Biotite is a common rock-forming mineral belonging to the mica group of sheet silicates. It is often referred to as “black mica” due to its typically dark appearance. Chemically, biotite is a complex potassium-magnesium-iron-aluminum silicate hydroxide with the approximate formula K(Mg, Fe)₃AlSi₃O₁₀(OH)₂. The presence of iron and magnesium atoms in its structure is responsible for its characteristic dark coloration, distinguishing it from lighter-colored micas. It is widespread, playing a significant role in the composition of many rocks globally.
The Defining Visuals: Color and Luster
The most immediate visual characteristic of biotite is its color, ranging from deep, opaque black to dark greenish-brown or blackish-brown. This variation is primarily due to the ratio of iron to magnesium within the crystal structure. Iron-rich varieties are black, while magnesium-rich forms, known as phlogopite, are lighter, sometimes appearing pale yellow to golden brown. Biotite generally forms in small, irregularly shaped flakes or thicker stacks of sheets called “books.” Its luster is vitreous (glassy) on fresh surfaces or pearly on cleavage faces, often giving the mineral a metallic or bronze-like sparkle. Biotite is typically opaque, though very thin flakes may be translucent.
The Crucial Sheet Structure
The defining property of biotite is its perfect basal cleavage, confirming its identity as a mica. The mineral splits cleanly along a single plane, resulting in extremely thin, parallel sheets. This behavior results from its internal atomic arrangement, where strong layers of aluminum silicate are separated by weaker bonds of potassium ions. The resulting flakes are elastic; they will bend but snap back to their original flat shape, distinguishing biotite from other dark minerals that crumble. Biotite is also relatively soft, scoring 2.5 to 3 on the Mohs scale of hardness, and this combination of perfect cleavage and low hardness is the most reliable way to identify black mica in hand samples.
Common Geological Contexts
Biotite is a common and widespread mineral found in a diverse range of rock types. It is a significant component in many igneous rocks, particularly silica-rich types such as granite, diorite, and syenite. In these rocks, it appears as small, dark flakes interspersed among lighter quartz and feldspar crystals. The mineral is also abundant in metamorphic rocks altered by heat and pressure, such as schists and gneisses, where its flaky crystals align to create the layered texture known as foliation. While durable, biotite is susceptible to chemical weathering over long periods, eventually breaking down into clay minerals.