SCHIST


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".

Manhattan schist, from Southeastern New York 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.

Schists are named for their prominent or perhaps unusual mineral constituents, such as garnet schist, tourmaline schist, glaucophane schist, etc.

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. 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 color fairly consistent.

Schist is a medium crystalline rock that has a highly developed schistosity, or tendency to split into layers. Unlike its close cousin gneiss, banding is poorly developed or absent. Most schists are composed largely of platy minerals such as muscovite, chlorite, talc, biotite, and graphite; feldspar and quartz are much less abundant in schist than in gneiss. The green color of many schists and their formation under a certain range of temperature and pressure has led to a distinction of the greenschist facies in the mineral facies classification of metamorphic rocks. The parallel orientation of the platy minerals and well-developed folding of many schists indicate formation under stresses that are not the same in all directions. The mineralogy and high water content of the minerals indicate that they were formed under conditions of relatively low temperature and pressure.

Schists are usually classified on the basis of their mineralogy, with varietal names that indicate the characteristic mineral present. Talc schist contains abundant talc; it has a greasy feel, a well-developed schistosity, and a grayish-green colour. Mica schist often contains muscovite mica rather than biotite, although both minerals are common. It represents a somewhat higher grade of metamorphism than talc schist and is more coarse-grained; individual flakes of mica can be seen.
Schist is a metamorphic layer rock that is not typically dimensioned, with a split face.

Alpine schist is multicolored featuring green, grey and whites while Waitaha schist is sparkling grey in colour while Westland schist has gold flecks and is predominantly grey they are all extremely durable.

Waitaha and Alpine Schist are both derived from the West coast of New Zealand's South island rivers.
Westland schist is from the West Coast region of the south island.

Schist lends itself towards landscaping and walling applications which creates detail and beauty. It can also be used for flooring, paving, driveways.

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