Schist << shihst >> is a medium- to coarse-grained rock that can easily be split into parallel layers. Schist is a metamorphic rock (see Metamorphic rock). It forms when heat and pressure alter the shape and mineral content of solid rock far below the surface of the earth. A variety of minerals occur in schist, depending on the chemical makeup of the original rock and the amount of heat and pressure that causes the change. Minerals commonly found in schist include micas, chlorite, quartz, and feldspar. Garnet, hornblende, staurolite, and kyanite or andalusite also occur in schist.
Schist commonly has flaky or elongated minerals lined up in parallel layers, along which the rock readily breaks. This structure is known as schistosity. The tendency to break along parallel layers is called rock cleavage. Schist that contains a large amount of mica is likely to cleave. Such rock has shiny breakage surfaces because mica flakes reflect light.
See also Rock (table: Common rocks).
