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schist
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{{short description|Medium grade metamorphic rock with lamellar grain}}
missing image!
- Schist detail.jpg -
Schist specimen showing the characteristic "scaly" schistose texture, caused by platy micas
Schist (pronounced {{IPAc-en|ʃ|ɪ|s|t}} {{respell|SHIST|'}}) is a medium-grade metamorphic rock formed from mudstone or shale.WEB,weblink Schist definition, Dictionary of Geology, 2013-07-12, Schist has medium to large, flat, sheet-like grains in a preferred orientation (nearby grains are roughly parallel). It is defined by having more than 50% platy and elongated minerals (such as micas or talc),BOOK,weblink Glossary of Geology, Springer, Jackson J.A., Mehl J.P. & Neuendorf K.K.E., 2005, 577, 9780922152766, often finely interleaved with quartz and feldspar.BOOK,weblink Cambridge Guide to Minerals, Rocks and Fossils, Cambridge University Press, Bishop A.C., Woolley A.R. & Hamilton W.R., 1999, 153, 9780521778817, These lamellar (flat, planar) minerals include micas, chlorite, talc, hornblende, graphite, and others. Quartz often occurs in drawn-out grains to such an extent that a particular form called quartz schist is produced. Schist is often garnetiferous. Schist forms at a higher temperature and has larger grains than phyllite. Geological foliation (metamorphic arrangement in layers) with medium to large grained flakes in a preferred sheetlike orientation is called schistosity.Essentials of Geology, 3rd Ed, Stephen MarshakThe names of various schists are derived from their mineral constituents. For example, schists primarily composed of biotite and muscovite are called mica schists.BOOK,weblink Earth science, J., Tarbuck, Edward, 2012, Prentice Hall/Pearson, Lutgens, Frederick K., 0321688503, 13th, Upper Saddle River, N.J., 693684089, Most schists are mica schists, but graphite and chlorite schists are also common. Schists are also named for their prominent or perhaps unusual mineral constituents, as in the case of garnet schist, tourmaline schist, and glaucophane schist.The individual mineral grains in schist, drawn out into flaky scales by heat and pressure, can be seen with the naked eye. Schist is characteristically foliated, meaning that the individual mineral grains split off easily into flakes or slabs. The word schist is derived ultimately from the Greek word σχίζειν (schízein) meaning "to split",WEB,weblink Schist, Oxford University Press, English Dictionary, 10 February 2014, which is a reference to the ease with which schists can be split along the plane in which the platy minerals lie.Most schists are derived from clays and muds that have passed through a series of metamorphic processes involving the production of shales, slates and phyllites as intermediate steps. Certain schists are derived from fine-grained igneous rocks such as basalts and tuffs.

Historical mining terminology

Before the mid-18th century, the terms slate, shale and schist were not sharply differentiated by those involved with mining.R. W. Raymond, Slate, A Glossary of Mining and Metallurigical Terms, American Institute of Mining Engineers, 1881, p. 78.

Formation

During metamorphism, rocks which were originally sedimentary, igneous or metamorphic are converted into schists and gneisses. If the composition of the rocks was originally similar, they may be very difficult to distinguish from one another if the metamorphism has been great. A quartz-porphyry, for example, and a fine grained feldspathic sandstone, may both be converted into a grey or pink mica-schist. Usually, however, it is possible to distinguish between sedimentary and igneous schists and gneisses. If, for example, the whole district occupied by these rocks has traces of bedding, clastic structure, or unconformability, then it may be a sign that the original rock was sedimentary. In other cases intrusive junctions, chilled edges, contact alteration or porphyritic structure may prove that in its original condition a metamorphic gneiss was an igneous rock. The last appeal is often to the chemistry, for there are certain rock types which occur only as sediments, while others are found only among igneous masses, and however advanced the metamorphism may be, it rarely modifies the chemical composition of the mass very greatly. Such rocks as limestones, dolomites, quartzites, and aluminous shales have very definite chemical characteristics that distinguish them even when completely recrystallized.{{EB1911|wstitle=Petrology |volume=21 |page=333 |inline=1 |first=John Smith |last=Flett}}The schists are classified principally according to the minerals they consist of and on their chemical composition. For example, many metamorphic limestones, marbles, and calc-schists, with crystalline dolomites, contain silicate minerals such as mica, tremolite, diopside, scapolite, quartz and feldspar. They are derived from calcareous sediments of different degrees of purity. Another group is rich in quartz (quartzites, quartz schists and quartzose gneisses), with variable amounts of white and black mica, garnet, feldspar, zoisite and hornblende. These were once sandstones and arenaceous rocks. The graphitic schists may readily be believed to represent sediments once containing coal or plant remains; there are also schistose ironstones (hematite-schists), but metamorphic beds of salt or gypsum are exceedingly uncommon. Among schists of igneous origin there are the silky calc-schists, the foliated serpentines (once ultramafic masses rich in olivine), and the white mica-schists, porphyroids and banded halleflintas, which have been derived from rhyolites, quartz-porphyries and felsic tuffs. The majority of mica-schists, however, are altered claystones and shales, and pass into the normal sedimentary rocks through various types of phyllite and mica-slates. They are among the most common metamorphic rocks; some of them are graphitic and others calcareous. The diversity in appearance and composition is very great, but they form a well-defined group not difficult to recognize, from the abundance of black and white micas and their thin, foliated, schistose character. A subgroup is the andalusite-, staurolite-, kyanite- and sillimanite-schists which usually make their appearance in the vicinity of gneissose granites, and have presumably been affected by contact metamorphism.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-schistSchist.jpg|Manhattan schist from southeastern New York StateNY-Central-Park-Rock-7333.jpg|Manhattan schist outcropping in New York City's Central ParkTalc-schist - Collezione mineralogica - Università dell’Insubria.jpg|Talc-scist from Saint-Marcel, France

Engineering considerations

In geotechnical engineering a schistosity plane often forms a discontinuity that may have a large influence on the mechanical behavior (strength, deformation, etc.) of rock masses in, for example, tunnel, foundation, or slope construction.

See also

  • {{annotated link|List of rock textures}}
  • {{annotated link|Greenschist}}
  • {{annotated link|Pelite}}

References

{{reflist}}

External links

{{Commons category}}
  • An Examination of Mica Schist by Andrea Samuels, Micscape magazine. Photographs of Manhattan schist.
  • {{deadlink|date=September 2019}}weblink" title="https:/-/web.archive.org/web/20161114165600weblink">weblink by USGS: Idaho, Univ. of Idaho, Moscow, articles cited.
{{Metamorphic petrology}}{{Authority control}}

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