Biotite and muscovite are the types of mica found in schist rock.
Micas are any of several minerals known for their ability to cleave into thin flexible sheets. They are abundant around the world in many colors and common to many kinds of rocks. The most common source for mica is mica schist, but in fact schist contains only two types of mica, biotite or muscovite. Others include ruby, green and phologopite (or amber) mica.
Biotite
Biotite is a mica common to many igneous and metamorphic rocks, including schists of all kinds, where its distinctive dark color is visible to the naked eye. Biotite is usually jet black, though it can also be dark green or brown. It is also notable for its perfect cleavage into thin, flexible black strips. Biotite schist is usually all biotite with some muscovite.
Muscovite
Muscovite is also common to many igneous and metamorphic rocks, including all kinds of schists. This mica is lighter in color, typically yellow, brown, green, red or transparent. It, like all micas, has perfect cleavage into thin, transparent sheets. Muscovite schist is light to dark gray in color, but its large deposits of muscovite makes it glitter.
How Mica Gets Inside Schist
Schist forms when shale, clay, lime and igneous rocks are exposed to heat and pressure over a long period of time, usually on the ocean floor, making it a metamorphic rock. It is not under these extreme conditions enough for all the components to blend into one homogeneous rock, as in slate, leaving large chunks of mica and other minerals in its composition. These large crystals are also what makes schist so easy to split.
Other Minerals in Schist
Other common schists include garnet schist, chlorite schist, blue schist and green schist. The composition and quantity of minerals depends on the type of schist, but most schists contain quartz, feldspar, chlorite, garnet, hornblende, actinolite, kyanite, magnetite and pyrite (also known as fool's gold) among many others.
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