It occurs in plutonic igneous rocks ranging from granites to gabbronorite to syenites. Large crystals may form in pegmatites. It can form phenocrysts in lava flows of dacitic or andesitic composition. Biotite is also common in metamorphic rocks where it is a frequent component of schists, gneisses and phyllites. Biotite survives a certain amount of weathering and is fouund in soils, sediments and sedimentary rocks.
Weathered biotite becomes relatively brassy of bronzy in color and has been mistaken for gold.
Its lower density, cleavage, and other properties are soon apparent to the careful observer. Finally biotite can form as a result of hydrothermal processes, especially wall-rock alteration around ore veins. The localities listed below highlight typical or remarkable biotite occurences in the state. Biotite is a widespread component of the fine-grained schists, phyllites and slates of the Tyler Formation throughout the region. Biotite is common in schist along Cunningham Creek, east of Highway 73-95, in NW SW Sec.