View source for Perthite

{{Short description|Intergrowth of two feldspars}}
[[File:Perthite textures.png|thumb|300px|Perthitic textures: antiperthite (1), perthite (2), and mesoperthite (3)]]
'''Perthite''' or '''perthitic texture''' is used to describe an intergrowth of two [[feldspar]]s: a host grain of [[potassium]]-rich [[alkali feldspar]] (near K-feldspar, KAlSi<sub>3</sub>O<sub>8</sub>, in composition) includes exsolved [[lamella (materials)|lamellae]] or irregular intergrowths of sodic alkali feldspar (near [[albite]], NaAlSi<sub>3</sub>O<sub>8</sub>, in composition). Typically, the host grain is [[orthoclase]] or [[microcline]], and the lamellae are albite. If sodic feldspar is the dominant phase, the result is an '''antiperthite''' and where the feldspars are in roughly equal proportions the result is a '''mesoperthite'''.<ref name="Le_Maitre_etal_2005">{{Cite book |url=https://books.google.com/books?id=u2tVu6Sbc4kC&q=le+maitre+classification+igneous+charnockite&pg=PA20 |title=Igneous Rocks: A Classification and Glossary of Terms: Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks |last1=Le Maitre R.W. |last2=Streckeisen A. |last3=Zanettin B. |last4=Le Bas M.J. |last5=Bonin B. |last6=Bateman P. |publisher=Cambridge University Press |year=2005 |isbn=9781139439398 |edition=2 |pages=20}}</ref>
The intergrowth forms by [[exsolution]] due to cooling of a grain of alkali feldspar with a composition intermediate between K-feldspar and albite. There is complete [[solid solution]] between albite and K-feldspar at temperatures near 700&nbsp;°C and pressures like those within the crust of the Earth, but a [[miscibility gap]] is present at lower temperatures. If an alkali feldspar grain with an intermediate composition cools slowly enough, K-rich and more Na-rich feldspar domains separate from one another. In the presence of water, the process occurs quickly.
[[File:Perthite 0.4mm.jpg|thumb|left|A [[photomicrograph]] of a perthite part of a feldspar grain in [[thin section]] as viewed through a [[petrographic microscope]] and with a first-order red plate. The K-feldspar host ([[orthoclase]]) appears orange, and [[albite]] [[exsolution]] [[lamella (materials)|lamellae]] appear yellow. The longer dimension of the field is 0.4&nbsp;mm.]]
When megascopically developed, the texture may consist of distinct pink and white lamellae representing exsolved white albite (NaAlSi<sub>3</sub>O<sub>8</sub>) in pink [[microcline]]. The intergrowths in perthite have a great variety of shapes. If cooling is sufficiently slow, the alkali feldspar may exsolve to form separate grains with near-endmember albite and K-feldspar compositions. The largest documented single crystal of perthite was found in Hugo Mine in [[South Dakota]] and measured about {{convert|10.7|m|abbr=on}} × {{convert|4.6|m|abbr=on}} × {{convert|1.8|m|abbr=on}}.<ref>{{cite journal| url = http://www.minsocam.org/ammin/AM66/AM66_885.pdf| journal = American Mineralogist| volume = 66| pages = 885–907| year= 1981| title= The largest crystals| author = P. C. Rickwood}}</ref>
The gem varieties of potassium feldspar, [[amazonite]] and [[moonstone (gemstone)|moonstone]] are variant colored perthites.
==References==
{{reflist}}
==External links==
*[https://web.archive.org/web/20070608214131/http://www.cst.cmich.edu/users/dietr1rv/perthite.htm R.V. Dietrich – Perthite]
[[Category:Feldspar]]
[[Category:Petrology]]