Biotite Mica

Hardness: 2.5-3 (Mohs scale), Color: Dark brown, black, dark green; transparent in thin sheets, Luster: Vitreous to pearly, Crystal structure: Monoclinic, typically in tabular or pseudo-hexagonal crystals, Cleavage: Perfect basal cleavage (splits into thin, flexible sheets), Specific gravity: 2.7-3.3

Biotite is a common rock-forming mineral found in a wide range of igneous and metamorphic rocks. It forms during the cooling of magma (e.g., granite, diorite, gabbro) and during regional or contact metamorphism of shales and other fine-grained sedimentary rocks. It is present in rocks of all geological ages, from Precambrian to Cenozoic.

While not a primary ore, biotite has some industrial uses, primarily as a filler and extender in paints, rubber, and plastics. Crushed mica, including biotite, is used in drilling muds and as a decorative aggregate. It is less commonly used in electrical insulation compared to muscovite due to its iron content.

Biotite is a member of the mica group, characterized by its distinctive perfect basal cleavage which allows it to be split into very thin, flexible, and elastic sheets. It was named after the French physicist Jean-Baptiste Biot, who researched the optical properties of mica. The presence of iron in biotite is responsible for its typically dark color, distinguishing it from the lighter muscovite mica.

Biotite is easily identified in the field by its dark color, flaky or platy habit, and perfect basal cleavage, allowing it to be peeled into thin, flexible flakes with a fingernail or knife. It often appears as small, shining black or dark brown flakes in granites, schists, and gneisses. Common locations include pegmatites, granitic intrusions, and regionally metamorphosed terranes worldwide. Collectors often appreciate specimens with large, well-formed crystals or those embedded in interesting rock matrices. Be careful when handling as the thin flakes can be sharp.