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Geology is the Way

Schist

Schist is a low- to medium-grade metamorphic rock with a well-defined foliation and containing minerals that are visible with the unaided eye. The name derives from the Greek schizo, ‘to tear’, in reference to its very strong fissility. Indeed, schists display a penetrative foliation – known as schistosity – defined by platy and elongated minerals oriented parallel to each other. Schistosity-defining minerals are, in the vast majority of cases, phyllosilicates (white mica, biotite, chlorite, talc…) and/or amphiboles. Many schists contain a very fine-grained layering with domains containing foliation-defining minerals interlayered with domains with granular phases (like quartz, feldspar, or pyroxenes). Schists may contain a wide range of silicates (e.g. garnet, chloritoid, staurolite, kyanite…) and accessory minerals (graphite, ilmenite…), depending on the chemical composition and the metamorphic grade. Any distinctive mineral present should be specified in the name of the rock (e.g. andalusite schist).

The term ‘schist’ is not restricted to coarse-grained schistose rocks but represents also a general term encompassing fine-grained varieties, like slate and phyllite. The boundary with the higher-grade rock gneiss is defined based on the spacing of foliation, which is less than 1 cm in schists and more than 1 cm in gneisses. In case of doubt, the term ‘gneiss’ should be used in preference for rocks that display well-defined compositional bands and very coarse grain size. Schists derive from the metamorphism of sedimentary rocks (mudstone, sandstone, marl…), volcanic rocks (like tuffs), and mafic igneous rocks (e.g. basalt). They are the medium-grade equivalents of phyllites and slates.

Well-foliated schists with deformed quartz veins (white), New Zealand. Photo by Marli Miller.

Outcrop of schist. Note the penetrative foliation. Southern Alps, New Zealand. Photo by Marli Miller.

An outcrop of staurolite-bearing schist. Tohmajärvi, Finland. Photo by Siim Sepp via Sandatlas.org.

Schist with large crystals of garnet. Syros, Greece. Photo by Graeme Churchard.

A sample of graphite schist from Trælen, Norway. Width of specimen 27 cm. Photo by Siim Sepp via Sandatlas.org.

Biotite – mica schist. The foliation is defined primarily by the preferred orientation of platy phyllosilicates. White grains are quartz and feldspars. Manhattan Island, New York City, USA. Width: 4.6 cm. Photo by James St. John.

A sample of muscovite-garnet-staurolite schist containing grains of garnet (red, equant), kyanite (blue) and staurolite (dark, elongated). Width of sample 7 cm. Photo by Siim Sepp via Sandatlas.org.

The foliation in this schist is defined by actinolite, a type of calcic amphibole. Width about 12 centimeters. Photo by James St. John.

References
Laird, J., & Albee, A. L. (1981). High-pressure metamorphism in mafic schist from northern Vermont. American Journal of Science281(2), 97-126.
Mortimer, N. (1993). Jurassic tectonic history of the Otago schist, New Zealand. Tectonics12(1), 237-244.

        

See also
Schist – Sandatlas.org
Schist – Alexstrekeisen.it
Rocks and Minerals in Thin Section – a Colour Atlas

it_IT Italiano
Metamorphic Minerals
Metamorphic Structures
Metamorphic Rocks

 

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