schist
schist, megascopically crystalline rock that has a highly developed schistosity, or tendency to split into layers. Banding (foliation) is typically poorly developed or absent. Most schists are composed largely of platy minerals such as muscovite, chlorite, talc, sericite, biotite, and graphite; feldspar and quartz are much less abundant in schist than in gneiss. The green colour of many schists and their formation under a certain range of temperature and pressure has led to a distinction of the greenschist facies in the mineral facies classification of metamorphic rocks. The parallel orientation of the platy minerals and well-developed folding of many schists indicate formation under stresses that are not the same in all directions. The mineralogy and high water content of the minerals indicate that they were formed under conditions of relatively low temperature and pressure.
Schists are usually classified on the basis of their mineralogy, with varietal names that indicate the characteristic mineral present. Talc schist contains abundant talc; it has a greasy feel, a well-developed schistosity, and a grayish-green colour. Mica schist often contains muscovite mica rather than biotite, although both minerals are common. It represents a somewhat higher grade of metamorphism than talc schist and is more coarse-grained; individual flakes of mica can be seen.