biotiitin

Biotite is a common name for a group of dark brown to black mica minerals. It belongs to the monoclinic crystal system and is characterized by its perfect basal cleavage, which allows it to be easily split into thin, flexible sheets. Biotite is a phyllosilicate mineral, meaning its structure consists of layers of silica tetrahedra. The general chemical formula for biotite is K(Mg,Fe)3(AlSi3O10)(OH,F)2, indicating that it contains potassium, magnesium, iron, aluminum, silicon, oxygen, and hydroxyl or fluorine ions. The ratio of magnesium to iron varies significantly, leading to different end-members within the biotite group, such as phlogopite (rich in magnesium) and annite (rich in iron).

Biotite is a significant component of many igneous rocks, including granites, diorites, and basalts. It is also found in metamorphic rocks like schists and gneisses, and can occur in sedimentary rocks as well. Its presence often indicates the conditions under which the rock formed. Due to its dark color and ability to form large crystals, biotite is sometimes used in decorative applications, though its tendency to flake can limit its use. In geological studies, biotite can be used as an indicator of temperature and pressure conditions during rock formation, and isotopic dating of biotite can help determine the age of rocks.