Biotite Mineral Geology and Uses

Discovery and Properties

Humans have known about and used mica since prehistoric times. In 1847, German mineralogist J.F.L. Hausmann named the mineral biotite in honor of French physicist Jean-Baptiste Biot, who explored the optical properties of mica.

Many minerals in the Earth's crust are silicates, but mica is distinct in the way it forms monoclinic crystals stacked to form hexagons. The flat faces of the hexagonal crystals give mica a glassy, pearlescent appearance. It's a soft mineral, with a Mohs hardness of 2.5 to 3 for biotite.

Biotite forms sheets of iron, silicon, magnesium, aluminum, and hydrogen weakly bonded by potassium ions. Stacks of sheets form what are called "books" because of their resemblance to pages. Iron is the key element in biotite, giving it a dark or black appearance, while most forms of mica are pale in color. This gives rise to the common names of biotite, which are "dark mica" and "black mica." Black mica and "white mica" (muscovite) often occur together within a rock and may even be found side-by-side.

Biotite isn't always black. It can be dark brown or brownish-green. Lighter colors also occur, including yellow and white.

Like other types of mica, biotite is a dielectric insulator. It is lightweight, reflective, refractive, flexible, and elastic. Biotite may be either translucent or opaque. It resists degradation from temperature, moisture, light, or electrical discharge. Mica dust is considered a workplace hazard because inhaling the tiny silicate particles can lead to lung damage.

Where to Find Biotite

Biotite is found in igneous and metamorphic rocks. It forms over a range of temperatures and pressures when the aluminosilicate crystallizes. It is an abundant mineral, calculated to account for about 7 percent of the continental crust. It is found in the lava from Mount Vesuvius, the Monzoni intrusive complex of the Dolomites, and in granite, pegmatite, and schist. Biotite is so common that it's considered a rock-forming mineral. If you pick up a rock and see glittery flashes, there's a good chance the sparkles come from biotite.

Biotite and most mica occur as small flakes in rocks. However, large crystals have been found. The biggest single crystal of biotite measured about 7 square meters (75 square feet), from Iveland, Norway.

Uses of Biotite

Biotite is used to determine the age of rock through the process of argon-argon dating or potassium-argon dating. Biotite can be used to determine the minimum age of rock and profile its temperature history.

Sheet mica is important in the electronics industry as an electrical and thermal insulator. Mica is birefringent, making it useful to make wave plates. Because the mineral flakes into ultra-flat sheets, it can be used as an imaging substrate in atomic force microscopy. Large sheets may also be used for decorative purposes.

All forms of mica, including biotite, may be ground and mixed. The main use of ground mica is to make gypsum board or drywall for construction. It is also used as an additive to drilling fluid in the petrochemical industry, as a filler in the plastics industry, to make pearlescent paint in the automotive industry, and to make asphalt and roofing shingles. Mica is used in Ayurveda to prepare Abhraka bhasma for the treatment of digestive and respiratory ailments.

Because of its dark coloration, biotite isn't used as extensively as other forms of mica for optical purposes or to make glitter, pigments, toothpaste, and cosmetics. 

Key Takeaways

• Biotite is a dark-colored mica. It is an aluminosilicate mineral that forms sheets or flakes.
• Although biotite is sometimes called black mica, it occurs in other colors, including brown, greenish-brown, yellow, and even white.
• Biotite occurs with other types of mica, even within a single rock.
• The primary use of biotite is to date the minimum age of rocks and geological features.

Sources

• Carmichael, I.S.; Turner, F.J.; Verhoogen, J. (1974). Igneous Petrology. New York: McGraw-Hill. p. 250.
• P. C. Rickwood (1981). "The largest crystals" (PDF). American Mineralogist. 66: 885–907.
• W. A. Deer, R. A. Howie and J. Zussman (1966) An Introduction to the Rock Forming Minerals, Longman.