Schists are a group of metamorphic rocks that have a visibly foliated crystalline structure. Due to this structure and the platy minerals the rock contains, schists tend to split easily into perfectly thin, flexible sheets along their layers. The minerals that are found in schists vary significantly, but talc, chlorite, muscovite, biotite, and graphite are some of the more common constituents. In order to more explicitly identify samples of the rocks, schists are typically classified and described based upon their mineralogical composition. Hornblende schist, for example, is a variety of schist rich in the amphibole mineral hornblende, though the rock may also contain an abundance of plagioclase feldspar and other substances as well.