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The schists form a group of medium-grade metamorphic rocks, chiefly notable for the preponderance of lamellar minerals such as micas, 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.
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 foliated, meaning the individual mineral grains split off easily into flakes or slabs. The characteristic flaky texture of schist gives rise to the adjective "schistose".
Most schists have in all probability been derived from clays and muds which have passed through a series of metamorphic processes involving the production of shales, slates and phyllites as intermediate steps. Certain schists have been derived from fine-grained igneous rocks such as basalts and tuffs. Most schists are mica schists, but graphite and chlorite schists are also common.
The word schist is derived from the Greek meaning "to split", which is in reference to the ease with which schists can be split along the plane in which the platy minerals lie.
Schists are frequently used in building houses or walls, as many are quite durable and strong. However it should be noted that many foundation problems with buildings both large and small are due to the schist decaying or even the 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 within the New York city area used schist (the predominant rock that Manhattan is made of is Manhattan schist, and schist could also be easily found elsewhere in the area). Decorative rock walls on houses in the area also used a schist called "Yonkers Stone" which is no longer available. This schist is particularly hard and the color is fairly consistent.
During metamorphism, rocks which were originally sedimentary and rocks which were undoubtedly igneous are converted into schists and gneisses, and if originally of similar composition 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, we may distinguish between sedimentary and igneous schists and gneisses. Often the metamorphism is progressive, and if the whole district occupied by these rocks be searched for traces of bedding, of clastic structure, unconformability or other evidence may be obtained showing that we are dealing with a group of altered sediments. 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 limestones, calc-schists, dolomites, quartzites and aluminous shales have very definite chemical characters which distinguish them even when completely recrystallized.
The schists and gneisses are classified according to the minerals they consist of, and this depends principally on their chemical composition. We have, 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. Another 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 readily be believed to represent sediments once containing coaly matter or plant remains; there are also schistose ironstones (hematite-schists), but metamorphic beds of salt or gypsum are exceedingly uncommon. Among schists of igneous origin we may mention 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-shists, however, 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. As a special subgroup we have the andalusite-,staurolite-, kyanite- and sillimanite-schists, together with the cordierite-gneisses, which usually make their appearance 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 in appearance. They contain quartz, one or more varieties of feldspar, and usually mica, hornblende or augite, often garnet, iron oxides, etc. 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; the foliation of the matrix winds around these augen, closing in on each side. Most of these augen gneisses are metamorphic granites, but sometimes a conglomerate bed simulates a gneiss of this kind rather closely. There are onter gneisses, which were derived from feldspathic sandstones, grits, arkoses and sediments of that order; they mostly contain biotite and muscovite, but the hornblende and pyroxene gneisses are usually igneous rocks allied in composition to the hornblende-granites and quartz-diorites. The metamorphic forms of dolerite, basalt and the 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.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Schist". A list of authors is available in Wikipedia.|