Biotite Mica

Hardness: 2.5-3 (Mohs), easily scratched with a fingernail. Color: Dark brown, black, dark green. Luster: Vitreous to pearly on cleavage surfaces, duller on other surfaces. Crystal structure: Monoclinic, typically forms in thin, flexible sheets or books. Cleavage: Perfect basal cleavage in one direction, producing thin flakes. Specific gravity: 2.7-3.3.

Forms in a wide range of igneous rocks (e.g., granite, diorite, gabbro), metamorphic rocks (e.g., gneiss, schist), and less commonly in some sedimentary rocks. Crystallizes during the cooling of magma or during regional or contact metamorphism. Its geological age varies greatly depending on the rock formation it is found in, ranging from Precambrian to Cenozoic.

While not a primary ore mineral, biotite is used in some niche applications. Ground mica, which can include biotite, is used as a filler and extender in paints, plastics, and rubber. It's also used in drilling muds and as an insulating material due to its heat-resistant properties. While generally not a gemstone, large, well-formed crystals can be collected.

Biotite is part of the mica group and is often referred to as 'iron mica' or 'black mica' due to its typically dark color and iron content. It is known for its excellent cleavage, allowing it to be split into very thin, flexible, and elastic sheets. These sheets are often transparent to translucent when thin. Biotite weathers relatively easily compared to other rock-forming minerals, altering to chlorite or clay minerals.

In the field, biotite is identified by its dark color, platy or flaky habit, and flexible, elastic sheets that can be easily peeled off with a fingernail or knife. It almost always occurs in igneous and metamorphic rocks. Its perfect basal cleavage is a key diagnostic feature. It's commonly found throughout the world wherever granitic, metamorphic, and some volcanic rocks are present. Collectors often seek out large, well-formed 'books' of biotite.