Schist

The schists form a group of metamorphic rocks chiefly notable for the preponderance of lamellar minerals such as micas, chlorite, talc, hornblende, graphite, etc. Quartz often occurs in drawn-out grains to such an extent that quartz schist is produced. Most schists have in all probability been derived from clays and muds which have passed through a series of metamorphic processes involving the production of shales, slates and phyllites as intermediate steps. Certain schists have been derived from fine-grained igneous rocks such as lavas and tuffs. Most schists are mica schists, but graphite and chlorite schists are common. Schists are named for their prominent or perhaps unusual mineral constituents, such as garnet schist, tourmaline schist, glaucophane schist, etc. The word schist is derived from the Greek meaning to split, which is in reference to the ease with which schists can be split along the plane in which the platy minerals lie.