The most distinctive properties of biotite are the combination of strong pleochroism and parallel extinction. Although hornblende has similar pleochroic colors and can be confused with biotite, hornblende has inclined extinction, not parallel extinction. Biotite is a common mineral in siliceous intrusive rocks such as granite, quartz monzonite, granodiorite, and tonalite. It is less common in siliceous volcanic rocks.
Biotite appears in shale, phyllite, and schist derived from shale protoliths.