Schist is a medium-grained metamorphic rock showing pronounced schistosity. This means that the rock is composed of mineral grains easily seen with a low-power hand lens, oriented in such a way that the rock is easily split into thin flakes or plates. This texture reflects a high content of platy minerals, such as micas, talc, chlorite, or graphite. These are often interleaved with more granular minerals, such as feldspar or quartz.
Schist typically forms during regional metamorphism accompanying the process of mountain building and usually reflects a medium grade of metamorphism. Schist can form from many different kinds of rocks, including sedimentary rocks such as mudstones and igneous rocks such as tuffs. Schist metamorphosed from mudstone is particularly common and is often very rich in mica. Where the type of the original rock is discernible, the schist is usually given a name reflecting its protolith, such as schistose metasandstone. Otherwise, the names of the consitutent minerals will be included in the rock name, such as quartz-felspar-biotite schist.
Schist bedrock can pose a challenge for civil engineering because of its pronounced planes of weakness.