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Schist

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A schist showing characteristic scaly schistose texture caused by platymicas

Schist(/ˈʃɪst/SHIST) is a medium-grainedmetamorphic rockshowing pronouncedschistosity(named for the rock). This means that the rock is composed ofmineralgrains easily seen with a low-powerhand lens,oriented in such a way that the rock is easily split into thin flakes or plates. Thistexturereflects a high content of platy minerals, such asmica,talc,chlorite,orgraphite.These are often interleaved with more granular minerals, such asfeldsparorquartz.

Schist typically forms during regionalmetamorphismaccompanying the process of mountain building (orogeny) and usually reflects a mediumgradeof metamorphism. Schist can form from many different kinds of rocks, includingsedimentary rockssuch asmudstonesandigneous rockssuch astuffs.Schist metamorphosed from mudstone is particularly common and is often very rich in mica (amica schist). Where the type of the original rock (theprotolith) is discernible, the schist is usually given a name reflecting its protolith, such asschistose metasandstone.Otherwise, the names of the constituent minerals will be included in the rock name, such asquartz-felspar-biotite schist.

Schistbedrockcan pose a challenge forcivil engineeringbecause of its pronouncedplanes of weakness.

Etymology

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The wordschistis derived ultimately from theGreekword σχίζειν (schízein), meaning "to split",[1]which refers to the ease with which schists can be split along the plane in which the platy minerals lie.

Definition

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Before the mid-19th century, the termsslate,shaleand schist were not sharply differentiated by those involved with mining.[2]Geologists define schist as medium-grainedmetamorphic rockthat shows well-developed schistosity.[3]Schistosity is a thin layering of the rock produced bymetamorphism(afoliation) that permits the rock to easily be split into flakes or slabs less than 5 to 10 millimeters (0.2 to 0.4 in) thick.[4][5]The mineral grains in a schist are typically from 0.25 to 2 millimeters (0.01 to 0.08 in) in size[6]and so are easily seen with a 10×hand lens.[7]Typically, over half the mineral grains in a schist show a preferred orientation. Schists make up one of the three divisions of metamorphic rock bytexture,with the other two divisions beinggneiss,which has poorly developed schistosity and thicker layering, andgranofels,which has no discernible schistosity.[4][8]

Schists are defined by their texture without reference to their composition,[9][4]and while most are a result of medium-grade metamorphism, they can vary greatly in mineral makeup.[10]However, schistosity normally develops only when the rock contains abundant platy minerals, such asmicaorchlorite.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.[4]Though not a defining characteristic, schists very often containporphyroblasts(individual crystals of unusual size) of distinctive minerals, such asgarnet,staurolite,kyanite,sillimanite,orcordierite.[11]

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 (theprotolith) is unknown and its mineral content is not yet determined. Otherwise, the modifierschistosewill be applied to a more precise type name, such asschistosesemipelite(when the rock is known to contain moderate amounts of mica) or aschistose metasandstone(if the protolith is known to have been asandstone).[12]If all that is known is that the protolith was a sedimentary rock, the schist will be described as aparaschist,while if the protolith was an igneous rock, the schist will be described as anorthoschist.[13]Mineral qualifiers are important when naming a schist. For example, a quartz-feldspar-biotite schist is a schist of uncertain protolith that containsbiotitemica,feldspar,andquartzin order of apparent decreasing abundance.[14]

Lineated schisthas a strong linear fabric in a rock which otherwise has well-developed schistosity.[10]

Formation

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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 (anorogenic 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.[15]While platy or elongated minerals are most obviously reoriented, even quartz or calcite may take up preferred orientations.[16]At the microscopic level, schistosity is divided intointernal schistosity,in which inclusions within porphyroblasts take a preferred orientation, andexternal schistosity,which is the orientation of grains in the surrounding medium-grained rock.[17]

The composition of the rock must permit formation of abundant platy minerals. For example, theclay mineralsin mudstone are metamorphosed to mica, producing a mica schist.[18]Early stages of metamorphism convert mudstone to a very fine-grained metamorphic rock calledslate,which with further metamorphism becomes fine-grainedphyllite.Further recrystallization produces medium-grained mica schist. If the metamorphism proceeds further, the mica schist experiencesdehydration reactionsthat convert platy minerals to granular minerals such as feldspars, decreasing schistosity and turning the rock into a gneiss.[11]

Other platy minerals found in schists include chlorite, talc, and graphite. Chlorite schist is typically formed by metamorphism ofultramaficigneous rocks,[19][20]as is talc schist.[21]Talc schist also forms from metamorphosis of talc-bearingcarbonate rocksformed byhydrothermal alteration.[22]Graphite schist is uncommon but can form from metamorphosis of sedimentary beds containing abundant organiccarbon.[23]This may be ofalgalorigin.[24]Graphite schist is known to have experiencedgreenschistfacies metamorphism,for example in the northernAndes.[25]

Metamorphosis offelsicvolcanic rock,such as tuff, can produce quartz-muscoviteschist.[26]

  • 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).
    Microscopic view of garnet-mica-schist inthin sectionunder 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).
  • View of cut garnet-mica-schist
    View of cut garnet-mica-schist
  • Manhattan schist from southeastern New York State
    Manhattan schistfrom southeastern New York State
  • Manhattan schist outcropping in New York City's Central Park
    Manhattan schistoutcropping in New York City'sCentral Park
  • Talc-scist from Saint-Marcel, Valle d'Aosta, Italy
    Talc-scist from Saint-Marcel, Valle d'Aosta, Italy
  • Amphibole epidote schist with slickensides from Benguet, Philippines, showing epidote lens
    Amphibole epidote schist withslickensidesfromBenguet,Philippines, showing epidote lens

Engineering considerations

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Ingeotechnical engineeringa schistosity plane often forms adiscontinuitythat may have a large influence on the mechanical behavior (strength, deformation, etc.) of rock masses in, for example,tunnel,foundation,orslopeconstruction.[27]A hazard may exist even in undisturbed terrain. On August 17, 1959, amagnitude 7.2 earthquakedestabilized a mountain slope nearHebgen Lake,Montana, composed of schist. This caused a massive landslide that killed 26 people camping in the area.[28]

Road cut inVadito Groupmuscovite 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

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References

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  1. ^"schist".LexicoUK English Dictionary.Oxford University Press.Archived fromthe originalon 2020-01-27.
  2. ^Raymond, R. W. (1881)."Slate".A Glossary of Mining and Metallurgical Terms.American Institute of Mining Engineers. p. 78.
  3. ^British Geological Survey 1999,p. 3.
  4. ^abcdSchmid, R.; Fettes, D.; Harte, B.; Davis, E.; Desmons, J. (2007). "How to name a metamorphic rock.".Metamorphic Rocks: A Classification and Glossary of Terms: Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Metamorphic Rocks(PDF).Cambridge: Cambridge University Press. p. 7.Archived(PDF)from the original on 2021-06-24.Retrieved28 February2021.
  5. ^Robertson, S. (1999)."BGS Rock Classification Scheme, Volume 2: Classification of metamorphic rocks"(PDF).British Geological Survey Research Report.RR 99-02: 5.Archived(PDF)from the original on 2018-04-03.Retrieved27 February2021.
  6. ^British Geological Survey 1999,p. 24.
  7. ^Blatt, Harvey; Tracy, Robert J. (1996).Petrology: igneous, sedimentary, and metamorphic(2nd ed.). New York: W.H. Freeman. p. 360.ISBN0716724383.
  8. ^British Geological Survey 1999,pp. 5–6.
  9. ^Jackson, Julia A., ed. (1997). "schist".Glossary of geology(Fourth ed.). Alexandria, Virginia: American Geological Institute.ISBN0922152349.
  10. ^abBritish Geological Survey 1999,p. 5.
  11. ^abBlatt & Tracy 1996,p. 365.
  12. ^British Geological Survey 1999,pp. 3–4.
  13. ^British Geological Survey 1999,pp. 5–7.
  14. ^British Geological Survey 1999,p. 8.
  15. ^Blatt & Tracy 1996,p. 359.
  16. ^Yardley, B. W. D. (1989).An introduction to metamorphic petrology.Harlow, Essex, England: Longman Scientific & Technical. pp. 168–169.ISBN0582300967.
  17. ^Yardley 1989,p. 171.
  18. ^Potter, Paul Edwin; Maynard, J. Barry; Pryor, Wayne A. (1980).Sedimentology of shale: study guide and reference source.New York: Springer-Verlag. p. 17.ISBN0387904301.
  19. ^Nokleberg, Warren J.; Jones, David L.; Silberling, Norman J. (1 October 1985). "Origin and tectonic evolution of the Maclaren and Wrangellia terranes, eastern Alaska Range, Alaska".GSA Bulletin.96(10): 1251–1270.Bibcode:1985GSAB...96.1251N.doi:10.1130/0016-7606(1985)96<1251:OATEOT>2.0.CO;2.
  20. ^Esteban, J.J.; Cuevas, J.; Tubía, J.M.; Liati, A.; Seward, D.; Gebauer, D. (November 2007). "Timing and origin of zircon-bearing chlorite schists in the Ronda peridotites (Betic Cordilleras, Southern Spain)".Lithos.99(1–2): 121–135.Bibcode:2007Litho..99..121E.doi:10.1016/j.lithos.2007.06.006.
  21. ^Woguia, D.L.; Fagel, N.; Pirard, E.; Gourfi, A.; Ngo bidjeck, L.M.; El ouahabi, M. (June 2021). "Talc schist deposits from central Cameroon: Mineralogical and physico-chemical characterization".Journal of African Earth Sciences.178:104182.Bibcode:2021JAfES.17804182W.doi:10.1016/j.jafrearsci.2021.104182.hdl:2268/258382.S2CID233704877.
  22. ^Prochaska, W. (September 1989). "Geochemistry and genesis of Austrian talc deposits".Applied Geochemistry.4(5): 511–525.Bibcode:1989ApGC....4..511P.doi:10.1016/0883-2927(89)90008-5.
  23. ^Ukar, E.; Cloos, M. (April 2016). "Graphite-schist blocks in the Franciscan Mélange, San Simeon, California: Evidence of high- P metamorphism".Journal of Metamorphic Geology.34(3): 191–208.Bibcode:2016JMetG..34..191U.doi:10.1111/jmg.12174.S2CID131721852.
  24. ^Lumpkin, B.; Stoddard, E.; Blake, D. (1994). "The Raleigh graphite schist".Geology and Field Trip Guide, Western Flank of the Raleigh Metamorphic Belt, North Carolina. Carolina Geological Society Field Trip Guidebook(PDF).Raleigh, NC: North Carolina Geological Survey. pp. 19–24.Archived(PDF)from the original on 2021-01-23.Retrieved22 July2021.
  25. ^Bustamante, C.; Cardona, A."Is the Central Cordillera of Colombia a potential source of graphite?: Implications for the energy transition in Colombia".Andean Geology.51(2): 413–420.doi:10.5027/andgeoV51n2-3728.
  26. ^Bauer, Paul W. (2004)."Proterozoic rocks of the Pilar Cliffs, Picuris Mountains, New Mexico"(PDF).New Mexico Geological Society Field Conference Series.55:193–205.Archived(PDF)from the original on 2021-07-22.Retrieved15 April2020.
  27. ^Zhang, Xiao-Ping; Wong, Louis Ngai Yuen; Wang, Si-Jing; Han, Geng-You (August 2011). "Engineering properties of quartz mica schist".Engineering Geology.121(3–4): 135–149.doi:10.1016/j.enggeo.2011.04.020.
  28. ^"The Hebgen Lake, Montana, earthquake of August 17, 1959".U.S. Geological Survey Professional Paper.Professional Paper.435.1964.doi:10.3133/pp435.
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