Biotite Mica

Physical Properties

Hardness: 2.5–3.0 on Mohs scale; Color: Black, dark brown, or greenish black; Luster: Vitreous to pearly; Crystal structure: Monoclinic; Cleavage: Perfect basal cleavage (easily peels into thin flexible sheets); Specific gravity: 2.7–3.3

Formation & Geological History

Formed in a wide range of geological environments including igneous rocks like granite and pegmatites, and metamorphic rocks like schist and gneiss. It is a common rock-forming mineral found in various geological ages.

Uses & Applications

Used as a filler in paints, as a lubricant, in the manufacturing of rubber and asphalt shingles, and for geological age dating through potassium-argon techniques. It has low value in jewelry but is a common collector specimen.

Geological Facts

Biotite is known as 'black mica'. Its name was given in honor of the French physicist Jean-Baptiste Biot who studied the optical properties of micas. It is a vital component in determining the cooling history of rocks.

Field Identification & Locations

Identify in the field by its dark color, metallic or vitreous luster, and its ability to be split into paper-thin, flexible sheets using a knife or fingernail. It is found globally in mountainous terrains with exposed basement rock.

Identified on: 5/7/2026

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