Biotite Mica

Physical Properties

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

Formation & Geological History

Forms in a wide range of geological environments including igneous rocks like granite, metamorphic rocks like schist and gneiss, and occasionally as a primary mineral in sedimentary deposits through mechanical weathering.

Uses & Applications

Used as an additive in drilling mud, as a filler in rubber and plastics, in the manufacturing of asphalt shingles and rolled roofing, and as a decorative component in paints.

Geological Facts

Biotite is often called black mica to distinguish it from the lighter muscovite. It is an important index mineral used by geologists to determine the temperature and pressure history of rocks through potassium-argon dating.

Field Identification & Locations

Identified in the field by its dark color and the ease with which it can be peeled into flexible, elastic flakes with a fingernail. It is common in granitic regions and metamorphic terrains.

Identified on: 5/7/2026

Mode: Standard