Editing Schist

{{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]]'' (named for the rock). 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]], [[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 ==
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> 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]] 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}}

==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> Graphite schist is known to have experienced [[greenschist]] [[Metamorphic facies|facies metamorphism]], for example in the northern [[Andes]].<ref>{{Cite journal |title=Is the Central Cordillera of Colombia a potential source of graphite?: Implications for the energy transition in Colombia |journal=[[Andean Geology]] |url=http://www.andeangeology.cl/index.php/revista1/article/view/V51n2-3728/html |last=Bustamante |first=C. |issue=2 |volume=51 |pages=413–420 |last2=Cardona |first2=A. |doi=10.5027/andgeoV51n2-3728|doi-access=free }}</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-schist 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]]