Biotite

Biotite is a group of sheet silicate minerals belonging to the mica family, with a general chemical formula of K(Mg,Fe)₃AlSi₃O₁₀(F,OH)₂. It forms a solid-solution series, with annite as the iron-rich endmember and phlogopite as the magnesium-rich endmember; more aluminous varieties include siderophyllite and eastonite. Until 1998, the International Mineralogical Association treated biotite as a single mineral species, but it is now classified as a mineral group. The name is still used in the field for dark micas that haven’t been chemically analyzed. J.F.L. Hausmann named it in 1847 after French physicist Jean-Baptiste Biot, who studied mica’s optical properties.

Biotite’s structure consists of sheets of iron, magnesium, aluminum, silicon, oxygen, and hydrogen, weakly held together by potassium ions. Iron-rich biotite is sometimes called “iron mica,” but that term can also refer to a flaky, micaceous form of hematite; the field term “lepidomelane” avoids this confusion for unanalyzed iron-rich biotite. It is also known as “black mica,” in contrast to “white mica” (muscovite), and both can occur together in the same rocks, sometimes side by side.

**Properties** Like other micas, biotite has perfect basal cleavage and forms flexible sheets or lamellae that easily flake off. Its crystal system is monoclinic, with tabular to prismatic crystals ending in a prominent pinacoid; four prism faces and two pinacoid faces create a pseudohexagonal shape. Fracture is uneven, though hard to see due to cleavage. Color ranges from greenish to brown or black, turning yellow when weathered. It can be transparent to opaque, with a vitreous to pearly luster and a gray-white streak. Large crystals are called “books” because they resemble stacked pages. Hardness is 2.5–3 on the Mohs scale. Biotite dissolves in both acidic and alkaline solutions, with fastest dissolution at low pH, and dissolution is highly anisotropic—crystal edge surfaces react 45 to 132 times faster than basal surfaces.

**Optical Properties** In thin section, biotite shows moderate relief and a pale to deep greenish-brown or brown color, with moderate to strong pleochroism. It has high birefringence, often masked by its deep intrinsic color. Under cross-polarized light, it extinguishes roughly parallel to cleavage lines and may display a characteristic “bird’s eye maple” extinction—a mottled pattern from distortion of flexible lamellae during thin-section grinding. Basal sections are typically hexagonal and appear isotropic under cross-polarized light.

**Structure** Biotite’s crystal structure is TOT-c: parallel TOT layers weakly bonded by cations (c). Each TOT layer has two tetrahedral sheets (T) strongly bonded to both sides of a single octahedral sheet (O). Weak ionic bonding between TOT layers gives biotite its perfect basal cleavage. Tetrahedral sheets are made of silica tetrahedra (silicon surrounded by four oxygens), with one in four silicon atoms replaced by aluminum. Each tetrahedron shares three of its four oxygens with neighbors, forming a hexagonal sheet; the remaining apical oxygen bonds to the octahedral sheet. The octahedral sheet is trioctahedral, structured like a brucite sheet, with magnesium or ferrous iron as typical cations. Apical oxygens replace some hydroxyls from the brucite sheet, tightly bonding the tetrahedral and octahedral sheets. Tetrahedral sheets carry a strong negative charge (bulk composition AlSi₃O₁₀⁵⁻), while the trioctahedral sheet has a positive charge (M₃(OH)₂⁴⁺, where M is a divalent ion). The combined TOT layer has a residual negative charge (M₃(AlSi₃O₁₀)(OH)₂⁻), neutralized by interlayer potassium ions. Because the hexagons in the T and O sheets differ slightly in size, bonding distorts the sheets, breaking hexagonal symmetry to monoclinic, though the original symmetry is still visible in the pseudohexagonal crystal shape.

**Occurrence** Biotite group minerals occur in many igneous and metamorphic rocks. For example, biotite is found in lava from Mount Vesuvius and in the Monzoni intrusive complex of the western Dolomites. In granite, biotite tends to be poorer in magnesium than in its volcanic equivalent, rhyolite. It is an essential phenocryst in some lamprophyres. Large cleavable crystals are occasionally found in pegmatite veins, such as in New England, Virginia, and North Carolina, USA, and at Bancroft and Sudbury, Ontario, Canada. Biotite is a key component of many metamorphic schists and forms over a wide range of pressures and temperatures in suitable compositions. It is estimated to make up to 7% of the exposed continental crust. An igneous rock composed almost entirely of dark mica (biotite or phlogopite) is called glimmerite or biotitite. Biotite may be found alongside its common alteration product, chlorite.