Editing Gneiss (section)

==Description==
[[image:Orthogneiss Geopark.jpg|thumb|Orthogneiss from the Czech Republic]]
In traditional English and North American usage, a gneiss is a coarse-grained [[metamorphic rock]] showing compositional banding ([[gneissic banding]]) but poorly developed [[schistosity]] and indistinct [[Cleavage (geology)|cleavage]]. In other words, it is a metamorphic rock composed of mineral grains easily seen with the unaided eye, which form obvious compositional layers, but which has only a weak tendency to fracture along these layers. In Europe, the term has been more widely applied to any coarse, [[mica]]-poor, high-grade metamorphic rock.<ref>{{cite book |last1=Yardley |first1=B. W. D. |title=An introduction to metamorphic petrology |date=1989 |publisher=Longman Scientific & Technical |location=Harlow, Essex, England |isbn=0582300967 |page=22}}</ref>

The [[British Geological Survey]] (BGS) and the [[IUGS|International Union of Geological Sciences]] (IUGS) both use ''gneiss'' as a broad [[Texture (geology)|textural]] category for medium- to coarse-grained metamorphic rock that shows poorly developed schistosity, with compositional layering over {{convert|5|mm||sp=us}} thick<ref name="BGS">{{cite journal |last1=Robertson |first1=S. |title=BGS Rock Classification Scheme, Volume 2: Classification of metamorphic rocks |journal=British Geological Survey Research Report |date=1999 |volume=RR 99-02 |url=http://nora.nerc.ac.uk/id/eprint/3226/1/RR99002.pdf |access-date=27 February 2021 |ref={{harvid|British Geological Survey|1999}}}}</ref> and tending to split into plates over {{convert|1|cm||sp=us}} thick.<ref name="schid-etal=2007">{{cite book |last1=Schmid |first1=R. |last2=Fettes |first2=D. |last3=Harte |first3=B. |last4=Davis |first4=E. |last5=Desmons |first5=J. |year=2007 |chapter=How to name a metamorphic rock. |title=Metamorphic Rocks: A Classification and Glossary of Terms: Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Metamorphic Rocks |publisher=Cambridge University Press |location=Cambridge |pages=3–15 |url=https://stuff.mit.edu/afs/athena.mit.edu/course/12/12.115/www/12.114%20Papers/BGS/Metamorphic/DetailedMetamorphic/1%20How%20to%20name%20a%20metamorphic%20rock.pdf |access-date=28 February 2021}}</ref> Neither definition depends on composition or origin, though rocks poor in platy [[minerals]] are more likely to produce gneissose texture. Gneissose rocks thus are largely [[Recrystallization (geology)|recrystallized]] but do not carry large quantities of micas, [[Chlorite group|chlorite]] or other platy minerals.<ref>{{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=360}}</ref> Metamorphic rock showing stronger schistosity is classified as schist, while metamorphic rock devoid of schistosity is called a [[granofels]].<ref name="BGS"/><ref name="schid-etal=2007"/>

Gneisses that are metamorphosed igneous rocks or their equivalent are termed [[granite]] gneisses, [[diorite]] gneisses, and so forth. Gneiss rocks may also be named after a characteristic component such as [[garnet]] gneiss, [[biotite]] gneiss, [[albite]] gneiss, and so forth. '''Orthogneiss''' designates a gneiss derived from an [[igneous rock]], and '''paragneiss''' is one from a [[sedimentary rock]].<ref name="BGS"/><ref name="schid-etal=2007"/>
Both the BGS and the IUGS use ''gneissose'' to describe rocks with the texture of gneiss,<ref name="BGS"/><ref name="schid-etal=2007"/> though ''gneissic'' also remains in common use.<ref>{{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=Gneissic}}</ref> For example, a gneissose metagranite or a gneissic metagranite both mean a granite that has been metamorphosed and thereby acquired gneissose texture.

===Gneissic banding===
[[File:Pure shear.png|thumb|Pure shear deformation of rock producing gneissic banding. The undeformed rock is shown at upper left, and the result of pure shear deformation at upper right. At lower left is the stretching component of the deformation, which compresses the rock in one direction and stretches it in the other, as shown by the arrows. The rock is simultaneously rotated to produce the final configuration, repeated at lower right.]]
The minerals in gneiss are arranged into layers that appear as bands in cross section. This is called gneissic banding.<ref name="Marshak 2009">{{Cite book | last = Marshak | first = Stephen | year = 2013 | title = Essentials of Geology | edition = 4th | publisher = W.W. Norton | isbn = 978-0-393-91939-4 |pages=194–95; Figs. 7.6a–c}}</ref> The darker bands have relatively more [[mafic]] minerals (those containing more [[magnesium]] and [[iron]]). The lighter bands contain relatively more [[felsic]] minerals (minerals such as feldspar or [[quartz]], which contain more of the lighter elements, such as [[aluminium]], [[sodium]], and [[potassium]]).{{sfn|Yardley|1989|p=22}}

The banding is developed at high temperature when the rock is more strongly compressed in one direction than in other directions (''nonhydrostatic stress''). The bands develop perpendicular to the direction of greatest compression, also called the shortening direction, as platy minerals are rotated or recrystallized into parallel layers.{{sfn|Blatt|Tracy|1996|p=359}}

A common cause of nonhydrodynamic stress is the subjection of the [[protolith]] (the original rock material that undergoes metamorphism) to extreme shearing force, a sliding force similar to the pushing of the top of a deck of cards in one direction, and the bottom of the deck in the other direction.<ref name="Marshak 2009"/> These forces stretch out the rock like a [[Plasticity (physics)|plastic]], and the original material is spread out into sheets. Per the [[polar decomposition theorem]], the deformation produced by such shearing force is equivalent to rotation of the rock combined with shortening in one direction and extension in another.<ref>{{cite book |last1=Fossen |first1=Haakon |title=Structural geology |date=2016 |publisher=Cambridge University Press |location=Cambridge, United Kingdom |isbn=9781107057647 |page=38 |edition=Second}}</ref>

Some banding is formed from original rock material (protolith) that is subjected to extreme temperature and pressure and is composed of alternating layers of [[sandstone]] (lighter) and [[shale]] (darker), which is metamorphosed into bands of [[quartzite]] and mica.<ref name="Marshak 2009"/>

Another cause of banding is "metamorphic differentiation", which separates different materials into different layers through chemical reactions, a process not fully understood.<ref name="Marshak 2009"/>

===Augen gneiss===
<!-- This section is the target of a redirect from [[Augen gneiss]]. -->
{{See also|Augen}}
[[File:Augen-gneiss.jpg|thumb|Augen gneiss from {{Lang|pt|[[Leblon]]|italic=no}}, Rio de Janeiro City, Brazil]]
[[File:Orthogneiss oeillé - Ras Prat Cabrera (Canigou).jpg|thumb|Ordovician augen gneiss outcrop, [[Canigó]] massif, eastern Pyrenees, France]]
'''Augen gneiss''', from the {{langx|de|Augen}} {{IPA|de|ˈaʊɡən|}}, meaning "eyes", is a gneiss resulting from metamorphism of granite, which contains characteristic elliptic or [[Lens (geology)|lenticular]] shear-bound grains ([[porphyroclast]]s), normally [[feldspar]], surrounded by finer grained material. The finer grained material deforms around the more resistant feldspar grains to produce this texture.{{sfn|Blatt|Tracy|1996|pp=358-359}}

===Migmatite===
{{Main|Migmatite}}
Migmatite is a gneiss consisting of two or more distinct rock types, one of which has the appearance of an ordinary gneiss (the ''mesosome''), and another of which has the appearance of an [[intrusive rock]] such [[pegmatite]], [[aplite]], or granite (the ''leucosome''). The rock may also contain a ''melanosome'' of mafic rock complementary to the leucosome.{{sfn|British Geological Survey|1999|p=11}} Migmatites are often interpreted as rock that has been partially melted, with the leucosome representing the silica-rich melt, the melanosome the residual solid rock left after partial melting, and the mesosome the original rock that has not yet experienced partial melting.<ref>{{cite book |last1=Sawyer |first1=E. W. |title=Atlas of migmatites |date=2008 |publisher=NRC Research Press |location=Ottawa, Ontario |isbn=978-0660197876}}</ref>