A schist showing characteristic scaly schistose texture caused by platymicas
Schist (/ˈʃɪst/SHIST) is a medium-grainedmetamorphic rock showing pronouncedschistosity (named for the rock). This means that the rock is composed ofmineral grains easily seen with a low-powerhand lens, oriented in such a way that the rock is easily split into thin flakes or plates. Thistexture reflects a high content of platy minerals, such asmica,talc,chlorite, orgraphite. These are often interleaved with more granular minerals, such asfeldspar orquartz.
Schist typically forms during regionalmetamorphism accompanying the process of mountain building (orogeny) and usually reflects a mediumgrade of metamorphism. Schist can form from many different kinds of rocks, includingsedimentary rocks such asmudstones andigneous rocks such 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.
The wordschist is derived ultimately from theGreek 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.
Before the mid-19th century, the termsslate,shale and schist were not sharply differentiated by those involved with mining.[2] Geologists define schist as medium-grainedmetamorphic rock that 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 asmica orchlorite. 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 modifierschistose will 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 containsbiotite mica,feldspar, andquartz in order of apparent decreasing abundance.[14]
Lineated schist has a strong linear fabric in a rock which otherwise has well-developed schistosity.[10]
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 minerals in 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 reactions that 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 ofultramafic igneous rocks,[19][20] as is talc schist.[21] Talc schist also forms from metamorphosis of talc-bearingcarbonate rocks formed byhydrothermal alteration.[22] Graphite schist is uncommon but can form from metamorphosis of sedimentary beds containing abundant organiccarbon.[23] This may be ofalgal origin.[24] Graphite schist is known to have experiencedgreenschistfacies metamorphism, for example in the northernAndes.[25]
Microscopic view of garnet-mica-schist inthin 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).
Ingeotechnical engineering a schistosity plane often forms adiscontinuity that may have a large influence on the mechanical behavior (strength, deformation, etc.) of rock masses in, for example,tunnel,foundation, orslope construction.[27] A hazard may exist even in undisturbed terrain. On August 17, 1959, amagnitude 7.2 earthquake destabilized 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 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.
^Yardley, B. W. D. (1989).An introduction to metamorphic petrology. Harlow, Essex, England: Longman Scientific & Technical. pp. 168–169.ISBN0582300967.
^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.
^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.
