10-09 |url-status=live |access-date=29 July 2022}}</ref>
==Occurrence==
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