Schist

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Schist specimen showing the characteristic "scaly" schistose texture, caused by platy micas Schist detail.jpg
Schist specimen showing the characteristic "scaly" schistose texture, caused by platy micas

Schist ( /ʃɪst/ shist) is a medium-grained metamorphic rock showing pronounced schistosity. 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 reflects a high content of platy minerals, such as micas, talc, chlorite, or graphite. These are often interleaved with more granular minerals, such as feldspar or quartz.

Contents

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 consitutent 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.

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

Geologists define schist as medium-grained metamorphic rock that shows well-developed schistosity. [2] Schistosity is a thin layering of the rock produced by metamorphism (a foliation ) 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. [3] [4] The mineral grains in a schist are typically from 0.25 to 2 millimeters (0.01 to 0.08 in) in size [5] and so are easily seen with a 10× hand lens. [6] 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, with the other two divisions being gneiss, which has poorly developed schistosity and thicker layering, and granofels, which has no discernible schistosity. [3] [7]

Schists are defined by their texture, without reference to their composition, [8] [3] and while most are a result of medium-grade metamorphism, they can vary greatly in mineral makeup. [9] However, schistosity normally develops only when the rock contains abundant platy minerals, such as micas 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. [3] 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. [10]

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). [11] 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. [12] 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. [13]

Lineated schist has a strong linear fabric in a rock which otherwise has well-developed schistosity. [9]

Historical mining terminology

Before the mid-19th century, the terms slate, shale and schist were not sharply differentiated by those involved with mining. [14]

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 clay 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 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 reactions that convert platy minerals to granular minerals such as feldspars, decreasing schistosity and turning the rock into a gneiss. [10]

Other platy minerals found in schists include chlorite, talc, and graphite. Chlorite schist is typically formed by metamorphism of ultramafic igneous rocks, [19] [20] as is talc schist. [21] Talc schist also forms from metamorphosis of talc-bearing carbonate rocks formed by hydrothermal alteration. [22] Graphite schist is uncommon, but can form from metamorphosis of sedimentary beds containing abundant organic carbon. [23] This may be of algal origin. [24]

Metamorphosis of felsic volcanic rock, such as tuff, can produce quartz-muscovite schist. [25]


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]

Vadito schist wall.jpg
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

Related Research Articles

Gneiss Common high-grade metamorphic rock

Gneiss is a common and widely distributed type of metamorphic rock. Gneiss is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. Gneiss forms at higher temperatures and pressures than schist. Gneiss nearly always shows a banded texture characterized by alternating darker and lighter colored bands and without a distinct cleavage.

Metamorphic rock Rock that was subjected to heat and pressure

Metamorphic rocks arise from the transformation of existing rock to new types of rock, in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than 150 to 200 °C and, often, elevated pressure of 100 megapascals (1,000 bar) or more, causing profound physical or chemical changes. During this process, the rock remains mostly in the solid state, but gradually recrystallizes to a new texture or mineral composition. The protolith may be a sedimentary, igneous, or existing metamorphic rock.

Amphibolite A metamorphic rock containing mainly amphibole and plagioclase

Amphibolite is a metamorphic rock that contains amphibole, especially hornblende and actinolite, as well as plagioclase feldspar.

Lithology

The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Lithology may refer to either a detailed description of these characteristics, or a summary of the gross physical character of a rock. Examples of lithologies in the second sense include sandstone, slate, basalt, or limestone.

Phyllite

Phyllite is a type of foliated metamorphic rock created from slate that is further metamorphosed so that very fine grained white mica achieves a preferred orientation. It is primarily composed of quartz, sericite mica, and chlorite.

Metasomatism Chemical alteration of a rock by hydrothermal and other fluids

Metasomatism is the chemical alteration of a rock by hydrothermal and other fluids. It is the replacement of one rock by another of different mineralogical and chemical composition. The minerals which compose the rocks are dissolved and new mineral formations are deposited in their place. Dissolution and deposition occur simultaneously and the rock remains solid.

Granulite Class of high-grade medium to coarse grained metamorphic rocks

Granulites are a class of high-grade metamorphic rocks of the granulite facies that have experienced high-temperature and moderate-pressure metamorphism. They are medium to coarse–grained and mainly composed of feldspars sometimes associated with quartz and anhydrous ferromagnesian minerals, with granoblastic texture and gneissose to massive structure. They are of particular interest to geologists because many granulites represent samples of the deep continental crust. Some granulites experienced decompression from deep in the Earth to shallower crustal levels at high temperature; others cooled while remaining at depth in the Earth.

Hornfels

Hornfels is the group name for a set of contact metamorphic rocks that have been baked and hardened by the heat of intrusive igneous masses and have been rendered massive, hard, splintery, and in some cases exceedingly tough and durable. These properties are due to fine grained non-aligned crystals with platy or prismatic habits, characteristic of metamorphism at high temperature but without accompanying deformation. The term is derived from the German word Hornfels, meaning "hornstone", because of its exceptional toughness and texture both reminiscent of animal horns. These rocks were referred to by miners in northern England as whetstones.

Chlorite group Phyllosilicates: non-swelling 2:1 clay (TOT) whose interlayer is occupied by a brucite, Mg(OH)2, bridge

The chlorites are a group of phyllosilicate minerals. Chlorites can be described by the following four endmembers based on their chemistry via substitution of the following four elements in the silicate lattice; Mg, Fe, Ni, and Mn.

Blueschist Metavolcanic rock that forms by the metamorphism of basalt and rocks with similar composition

Blueschist, also called glaucophane schist, is a metavolcanic rock that forms by the metamorphism of basalt and rocks with similar composition at high pressures and low temperatures, approximately corresponding to a depth of 15–30 km (9.3–18.6 mi). The blue color of the rock comes from the presence of the predominant minerals glaucophane and lawsonite.

Greenschist

Greenschists are metamorphic rocks that formed under the lowest temperatures and pressures usually produced by regional metamorphism, typically 300–450 °C (570–840 °F) and 2–10 kilobars (14,500–58,000 psi). Greenschists commonly have an abundance of green minerals such as chlorite, serpentine, and epidote, and platy minerals such as muscovite and platy serpentine. The platiness causes the tendency to split, or have schistosity. Other common minerals include quartz, orthoclase, talc, carbonate minerals and amphibole (actinolite).

Spruce Pine Mining District

The Spruce Pine Mining District is a swath of the valley of the North Toe River in the Blue Ridge Mountains of northwestern North Carolina. The area is mined for its mica, kaolin, quartz and feldspar. Spruce Pine district is one of the largest suppliers of high-purity quartz, which is used in the manufacture of silicon chips. The district is named after the town of Spruce Pine, which is located in the middle of the region and is the hub of major mining activity there. The district is approximately 25 miles long and 5 miles wide.

Quartz-porphyry Type of volcanic rock containing large porphyritic crystals of quartz

Quartz-porphyry, in layman's terms, is a type of volcanic (igneous) rock containing large porphyritic crystals of quartz. These rocks are classified as hemi-crystalline acid rocks.

Porphyroblast

A porphyroblast is a large mineral crystal in a metamorphic rock which has grown within the finer grained matrix. Porphyroblasts are commonly euhedral crystals, but can also be partly to completely irregular in shape.

Foliation (geology)

Foliation in geology refers to repetitive layering in metamorphic rocks. Each layer can be as thin as a sheet of paper, or over a meter in thickness. The word comes from the Latin folium, meaning "leaf", and refers to the sheet-like planar structure. It is caused by shearing forces, or differential pressure. The layers form parallel to the direction of the shear, or perpendicular to the direction of higher pressure. Nonfoliated metamorphic rocks are typically formed in the absence of significant differential pressure or shear. Foliation is common in rocks affected by the regional metamorphic compression typical of areas of mountain belt formation.

Cleavage (geology)

Cleavage, in structural geology and petrology, describes a type of planar rock feature that develops as a result of deformation and metamorphism. The degree of deformation and metamorphism along with rock type determines the kind of cleavage feature that develops. Generally these structures are formed in fine grained rocks composed of minerals affected by pressure solution.

Metamorphic facies

A metamorphic facies is a set of mineral assemblages in metamorphic rocks formed under similar pressures and temperatures. The assemblage is typical of what is formed in conditions corresponding to an area on the two dimensional graph of temperature vs. pressure. Rocks which contain certain minerals can therefore be linked to certain tectonic settings, times and places in the geological history of the area. The boundaries between facies are wide because they are gradational and approximate. The area on the graph corresponding to rock formation at the lowest values of temperature and pressure is the range of formation of sedimentary rocks, as opposed to metamorphic rocks, in a process called diagenesis.

This glossary of geology is a list of definitions of terms and concepts relevant to geology, its sub-disciplines, and related fields. For other terms related to the Earth sciences, see Glossary of geography terms.

Lilesville Granite Body of granitic rock

The Lilesville Granite, also referred to as the Lilesville pluton, is a ring-shaped body of granitic rock that spans about 94 square miles (240 km2) in Anson, Richmond, and Montgomery Counties in southern North Carolina.

References

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