Biotite Mica

Hardness: 2.5-3 (Mohs Scale); Color: Dark brown, black, dark green; Luster: Vitreous to pearly; Crystal structure: Monoclinic, typically found in flaky layers or books; Cleavage: Perfect basal cleavage, splitting into thin, flexible sheets; Specific gravity: 2.7-3.3

Biotite forms in a wide range of igneous and metamorphic rocks. In igneous rocks, it crystallizes from magma as an essential mineral in granites, gabbros, and diorites, or as a phenocryst in volcanic rocks. In metamorphic rocks, it is a common mineral in schists, gneisses, and hornfels, forming under various pressure and temperature conditions. Its formation is generally associated with continental crust processes and can range from Precambrian to Cenozoic.

While not a primary ore, biotite is used as an insulating material in electrical applications due to its excellent dielectric properties. Finely ground biotite can be used as a filler in paints, plastics, and rubber. Due to its pearly luster, it is occasionally used in decorative applications or as a pigment, though less commonly than muscovite.

Biotite is a member of the mica group, notorious for its perfect basal cleavage, allowing it to be easily split into thin, flexible sheets. It is often referred to as 'iron mica' due to its significant iron content, which distinguishes it from muscovite (white mica). It is a key rock-forming mineral and an important indicator of metamorphic grade.

In the field, biotite is identified by its dark color (black, dark brown, or dark green), its characteristic pearly to submetallic luster, and most notably by its ability to split into very thin, flexible, and elastic sheets. It feels somewhat soft and can be scratched with a fingernail (though with some difficulty). It is commonly found in granites, pegmatites, schists, and gneisses worldwide. To identify it, look for these dark, flaky crystals in rock outcrops. While less common, larger 'books' of biotite can be found in pegmatites.