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{{Short description|Easily split medium-grained metamorphic rock}}
[[File:Schist detail.jpg|thumb|A schist showing characteristic scaly schistose texture caused by platy [[mica|micas]]]]
'''Schist''' ({{IPAc-en|'|ʃ|ɪ|s|t}} {{respell|SHIST}}) is a medium-grained [[metamorphic rock]] showing pronounced '''schistosity'''. This means that the rock is composed of [[mineral]] grains easily seen with a low-power [[hand lens]], oriented in such a way that the rock is easily split into thin flakes or plates. This [[texture (geology)|texture]] reflects a high content of platy minerals, such as [[mica]]s, [[talc]], [[chlorite group|chlorite]], or [[graphite]]. These are often interleaved with more granular minerals, such as [[feldspar]] or [[quartz]].

Schist typically forms during [[regional metamorphism]] accompanying the process of mountain building ([[orogeny]]) and usually reflects a medium [[Metamorphism#Metamorphic grades|grade]] of metamorphism. Schist can form from many different kinds of rocks, including [[sedimentary rock]]s such as [[mudstone]]s and [[igneous rock]]s such as [[tuff]]s. Schist metamorphosed from mudstone is particularly common and is often very rich in mica (a ''mica schist''). Where the type of the original rock (the [[protolith]]) is discernible, the schist is usually given a name reflecting its protolith, such as ''schistose metasandstone''. Otherwise, the names of the constituent minerals will be included in the rock name, such as ''quartz-felspar-biotite schist''.

Schist [[bedrock]] can pose a challenge for [[civil engineering]] because of its pronounced [[Fracture (geology)|planes of weakness]].

== Etymology ==
The word ''schist'' is derived ultimately from the [[Greek language|Greek]] word σχίζειν (''schízein''), meaning "to split",<ref name="OD">{{Cite dictionary |url=http://www.lexico.com/definition/schist |archive-url=https://web.archive.org/web/20200127112823/https://www.lexico.com/definition/schist |url-status=dead |archive-date=2020-01-27 |title=schist |dictionary=[[Lexico]] UK English Dictionary |publisher=[[Oxford University Press]]}}</ref> which refers to the ease with which schists can be split along the plane in which the platy minerals lie.

== Definition ==
Geologists define schist as medium-grained [[metamorphic rock]] that shows well-developed schistosity.{{sfn|British Geological Survey|1999 |p=3}} Schistosity is a thin layering of the rock produced by [[metamorphism]] (a ''[[foliation (geology)|foliation]]'') that permits the rock to easily be split into flakes or slabs less than {{convert|5 to 10|mm|1|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 |page=7 |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 |archive-url=https://web.archive.org/web/20210624213741/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 |archive-date=2021-06-24 |url-status=live |access-date=28 February 2021}}</ref><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 |archive-url=https://web.archive.org/web/20180403144017/http://nora.nerc.ac.uk/id/eprint/3226/1/RR99002.pdf |archive-date=2018-04-03 |url-status=live |access-date=27 February 2021 |ref={{harvid|British Geological Survey|1999}} |page=5}}</ref> The mineral grains in a schist are typically from {{convert|0.25 to 2|mm|2|sp=us}} in size{{sfn|British Geological Survey|1999 |p=24}} and so are easily seen with a 10× [[hand lens]].<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> Typically, over half the mineral grains in a schist show a preferred orientation. Schists make up one of the three divisions of metamorphic rock by [[Texture (geology)|texture]], with the other two divisions being [[gneiss]], which has poorly developed schistosity and thicker layering, and [[granofels]], which has no discernible schistosity.<ref name="schid-etal=2007"/>{{sfn|British Geological Survey|1999 |pp=5-6}}

Schists are defined by their texture, without reference to their composition,<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=schist}}</ref><ref name="schid-etal=2007"/> and while most are a result of medium-grade metamorphism, they can vary greatly in mineral makeup.{{sfn|British Geological Survey|1999 |p=5}} However, schistosity normally develops only when the rock contains abundant platy minerals, such as [[mica]]s or [[Chlorite group|chlorite]]. Grains of these minerals are strongly oriented in a preferred direction in schist, often also forming very thin parallel layers. The ease with which the rock splits along the aligned grains accounts for the schistosity.<ref name="schid-etal=2007"/> Though not a defining characteristic, schists very often contain ''[[porphyroblast]]s'' (individual crystals of unusual size) of distinctive minerals, such as [[garnet]], [[staurolite]], [[kyanite]], [[sillimanite]], or [[cordierite]].{{sfn|Blatt|Tracy|1996|p=365}}

{{anchor|Paraschist}}{{anchor|Orthoschist}}
Because schists are a very large class of metamorphic rock, geologists will formally describe a rock as a schist only when the original type of the rock prior to [[metamorphism]] (the [[protolith]]) is unknown and its mineral content is not yet determined. Otherwise, the modifier ''schistose'' will be applied to a more precise type name, such as ''schistose [[semipelite]]'' (when the rock is known to contain moderate amounts of [[mica]]) or a ''schistose metasandstone'' (if the protolith is known to have been a [[sandstone]]).{{sfn|British Geological Survey|1999 |pp=3-4}} If all that is known is that the protolith was a sedimentary rock, the schist will be described as a '''paraschist''', while if the protolith was an igneous rock, the schist will be described as an '''orthoschist'''.{{sfn|British Geological Survey|1999 |pp=5-7}} Mineral qualifiers are important when naming a schist. For example, a quartz-feldspar-biotite schist is a schist of uncertain protolith that contains [[biotite]] mica, [[feldspar]], and [[quartz]] in order of apparent decreasing abundance.{{sfn|British Geological Survey|1999 |p=8}}

''Lineated schist'' has a strong linear fabric in a rock which otherwise has well-developed schistosity.{{sfn|British Geological Survey|1999 |p=5}}

===Historical mining terminology===
Before the mid-19th century, the terms slate, shale and schist were not sharply differentiated by those involved with mining.<ref>{{cite book |first=R. W. |last=Raymond |chapter=Slate |chapter-url=https://books.google.com/books?id=FpqFAAAAIAAJ&pg=PA78 |title=A Glossary of Mining and Metallurgical Terms |publisher=American Institute of Mining Engineers |year=1881 |page=78}}</ref>

==Formation==
Schistosity is developed at elevated temperature when the rock is more strongly compressed in one direction than in other directions (''nonhydrostatic stress''). Nonhydrostatic stress is characteristic of [[regional metamorphism]] where mountain building is taking place (an [[orogenic belt]]). The schistosity develops 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}} While platy or elongated minerals are most obviously reoriented, even quartz or calcite may take up preferred orientations.<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 |pages=168–169}}</ref>

At the microscopic level, schistosity is divided into ''internal schistosity'', in which inclusions within porphyroblasts take a preferred orientation, and ''external schistosity'', which is the orientation of grains in the surrounding medium-grained rock.{{sfn|Yardley|1989|p=171}}

The composition of the rock must permit formation of abundant platy minerals. For example, the [[clay minerals]] in mudstone are metamorphosed to mica, producing a mica schist.<ref>{{cite book |last1=Potter |first1=Paul Edwin |last2=Maynard |first2=J. Barry |last3=Pryor |first3=Wayne A. |title=Sedimentology of shale : study guide and reference source |date=1980 |publisher=Springer-Verlag |location=New York |isbn=0387904301 |page=17}}</ref> Early stages of metamorphism convert mudstone to a very fine-grained metamorphic rock called ''[[slate]]'', which with further metamorphism becomes fine-grained ''[[phyllite]]''. Further recrystallization produces medium-grained mica schist. If the metamorphism proceeds further, the mica schist experiences [[dehydration reaction]]s that convert platy minerals to granular minerals such as feldspars, decreasing schistosity and turning the rock into a gneiss.{{sfn|Blatt|Tracy|1996|p=365}}

Other platy minerals found in schists include chlorite, talc, and graphite. Chlorite schist is typically formed by metamorphism of [[ultramafic]] igneous rocks,<ref>{{cite journal |last1=Nokleberg |first1=Warren J. |last2=Jones |first2=David L. |last3=Silberling |first3=Norman J. |title=Origin and tectonic evolution of the Maclaren and Wrangellia terranes, eastern Alaska Range, Alaska |journal=GSA Bulletin |date=1 October 1985 |volume=96 |issue=10 |pages=1251–1270 |doi=10.1130/0016-7606(1985)96<1251:OATEOT>2.0.CO;2|bibcode=1985GSAB...96.1251N }}</ref><ref>{{cite journal |last1=Esteban |first1=J.J. |last2=Cuevas |first2=J. |last3=Tubía |first3=J.M. |last4=Liati |first4=A. |last5=Seward |first5=D. |last6=Gebauer |first6=D. |title=Timing and origin of zircon-bearing chlorite schists in the Ronda peridotites (Betic Cordilleras, Southern Spain) |journal=Lithos |date=November 2007 |volume=99 |issue=1–2 |pages=121–135 |doi=10.1016/j.lithos.2007.06.006|bibcode=2007Litho..99..121E }}</ref> as is talc schist.<ref>{{cite journal |last1=Woguia |first1=D.L. |last2=Fagel |first2=N. |last3=Pirard |first3=E. |last4=Gourfi |first4=A. |last5=Ngo bidjeck |first5=L.M. |last6=El ouahabi |first6=M. |title=Talc schist deposits from central Cameroon: Mineralogical and physico-chemical characterization |journal=Journal of African Earth Sciences |date=June 2021 |volume=178 |pages=104182 |doi=10.1016/j.jafrearsci.2021.104182|bibcode=2021JAfES.17804182W |hdl=2268/258382 |s2cid=233704877 |hdl-access=free }}</ref> Talc schist also forms from metamorphosis of talc-bearing [[carbonate rock]]s formed by [[hydrothermal alteration]].<ref>{{cite journal |last1=Prochaska |first1=W. |title=Geochemistry and genesis of Austrian talc deposits |journal=Applied Geochemistry |date=September 1989 |volume=4 |issue=5 |pages=511–525 |doi=10.1016/0883-2927(89)90008-5|bibcode=1989ApGC....4..511P }}</ref> Graphite schist is uncommon, but can form from metamorphosis of sedimentary beds containing abundant organic [[carbon]].<ref>{{cite journal |last1=Ukar |first1=E. |last2=Cloos |first2=M. |title=Graphite-schist blocks in the Franciscan Mélange, San Simeon, California: Evidence of high- P metamorphism |journal=Journal of Metamorphic Geology |date=April 2016 |volume=34 |issue=3 |pages=191–208 |doi=10.1111/jmg.12174|bibcode=2016JMetG..34..191U |s2cid=131721852 }}</ref> This may be of [[algal]] origin.<ref>{{cite book |last1=Lumpkin |first1=B. |last2=Stoddard |first2=E. |last3=Blake |first3=D. |year=1994 |chapter=The Raleigh graphite schist |title=Geology and Field Trip Guide, Western Flank of the Raleigh Metamorphic Belt, North Carolina. Carolina Geological Society Field Trip Guidebook |publisher=North Carolina Geological Survey |location=Raleigh, NC |pages=19–24 |url=https://carolinageologicalsociety.org/1990s_files/gb%201994.pdf#page=25 |archive-url=https://web.archive.org/web/20210123041007/https://carolinageologicalsociety.org/1990s_files/gb%201994.pdf |archive-date=2021-01-23 |url-status=live |access-date=22 July 2021}}</ref>

Metamorphosis of [[felsic]] [[volcanic rock]], such as tuff, can produce [[quartz]]-[[muscovite]] schist.<ref>{{cite journal |last1=Bauer |first1=Paul W. |title=Proterozoic rocks of the Pilar Cliffs, Picuris Mountains, New Mexico |journal=New Mexico Geological Society Field Conference Series |date=2004 |volume=55 |pages=193–205 |url=https://nmgs.nmt.edu/publications/guidebooks/downloads/55/55_p0193_p0205.pdf |archive-url=https://web.archive.org/web/20210722185827/https://nmgs.nmt.edu/publications/guidebooks/downloads/55/55_p0193_p0205.pdf |archive-date=2021-07-22 |url-status=live |access-date=15 April 2020}}</ref>

<gallery widths="200px" heights="200px">
Thin section of garnet-mica-schist.jpg|Microscopic view of garnet-mica-schist in [[thin section]] under polarized light with a large garnet crystal (black) in a matrix of quartz and feldspar (white and gray grains) and parallel strands of mica (red, purple and brown).
Normal View of Garnet-Mica-Schist.JPG|View of cut garnet-mica-schist
Schist.jpg|[[Manhattan schist]] from southeastern New York State
NY-Central-Park-Rock-7333.jpg|[[Manhattan schist]] outcropping in New York City's [[Central Park]]
Talc-schist - Collezione mineralogica - Università dell’Insubria.jpg|Talc-scist from Saint-Marcel, Valle d'Aosta, Italy
Dalupirip schist 02.jpg|Amphibole epidote schist with [[slickensides]] from [[Benguet]], Philippines, showing epidote lens
</gallery>

==Engineering considerations==

In [[geotechnical engineering]] a schistosity plane often forms a [[Discontinuity (Geotechnical engineering)|discontinuity]] that may have a large influence on the mechanical behavior (strength, deformation, etc.) of rock masses in, for example, [[tunnel]], [[Foundation (engineering)|foundation]], or [[Slope stability analysis|slope]] construction.<ref name="Engineering properties of quartz mi">{{cite journal |last1=Zhang |first1=Xiao-Ping |last2=Wong |first2=Louis Ngai Yuen |last3=Wang |first3=Si-Jing |last4=Han |first4=Geng-You |title=Engineering properties of quartz mica schist |journal=Engineering Geology |date=August 2011 |volume=121 |issue=3–4 |pages=135–149 |doi=10.1016/j.enggeo.2011.04.020}}</ref> A hazard may exist even in undisturbed terrain. On August 17, 1959, a [[1959 Hebgen Lake earthquake|magnitude 7.2 earthquake]] destabilized a mountain slope near [[Hebgen Lake]], Montana, composed of schist. This caused a massive landslide that killed 26 people camping in the area.<ref>{{cite journal |title=The Hebgen Lake, Montana, earthquake of August 17, 1959 |journal=U.S. Geological Survey Professional Paper |series=Professional Paper |date=1964 |volume=435 |doi=10.3133/pp435|doi-access=free }}</ref>

{{Wide image |Vadito schist wall.jpg|1200|Road cut in [[Vadito Group]] muscovite schist. The cut has been angled to be nearly coincident with the plane of schistosity, reducing rockfall in the road. This also produces the appearance of a shining metal wall due to reflection of sunlight off the muscovite. The road and road cut are nearly straight; the curved appearance is an artifact of the panoramic photography.}}

==See also==
* {{annotated link|Blueschist}}
* {{annotated link|Greenschist}}
* {{annotated link|List of rock textures}}
* {{annotated link|Pelite}}
* {{annotated link|Whiteschist}}

==References==
{{reflist}}

==External links==
{{Commons category}}
* {{cite web |url=http://www.microscopy-uk.org.uk/mag/artnov08macro/Samuels/ |title=An Examination of Mica Schist |first=Andrea |last=Samuels |work=Micscape magazine |date=November 2008}} Photographs of Manhattan schist.
* {{cite web |title=Geologic units containing Schist |publisher=U.S. Geological Survey |url=https://mrdata.usgs.gov/geology/state/sgmc-lith.php?code=5.5}}

{{Metamorphic petrology}}
{{Rock type}}
{{Authority control}}

[[Category:Schist| ]]
[[Category:Metamorphic rocks]]
[[Category:Natural materials]]

Schist - Revision history

Ajay Kumar: Created a new article