Biotite Mica

Hardness: 2.5-3 (Mohs), easily scratched or flaked; Color: Black, dark brown, dark green; Luster: Vitreous to pearly; Crystal Structure: Monoclinic, typically forms pseudohexagonal flakes or books; Cleavage: Perfect basal cleavage, allowing it to split into thin, flexible, elastic sheets; Specific Gravity: 2.7-3.3

Biotite is a common rock-forming mineral found in a wide range of igneous and metamorphic rocks. In igneous rocks, it crystallizes from magma or lava, especially in felsic to intermediate compositions like granite, diorite, and syenite. In metamorphic rocks, it forms during regional metamorphism of shale and other clay-rich sediments, where it is a key component of schists and gneisses, often associated with minerals like garnet and staurolite. It can also occur in some pegmatites. The geological age varies widely depending on the rock formation.

Biotite itself has limited industrial uses due to its iron content making it conductive and less desirable than muscovite for electrical insulation. However, it is a valuable indicator mineral for geologists studying rock formation and metamorphic grades. It is not typically used in jewelry due to its softness and perfect cleavage, but aesthetic specimens can be collected. Its presence can be an indicator of potential ore deposits for other minerals.

Biotite is a member of the mica group, known for its distinct platy structure. It is often referred to as 'iron mica' or 'black mica' and is one of the most common mica minerals. Biotite can be altered by weathering into vermiculite and chlorite. Its dark color helps distinguish it from muscovite (white mica). It also exhibits pleochroism, meaning its color changes when viewed from different angles.

In the field, biotite is easily identified by its dark color, platy habit, and perfect basal cleavage, which allows it to be peeled into very thin, flexible, and elastic sheets. It feels somewhat soft and greasy. It is commonly found in igneous rocks such as granite, granodiorite, and syenite, as well as in many metamorphic rocks like gneiss and schist. Its association with lighter-colored minerals like quartz and feldspar (as seen in the image) is typical of many granitic or granodioritic rocks or pegmatites. Collectors often look for well-formed 'books' of biotite in pegmatite occurrences. The specimen in the image appears to be a large biotite crystal embedded in a lighter-colored, possibly quartz or feldspar, matrix.