Schist is a metamorphic rock that comes in almost infinite variety, but its main characteristic is hinted at in its name: schist
comes from the ancient Greek for "split," through Latin and French. Schist is a rock formed by dynamic metamorphism at high temperatures and high pressures that aligns the grains of mica, hornblende and other flat or elongated minerals into thin layers, or foliation. At least 50 percent of the mineral grains in schist are aligned this way (less than 50 percent makes it gneiss
). The rock may or may not be actually deformed in the direction of the foliation, although a strong foliation probably is a sign of high strain
Schists are commonly described in terms of their predominant minerals. This specimen from Manhattan, for example, would be called a mica schist because the flat, shiny grains of mica are so abundant. Other possibilities include blueschist (glaucophane schist) or amphibole schist.