Biotite (Black Mica)

Physical Properties

Hardness: 2.5–3 on Mohs scale; Color: Black, dark green, or dark brown; Luster: Vitreous to pearly; Crystal structure: Monoclinic; Cleavage: Perfect basal (peels into thin sheets).

Formation & Geological History

Forms in a wide range of igneous and metamorphic environments. It is a common constituent in granite, diorite, and schists, typically forming at high temperatures and pressures.

Uses & Applications

Primarily used in industry as an insulator in electrical equipment, as a lubricant, and in the manufacturing of rubber and paint products. It is also significant in scientific dating through argon-argon and potassium-argon methods.

Geological Facts

Biotite was named in honor of the French physicist Jean-Baptiste Biot, who researched the optical properties of micas in the early 19th century.

Field Identification & Locations

Identify by its dark color and the way it flakes into thin, flexible, elastic sheets. Common in many igneous mountain ranges and metamorphic belts globally.

Identified on: 5/7/2026

Mode: Standard