Schist 43530 222633627 2008-06-30T10:44:49Z Tomas e 5181812 Interwiki sv [[Image:SchistUSGOV.jpg|thumb|Schist]] The '''schists''' form a group of medium-grade [[metamorphic rock]]s, chiefly notable for the preponderance of lamellar [[mineral]]s such as [[mica]]s, [[Chlorite group|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. By definition, schist contains more than 50% platy and elongated minerals, often finely interleaved with quartz and [[feldspar]]. Schist is often [[garnet|garnetiferous]]. The individual [[mineral]] grains in schist, drawn out into flaky scales by heat and pressure, can be seen by the naked eye. Schist is characteristically ''[[Foliation (geology)|foliated]]'', meaning the individual mineral grains split off easily into flakes or slabs. The word schist is derived from the [[Greek language|Greek]] word ''σχίζειν'' meaning "to split", which is a reference to the ease with which schists can be split along the plane in which the platy minerals lie. [[Image:Schist.jpg|left|thumb|[[Manhattan schist]], from Southeastern New York]] Most schists have been derived from [[clay]]s and [[mud]]s which have passed through a series of metamorphic processes involving the production of [[shale]]s, [[slate]]s and [[phyllite]]s as intermediate steps. Certain schists have been derived from fine-grained [[igneous rock]]s such as [[basalt]]s and [[tuff]]s. Most schists are mica schists, but [[graphite]] and [[chlorite_group|chlorite]] schists are also common. Schists are named for their prominent or perhaps unusual mineral constituents, such as [[garnet]] schist, [[tourmaline]] schist, [[glaucophane]] schist, etc. Schists are frequently used as [[dimension stone]]. ==Formation== During metamorphism, rocks which were originally sedimentary or igneous are converted into schists and [[gneiss]]es. 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 the whole district, for example, occupied by these rocks have traces of bedding, [[clastic]] structure, or [[Unconformity|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, for example, as [[limestone]]s, [[dolomite]]s, [[quartzite]]s and aluminous shales have very definite chemical characters which distinguish them even when completely recrystallized. 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 [[ironstone]]s ([[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 [[serpentine]]s (once [[ultramafic]] masses rich in [[olivine]]), and the white mica-schists, porphyroids and banded [[halleflinta]]s, which have been derived from [[rhyolite]]s, quartz-porphyries and felsic [[tuff]]s. The majority of mica-schists, however, are altered [[claystone]]s 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 [[Graphite|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. As a special subgroup we have 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. <ref name=EB1911>{{1911|article=Petrology}}</ref> ==See also== * [[List of minerals]] * [[List of rock textures]] ==References== <references /> [[Category:Metamorphic rocks]] [[ar:الشيست]] [[cs:Svor (hornina)]] [[de:Glimmerschiefer]] [[es:Esquisto]] [[eo:Skisto]] [[fr:Micaschiste]] [[he:שיסט]] [[hu:Kristályos pala]] [[it:Scisto]] [[mk:Шкрилец]] [[nl:Schist]] [[ja:結晶片岩]] [[pt:Xisto]] [[ru:Сланцы (горные породы)]] [[sk:Kryštalická bridlica]] [[fi:Liuske]] [[sv:Glimmerskiffer]] [[zh:片岩]]