Schists are a group of metamorphic rocks which are medium to coarse-grained and of a fissile nature, meaning that at least one of the minerals in the rock crystallizes in platy form. Because of this, schists will split easily along these parallel layers. In fact, the word schist comes from a Greek word meaning “to split”.
What is a schist?
Schist is a foliated metamorphic rock made up of plate-shaped mineral grains that are large enough to see with an unaided eye. It usually forms on a continental side of a convergent plate boundary where sedimentary rocks, such as shales and mudstones, have been subjected to compressive forces, heat, and chemical activity. This metamorphic environment is intense enough to convert the clay minerals of the sedimentary rocks into platy metamorphic minerals such as muscovite, biotite, and chlorite. To become schist, a shale must be metamorphosed in steps through slate and then through phyllite. If the schist is metamorphosed further, it might become a granular rock known as gneiss.
A rock does not need a specific mineral composition to be called “schist.” It only needs to contain enough platy metamorphic minerals in alignment to exhibit distinct foliation. This texture allows the rock to be broken into thin slabs along the alignment direction of the platy mineral grains. This type of breakage is known as schistosity.
In rare cases the platy metamorphic minerals are not derived from the clay minerals of a shale. The platy minerals can be graphite, talc, or hornblende from carbonaceous, basaltic, or other sources.
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How Schists are Formed?
Schists are formed by regional metamorphism and are associated with tectonics and major mountain building events. Schists are found in regions composed mainly of metamorphic rocks, such as the Central Alps, the Himalayas, Scandinavia, the Highlands of Scotland and north-west of Ireland. They are produced by intense heat, pressure and folding. Schists may be formed from both igneous and sedimentary rocks. Often they are a metamorphosed form of shale. When shale goes through Barrovian metamorphism, it becomes slate first and with greater temperature and pressure will further morph into phyllite and then schist. The depth of burial and the amount of heat and pressure determines the degree of metamorphic changes the shale goes through.
Two categories of particular importance are the greenschists and the blueschists. These have similar parent rocks but are formed under different pressure (P) and temperature (T) conditions. What minerals will crystallize during metamorphosis depend on both P and T.
Greenschists form under high P and high T such as are found far below Earth’s crust ; blueschists form under high P and relatively low T. Both greenschists and blueschists are found in regionally metamorphosed landscapes—that is, land masses that have been submerged entirely in Earth’s interior and metamorphosed in bulk. Regional metamorphosis often occurs at subduction zones, those places where one tectonic plate is being driven edgewise into the mantle beneath another. A large, cool chunk of Earth’s crust takes a long time to reach ambient T when plunged into the mantle, but is raised to high P at once; a subducted mass that stays down long enough to achieve both high T and high P produces greenschist, while one that returns to the surface relatively quickly produces blueschist. One of the great unresolved puzzles of modern geology is that blueschist formation seems to have become globally more common in the last 300 million years, while greenschist formation has remained constant throughout Earth’s history.
Schist Types and Compositions
Schists are classified according to the types of rocks they are derived from. There are two major groups: paraschists, which are derived from sedimentary rock like shale; and orthoschists, which are metamorphosed from igneous rock such as basalt. Schists are commonly referred to based on their preponderant mineral composition, e.g. a mica schist would be a schist that consisted mostly of mica. Schists are often a mix of quartz, feldspar, mica, and sometimes amphibole. They may also contain chlorite, garnet or kyanite.
The schists and gneisses are classified according to the minerals they consist of, and this depends mainly on their chemical composition. There are, for example, a group of metamorphic limestones, marbles, calc-shists and cipolins, with crystalline dolomites; many of these contain silicate minerals such as mica, tremolite, diopside, scapolite, quartz and feldspar. They are derived from calcareous sediments of different degrees of purity. The color of schist depends greatly on its predominant minerals. Generally, schists may be gray, yellow, green, brown, white, or black.
A separate group is rich in quartz (quartzites, quartz schists and quartzone 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 represent sediments once containing coaly matter or plant remains. There are also schistose ironstones (hematite-schists), but metamorphic beds of salt or gypsum are uncommon.
Schists of igneous origin include 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 acid tuffs.
The majority of mica-schists are altered clays 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.
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A special subgroup consists of the andalusite-, staurolite-, kyanite-, and sillimanite-schists, together with the cordierite-gneisses. They usually appear in the vicinity of gneissose granites and have presumably been affected by contact alteration.
The more coarsely foliated gneisses are almost as frequent as the mica-schists, and present a great variety of types differing in composition and appearance. They contain quartz, one or more varieties of feldspar, and usually mica, hornblende or augite, often garnet, iron oxides, and so forth. Hence, in composition they resemble granite, differing principally in their foliated structure. Many of them have “augen” or large elliptical crystals, mostly feldspar but sometimes quartz, which are the crushed remains of porphyritic minerals. Most of these augen gneisses are metamorphic granites, but sometimes a conglomerate bed simulates a gneiss of this kind rather closely.
There are other gneisses, derived from feldspathic sandstones, grits, arkoses, and sediments of that type. They contain mostly biotite and muscovite. The hornblende and pyroxene gneisses are usually igneous rocks similar in composition to the hornblende-granites and quartz-diorites. The metamorphic forms of dolerite, basalt, and mafic igneous rocks generally have a distinctive facies, as their pyroxene and olivine are replaced by dark green hornblende, with often epidote, garnet and biotite. These rocks have a well-developed foliation, as the prismatic hornblendes lie side by side in parallel arrangement. The majority of amphibolites, hornblende-schists, foliated epidiorites and green schists belong to this group. Where they are least altered, they pass through chloritic schists into sheared diabases, flaser gabbros and other rocks in which remains of the original igneous minerals and structures occur in greater or less profusion.
image source: http://www.sandatlas.org/schist/
Where can we find Schists today?
Schist, with their layers of intermixed minerals, such as quartz, garnet and mica can be very attractive rocks with impressive amounts of shine. Muscovite (mica schist), for instance, is pearlized, reflective and somewhat transparent. Schists are often used in jewelry and other types of artwork.
Schists have been frequently used in building houses or walls, as many are quite strong and durable. However, many foundation problems with buildings both large and small are due to decay of the schist or failure of the mortar. This in turn lets water into the joints, thus weakening the schist further.
Most of the building foundations built in the 1920s and ’30s in the New York City area used schist. Decorative rock walls on houses in the area also used a schist called “Yonkers Stone,” which is no longer available. This schist was particularly hard and its color was fairly consistent.There are buildings at Princeton University and the University of Pennsylvania that were constructed using schist. Schists are often used as decorative stones and in veneers.
Schist is not a rock with numerous industrial uses. Its abundant mica grains and its schistosity make it a rock of low physical strength, usually unsuitable for use as a construction aggregate, building stone, or decorative stone. The only exception is for its use as a fill when the physical properties of the material are not critical.
info sources: http://geology.com/rocks/schist.shtml