biotite

Biotite is a common phyllosilicate mineral in the mica group with the ideal chemical formula K(Mg,Fe)3AlSi3O10(OH)2. It is a sheet silicate, consisting of stacked tetrahedral silica sheets separated by octahedral layers of magnesium, iron, and aluminum, with potassium ions situated between the sheets. This structure gives biotite its characteristic one-directional basal cleavage and makes the mineral capable of forming flexible, platy crystals.

Biotite occurs in a wide range of igneous and metamorphic rocks. It is a common constituent of granites, granodiorites, and rhyolites, and appears in high-temperature metamorphic rocks such as schists and gneisses. The iron-to-magnesium content controls its color, which ranges from brown to black, and it is typically transparent to translucent in thin flakes with a vitreous luster. Biotite has a Mohs hardness of about 2.5–3 and a specific gravity of roughly 2.7–3.4.

Two well-known varieties describe its chemical endmembers: annite, which is iron-rich, and phlogopite, which is magnesium-rich. Biotite can alter to other micas and clay minerals during weathering and metamorphism.

Geologically, biotite is significant for both petrology and geochronology. Its potassium content makes it useful for radiometric dating (K-Ar and Ar-Ar methods), aiding the determination of rock ages and metamorphic histories. As a relatively indicator mineral, biotite fragments help reconstruct conditions of formation and thermal histories in continental crust.