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==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,
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>
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Pegmatites form under conditions in which the rate of new crystal [[nucleation]] is much less than the rate of [[crystal growth]]. In other words, molecules in the fluid from which the crystals are growing are much more likely to join an existing crystal than to come together to start a new crystal. This favors growth of a few very large crystals. In normal igneous rocks, coarse texture is a result of slow cooling deep underground. However, the extraordinarily coarse texture of pegmatites must be a result of more than just slow cooling.{{sfn|Philpotts|Ague|2009|p=259}} In fact, it is not clear if pegmatite forms by slow or rapid cooling.{{sfn|Philpotts|Ague|2009|p=257}}
Pegmatites are widely believed to form from the last part of a magma body to crystallize. This final fluid fraction is highly enriched in [[Volatiles|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 0.5 [[GPa]], but only 1.5 wt% at 0.1 GPa, for phase separation to take place.{{sfn|Philpotts|Ague|2009|p=259}}
The volatiles (primarily water, [[boron]], [[fluorine]], [[chlorine]], and [[phosphorus]]) are concentrated in the hydrous phase, greatly lowering its viscosity.<ref name=KleinHurlbut1993/> The silica in the hydrous phase is completely depolymerized, existing almost entirely as [[orthosilicate]], with all oxygen bridges between silicon ions broken.{{sfn|Philpotts|Ague|2009|p=255}} The very low viscosity promotes rapid diffusion of molecules through the fluid, enhancing crystal growth rates and allowing very large crystals to grow.<ref name=KleinHurlbut1993/>
When this hydrous fluid is injected into the surrounding [[country rock (geology)|country rock]], minerals crystallize from the outside in to form a zoned pegmatite,<ref name=KleinHurlbut1993/> with different minerals predominating in concentric zones.{{sfn|Philpotts|Ague|2009|p=255}} A typical sequence of deposition begins with [[microcline]] and quartz, with minor [[schorl]] and [[garnet]]. This is followed by deposition of [[albite]], [[lepidolite]], gem [[tourmaline]], beryl, spodumene, [[amblygonite]], [[topaz]], [[apatite]], and [[fluorite]], which may partially replace some of the minerals in the earlier zone.<ref name=KleinHurlbut1993/> The center of the pegmatite may have cavities lined with spectacular gemstone crystals.<ref>{{cite book |last1=Sinkankas |first1=John |title=Mineralogy for amateurs. |date=1964 |publisher=Van Nostrand |location=Princeton, N.J. |isbn=0442276249 |pages=90–91}}</ref>
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==Mineralogy==
[[Image:PegmatiticGranite.JPG|thumb|Pegmatitic granite with pink potassium feldspar crystals, surrounding a finer-grained [[cumulate]]-filled [[Enclave (geology)|enclave]], Rock Creek Canyon, eastern [[Sierra Nevada (U.S.)|Sierra Nevada]], California]]
Most pegmatites have a simple composition, often being composed entirely of minerals common in granite, such as feldspar, mica, and quartz.<ref name=Jackson1997/> The feldspar and quartz often show [[graphic texture]].<ref name=KleinHurlbut1993/> Rarely, pegmatites are extremely enriched in [[incompatible element]]s, such as [[lithium]], [[caesium]], [[beryllium]], [[tin]], [[niobium]], [[zirconium]], [[uranium]], [[thorium]], boron, phosphorus, and fluorine. These complex pegmatites contain unusual minerals of these elements, such as beryl, spodumene,<ref name="McBirney1984"/> lepidolite, amblygonite, topaz, apatite, fluorite, tourmaline, [[triphylite]], [[columbite]], [[monazite]], and [[molybdenite]]. Some of these can be important [[ore]] minerals.<ref name=KleinHurlbut1993/> Some [[gemstones]], such as [[emerald]], are found almost exclusively in pegmatites.
Nepheline syenite pegmatites typically contain zirconium, [[titanium]], and [[rare earth element]] minerals.<ref name=KleinHurlbut1993/>
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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.
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).
Intermediate pegmatites (NYF + LCT pegmatites) are known and may have formed by contamination of an initially NYF magma body with melted undepleted supracrustral rock.<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 are important because they often contain rare earth minerals 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|issn=1811-5209}}</ref> such as [[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|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
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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]],
==Occurrence==
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==External links==
*{{
{{igneous rocks}}
{{Authority control}}
[[Category:Igneous rocks]]
[[Category:Economic geology]]
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