Schist
Schist (.
Schist typically forms during regional metamorphism accompanying the process of mountain building (orogeny) and usually reflects a medium grade of metamorphism. Schist can form from many different kinds of rocks, including sedimentary rocks such as mudstones and igneous rocks such as tuffs. 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 planes of weakness.
Etymology
The word schist is derived ultimately from the Greek word 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
Before the mid-19th century, the terms
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 as mica or 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.[4] Though not a defining characteristic, schists very often contain porphyroblasts (individual crystals of unusual size) of distinctive minerals, such as garnet, staurolite, kyanite, sillimanite, or cordierite.[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 (the
Lineated schist has a strong linear fabric in a rock which otherwise has well-developed schistosity.[10]
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.[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 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.[17]
The composition of the rock must permit formation of abundant platy minerals. For example, the
Other platy minerals found in schists include chlorite, talc, and graphite. Chlorite schist is typically formed by metamorphism of
Metamorphosis of felsic volcanic rock, such as tuff, can produce quartz-muscovite schist.[25]
-
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).
-
View of cut garnet-mica-schist
-
Manhattan schistfrom southeastern New York State
-
Manhattan schist outcropping in New York City's Central Park
-
Talc-scist from Saint-Marcel, Valle d'Aosta, Italy
-
Amphibole epidote schist withslickensides from Benguet, Philippines, showing epidote lens
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.[26] A hazard may exist even in undisturbed terrain. On August 17, 1959, a 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.[27]
See also
- Blueschist – Type of metavolcanic rock
- Greenschist – Metamorphic rock
- List of rock textures – List of rock textural and morphological terms
- Pelite – Metamorphic rock
- Whiteschist
References
- ^ "schist". Lexico UK English Dictionary. Oxford University Press. Archived from the original on 2020-01-27.
- ^ Raymond, R. W. (1881). "Slate". A Glossary of Mining and Metallurgical Terms. American Institute of Mining Engineers. p. 78.
- ^ British Geological Survey 1999, p. 3.
- ^ a b c d Schmid, 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. Retrieved 28 February 2021.
- ^ 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. Retrieved 27 February 2021.
- ^ British Geological Survey 1999, p. 24.
- ISBN 0716724383.
- ^ British Geological Survey 1999, pp. 5–6.
- ISBN 0922152349.
- ^ a b British Geological Survey 1999, p. 5.
- ^ a b Blatt & Tracy 1996, p. 365.
- ^ British Geological Survey 1999, pp. 3–4.
- ^ British Geological Survey 1999, pp. 5–7.
- ^ British Geological Survey 1999, p. 8.
- ^ Blatt & Tracy 1996, p. 359.
- ISBN 0582300967.
- ^ Yardley 1989, p. 171.
- ISBN 0387904301.
- .
- .
- S2CID 233704877.
- .
- S2CID 131721852.
- ^ 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. Retrieved 22 July 2021.
- ^ 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. Retrieved 15 April 2020.
- .
- doi:10.3133/pp435.
External links
- Samuels, Andrea (November 2008). "An Examination of Mica Schist". Micscape magazine. Photographs of Manhattan schist.
- "Geologic units containing Schist". U.S. Geological Survey.