Editing Pegmatite (section)

==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}}

The volatiles (primarily water, [[borate]]s, [[fluoride]]s, [[chloride]]s, and [[phosphate]]s) 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 low viscosity promotes rapid diffusion through the fluid, allowing growth of large crystals.<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>

Some pegmatites have more complex zoning. Five distinct zones are recognized in the [[Harding Pegmatite Mine|Harding Pegmatite]] in the [[Picuris Mountains]] of northern [[New Mexico]], US. These are:<ref name="je1976">{{cite journal |last1=Jahns |first1=Richard H. |last2=Ewing |first2=Rodney C. |title=The Harding Mine Taos County New Mexico |journal=New Mexico Geological Society Field Conference Series |date=1976 |volume=27 |page=263 |url=https://earth315.earth.lsa.umich.edu/Earth_315_Field_Trip_-_New_Mexico/References_files/Jahns76_NWGS_HardingMine.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://earth315.earth.lsa.umich.edu/Earth_315_Field_Trip_-_New_Mexico/References_files/Jahns76_NWGS_HardingMine.pdf |archive-date=2022-10-09 |url-status=live}}</ref>
* A white border rind of fine-grained quartz-albite [[muscovite]] pegmatite.
* A continuous layer of very coarse quartz, albite, and muscovite. This zone also contains microcline, and has abundant accessory apatite, beryl, and [[tantalite]]. Beryl is occasionally very coarse and abundant.
* A continuous layer of massive quartz. This zone is also rich in muscovite, microcline, and [[cleavelandite]].
* A spectacular quartz and lath-spodumene zone. The spodumene occurs as blade-like crystals, sometimes of enormous size, mostly oriented at random but sometimes arranged to form a comb-like structure. Accessory minerals are beryl, apatite, microcline, and tantalum-niobium minerals, especially in the lower part of this zone. There is some pseudomorphic replacement of spodumene by rose muscovite and quartz by cleavelandite.
* The core of the pegmatite, known as "spotted rock", which is relatively fine-grained spodumene, microcline, and quartz, with accompanying finer-grained albite, lithium-bearing muscovite, lepidolite, [[microlite]], and tantalite. Much of the spodumene and microcline have been extensively corroded and replaced by fine-grained micas.

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/>