Editing Syenite

{{short description|Intrusive igneous rock}}
{{Infobox rock|name=Syenite|image=Porphyritic syenite (Wausau Syenite Complex, Mesoproterozoic; Wausau, Wisconsin, USA) 2 - 52188157706.jpg|type=Igneous|composition=Plagioclase and K-Feldspars, depleted in quartz|texture=Phaneritic|Equivalent=[[Trachyte]] extrusively|class=Felsic}}
[[File:Syénite Tretorre.jpg|thumb|Syenite from Corsica]]
[[File:QAPFsyenite.gif|thumb|upright=1.5|[[QAPF diagram]] that shows the [[quartz]] (Q), [[alkali feldspar]] (A), and [[plagioclase]] (P) composition of syenite]]
[[Image:Nepheline-syenite-2005.jpg|thumb|[[Leucocratic]] variety of [[nepheline]] syenite from [[Sweden]] ([[särnaite]])]]
'''Syenite''' is a coarse-grained [[Intrusive rock|intrusive]] [[igneous rock]] with a general composition similar to that of [[granite]], but deficient in [[quartz]], which, if present at all, occurs in relatively small concentrations (< 5%). It is considered a [[granitoid]]. Some syenites contain larger proportions of [[mafic]] components and smaller amounts of [[felsic]] material than most granites; those are classed as being of [[intermediate composition]].

The [[Extrusive rock|extrusive]] equivalent of syenite is [[trachyte]].<ref>{{Cite book|last=Carracedo|first=J. C.|url=https://www.worldcat.org/oclc/951031503|title=The geology of the Canary Islands|date=2016|others=V. R. Troll|isbn=978-0-12-809664-2|location=Amsterdam, Netherlands|oclc=951031503}}</ref>

==Composition==
The [[feldspar]] component of syenite is predominantly [[alkaline]] in character (usually [[orthoclase]]). [[Plagioclase]] feldspars may be present in small proportions, between 10% and 35% of the feldspar content. Such feldspars often are interleaved as [[perthite|perthitic]] components of the rock.

When [[Mafic|ferromagnesian]] minerals are present in syenite at all, they usually occur in the form of [[amphibole]] (typically [[hornblende]]) and [[clinopyroxene]]. [[Biotite]] is rare, because in a syenite magma the formation of feldspar consumes nearly all the aluminium. However less Al-rich [[phyllosilicates]] may be included, such as [[annite]].

Other common accessory minerals are [[apatite]], [[titanite]], [[zircon]] and other opaques.

Most syenites are either [[peralkaline]] with high proportions of alkali elements relative to aluminum, or [[peraluminous]] with a higher concentration of aluminum relative to [[Alkali metal|alkali]] (predominantly K and Na) and [[Alkaline earth metal|earth-alkali]] (predominantly Ca) elements.

==Formation==

=== Partial melting ===
Syenites are products of alkaline igneous activity, generally formed in thick [[continental crust]]al areas, or in Cordilleran [[subduction]] zones. The formation of syenites can be theorized to be from the melt of granitic or igneous [[protolith]] to a fairly low degree of [[partial melting]]. This is required because potassium is an incompatible element and tends to enter a melt first, whereas higher degrees of partial melting will liberate more calcium and sodium, which produce [[plagioclase]], and hence a [[granite]], [[granodiorite]] or [[tonalite]].

At very low degrees of partial melting a [[silica]] undersaturated melt is produced, forming a [[nepheline syenite]], where orthoclase is replaced by a [[feldspathoid]] such as [[leucite]], [[nepheline]] or [[analcime]].

Conversely in certain conditions, large volumes of [[anorthite]] crystals may precipitate from thoroughly molten magma in a [[cumulate rock|cumulate]] process as it cools. This leaves a drastically reduced concentration of silica in the remainder of the melt. The segregation of the silica from the melt leaves it in a state that may favour syenite formation.{{citation needed|date=September 2022}}

=== Fractional crystallization ===
Some syenites are also theorized to be the product of the [[Fractional crystallization (geology)|fractional crystallization]] of basaltic magmas.<ref>{{cite web |title=ALEX STREKEISEN-Syenite- |url=http://www.alexstrekeisen.it/english/pluto/syenite.php}}</ref>

==Occurrence==
[[File:Bulletin 426 Plate XVI B Bowlder Outcrop of Syenite.jpg|thumb|right|Boulders of syenite near [[Concord, North Carolina|Concord]], North Carolina, c. 1910]]
Syenite is not a common rock. Regions where it occurs in significant quantities include the following.
*In the [[Kola Peninsula]] of Russia two giant [[nepheline]] syenite bodies exists making up the [[Lovozero Massif]] and the [[Khibiny Mountains]]. These syenites are part of the [[Kola Alkaline Province]].<ref>{{cite journal |last1=Downes |first1=Hilary |last2=Balaganskaya |first2=Elena |last3=Beard |first3=Andrew|last4=Liferovich |first4=Ruslan |last5=Demaiffe |first5=Daniel |date=2005 |title=Petrogenetic processes in the ultramafic, alkaline and carbonatitic magmatism in the Kola Alkaline Province: a review  |url=http://eprints.bbk.ac.uk/157/1/Downes4_Figs2-12.pdf |archive-url=https://web.archive.org/web/20170110145028/http://eprints.bbk.ac.uk/157/1/Downes4_Figs2-12.pdf |archive-date=2017-01-10 |url-status=live |journal=Lithos |volume=85 |issue= 1–4|pages=48–75 |doi= 10.1016/j.lithos.2005.03.020|bibcode=2005Litho..85...48D }}</ref> 
* In North America syenite occurs in [[Arkansas]] and [[Montana]]. Regions in [[New England]] have sizable amounts, and in [[New York (state)|New York]] syenite [[gneiss]]es occur.{{citation needed|date=December 2014}} The "great syenite dyke" extends from [[Hanging Rock, South Carolina]], through [[Taxahaw, South Carolina]], to the Brewer and Edgeworth mine in [[Chesterfield, South Carolina]].<ref name=Lieber>{{cite book|last1=Lieber|first1=Oscar Montgomery|title=Report on the Survey of South Carolina|date=1856|publisher=[[South Carolina General Assembly]]|page=32|isbn=9785880484188|url=https://books.google.com/books?id=xb8SAwAAQBAJ&q=taxahaw%20sc&pg=PA32|access-date=2 December 2014}}</ref> Syenite pebbles, containing [[fluorescent]] [[sodalite]], were moved from Canada to Michigan by glaciers;<ref>{{cite web | url=http://www.wilx.com/content/news/Yooperlite-a-new-rock-found-in-the-Upper-Peninsula-492732351.html | title='Yooperlite' a new rock found in the Upper Peninsula | publisher=wilx.com | date=September 7, 2018 | access-date=8 September 2018}}</ref> these [[glacial erratic]] pebbles have been given the trade name "yooperlite".<ref name="Yooperlite">{{cite web | url=https://www.mindat.org/min-53133.html | title=Yooperlite | publisher=Mindat.org | access-date=9 November 2019}}</ref> In other parts of the world, these types of rocks are known as sodalite-syenite and occur in Canada, India, other US states, Greenland, Malawi, and Russia.{{Citation needed|date=November 2019}}
* In Europe syenite may be found in parts of [[Switzerland]], [[Germany]], [[Norway]], [[Portugal]], [[Sweden]], [[Ben Loyal|Scotland]],<ref>{{cite book| last = Gillen | first = Con | title = Geology and landscapes of Scotland| publisher = Dunedin| edition = 2nd | date = 2013 | page = 188 | isbn = 9781780460093}}</ref> in [[Plovdiv]], Bulgaria and in [[Ditrău]], Romania. 
*In Africa there are syenite formations in [[Aswan|Aswan, Egypt]], and in [[Malawi]] in the [[Mulanje Mountain Forest Reserve]]. Syenite rock was used to make the [[Quay with Sphinxes]].
*In Australia syenite occurs as small intrusive bodies in nearly every state. In [[New South Wales]], a large syenite intruded during the breakup of [[Gondwana]] in the [[Cretaceous]].
*[[Paatusoq]] and [[Kangerluluk]] fjords in southeastern [[Greenland]], where a bay within the latter ''(Syenitbugt)'' and a headland ''(Syenitnæs)'' are named after the rock.<ref name="mapcarta">{{cite web|url=http://mapcarta.com/19184706|title=Syenitnaes|work=Mapcarta|access-date=6 May 2016}}</ref>

==Etymology==
The term ''syenite'' was originally applied to hornblende granite like that of [[Syene]] (now Aswan) in [[Egypt]], from which the name is derived.

==Episyenite==
'''Episyenite''' (or ''epi-syenite'') is a term used in [[petrology]] to describe veins, pods, or lenses of rock originally rich in [[silicon dioxide]] (SiO<sub>2</sub>) from which quartz has been severely depleted.<ref name=RecioEtal>{{cite journal |last1=Recio |first1=C. |last2=Fallick |first2=A.E. |last3=Ugidos |first3=J.M. |last4=Stephens |first4=W.E. |title=Characterization of multiple fluid-granite interaction processes in the episyenites of Avila-Béjar, Central Iberian Massif, Spain |journal=Chemical Geology |date=December 1997 |volume=143 |issue=3–4 |pages=127–144 |doi=10.1016/S0009-2541(97)00106-X|bibcode=1997ChGeo.143..127R }}</ref> This is often accompanied by strong enrichment in potassium and [[rare earth elements]], leaving the altered rock a distinctive brick red color,<ref name="McLemore-2016">{{cite journal |last1=McLemore |first1=Virginia T. |title=Episyenites in the Sevilleta National Wildlife Refuge, Socorro County, New Mexico: preliminary results |journal=New Mexico Geological Society Field Conference Series |date=2016 |volume=67 |pages=255–262 |url=https://nmgs.nmt.edu/publications/guidebooks/downloads/67/67_p0255_p0262.pdf |archive-url=https://web.archive.org/web/20200611140309/https://nmgs.nmt.edu/publications/guidebooks/downloads/67/67_p0255_p0262.pdf |archive-date=2020-06-11 |url-status=live |access-date=11 June 2020}}</ref> or by albitization (enrichment in sodium), leaving the altered rock a conspicuous white color.<ref name=SuikkanenRamo2019>{{cite journal |last1=Suikkanen |first1=E. |last2=Rämö |first2=O. T. |title=Episyenites—Characteristics, Genetic Constraints, and Mineral Potential |journal=Mining, Metallurgy & Exploration |date=October 2019 |volume=36 |issue=5 |pages=861–878 |doi=10.1007/s42461-019-00120-9|hdl=10138/306792 |s2cid=201313988 |hdl-access=free }}</ref>

Episyenites are heterogenous in their properties, but all have experienced nearly complete disappearance of quartz at sub-[[Solidus (chemistry)|solidus]] temperatures; that is, at temperatures below the melting point of the host rock. The formation of episyenites (''episyenitization'') typically takes place through leaching of quartz by mildly saline hydrothermal fluids, typically near a cooling [[intrusion]]. Because episyenitization usually takes place in [[granitoid]] rock and usually involves alkaline [[metasomatism]] (addition of alkali metal oxides to the rock) the result is a rock that has the mineral composition of an igneous syenite.<ref name=SuikkanenRamo2019/>

In addition to rare earth elements,<ref name="McLemore-2016"/> episyenites may be important sources of [[uranium]]<ref>{{cite journal |last1=Leroy |first1=J. |title=The Margnac and Fanay uranium deposits of the La Crouzille District (western Massif Central, France); geologic and fluid inclusion studies |journal=Economic Geology |date=1 December 1978 |volume=73 |issue=8 |pages=1611–1634 |doi=10.2113/gsecongeo.73.8.1611|bibcode=1978EcGeo..73.1611L }}</ref> and other valuable metals.<ref name=SuikkanenRamo2019/>

Notable occurrences of episyenite are found in the [[Hesperian Massif|Central Iberian Massif]] of Spain,<ref name=RecioEtal/> in [[Cambrian]] to [[Ordovician]] beds of New Mexico and Colorado,<ref name="McLemore-2016"/> Scandinavia, Brazil, and Ukraine.<ref name=SuikkanenRamo2019/>

==See also==
* [[List of rock types]]

==References==
* E. Wm. Heinrich. Microscopic Petrography, McGraw-Hill, 1956
{{Reflist}}

{{Rock type}}
{{Authority control}}

[[Category:Granitic rocks]]
[[Category:Industrial minerals]]