Schist

This texture reflects a high content of platy minerals, such as mica, talc, chlorite, or graphite.

Schist bedrock can pose a challenge for civil engineering because of its pronounced planes of weakness.

[2] Geologists define schist as medium-grained metamorphic rock that shows well-developed schistosity.

[4][8] Schists are defined by their texture without reference to their composition,[9][4] and while most are a result of medium-grade metamorphism, they can vary greatly in mineral makeup.

Grains of these minerals are strongly oriented in a preferred direction in schist, often also forming very thin parallel layers.

[4] Though not a defining characteristic, schists very often contain porphyroblasts (individual crystals of unusual size) of distinctive minerals, such as garnet, staurolite, kyanite, sillimanite, or cordierite.

Nonhydrostatic stress is characteristic of regional metamorphism where mountain building is taking place (an orogenic belt).

[22] Graphite schist is uncommon but can form from metamorphosis of sedimentary beds containing abundant organic carbon.

[26] In geotechnical engineering a schistosity plane often forms a discontinuity that may have a large influence on the mechanical behavior (strength, deformation, etc.)

On August 17, 1959, a magnitude 7.2 earthquake destabilized a mountain slope near Hebgen Lake, Montana, composed of schist.