Great South Gems & Minerals

• Biotite •
(Montana)

• Biotite •
(Montana)

Black mica - Basic patassium - magnesium - iron - aluminum silicate

Biotite is a black mica. Basic patassium, magnesium, iron, aluminum silicate. Hardness 2-1/2 to 3. Forms in thin layers or plates. Very elastic. Opaque to translucent.

Biotite is a sheet silicate. Iron, magnesium, aluminum, silicon, oxygen and hydrogen form sheets that are weekly bond together by potassium ions. Biotite is sometimes called "iron mica" and sometimes "black mica". Like other mica minerals, biotite has a highly perfect basal cleavage and consist of flexible sheets, which easily flake off. When biotite is found in large chunks, they are called "books" because it resembles a book with pages of many sheets.

The largest documented single crystals of biotite were about 75 square feet sheets found in Iveland, Norway. Biotite has a hardness of about 2-1/2 to 3. About the hardness of a fingernail. Biotite is named after the French physicist, Jean Baptiste Biot (1774 - 1862), who studied the optical properties of the micas.

Biotite is a common rock-forming silicate mineral. It is a mica found in may types of igneous rocks and in some metamorphic rocks.

From the Rocky Boy Indian Reservation, Havre, Montana.

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