Biotite

Biotite is a dark mica, a sheet silicate (phyllosilicate) of the mica group with the formula K(Mg,Fe)₃AlSi₃O₁₀(OH)₂, prized for its perfect basal cleavage into thin elastic flakes. Biotite crystallizes from silicate melts at temperatures between 700 and 900 degrees Celsius, making it one of the earliest minerals to form in cooling magma according to Bowen's reaction series. In granitic magmas, biotite nucleates when the melt is still above 750 degrees Celsius, incorporating potassium, iron, magnesium, and aluminum from the surrounding liquid. The sheet structure forms because silicon-oxygen tetrahedra link together in continuous two-dimensional planes, with layers of octahedrally coordinated iron and magnesium sandwiched between them. Potassium ions hold adjacent sheets together with weak electrostatic bonds.

In metamorphic rocks, biotite is a key index mineral. It first appears in the greenschist facies at temperatures around 400 degrees Celsius, marking the biotite isograd. Pelitic rocks (former mudstones and shales) develop biotite as clay minerals recrystallize under increasing temperature and pressure. The presence of biotite in a metamorphic rock tells geologists that the rock reached at least 400 degrees Celsius during metamorphism.

The largest biotite crystals grow in granitic pegmatites, where slow cooling and abundant volatiles (water, fluorine) allow crystals to reach enormous sizes. Biotite books exceeding one meter across have been recorded from pegmatites in Ontario, Canada. These giant crystals formed over thousands to tens of thousands of years as pegmatite fluids slowly cooled from roughly 600 to 400 degrees Celsius.

Biotite is one of the easiest minerals to identify in hand specimen. Look for dark brown to black, shiny flakes that peel apart into thin, flexible, elastic sheets along perfect basal cleavage. This is the defining feature: you can peel a flake off with a fingernail or knife tip, bend it, and it springs back to its original shape. At hardness 2.5 to 3, it scratches easily with a copper coin.

Distinguish it from muscovite (white or silver mica, same cleavage but much lighter color), phlogopite (brown mica, lighter color and higher magnesium content), and chlorite (green, sheets are flexible but not elastic, they stay bent). Unlike hornblende, which forms elongated prismatic crystals, biotite forms flat, tabular crystals or flaky aggregates. In rocks, biotite appears as shiny dark flecks that catch the light when the rock is rotated.