Home Minerals Biotite


Biotite is the most common mica mineral and also known as black mica, a silicate mineral in the common mica group. Approximate chemical formula K (Mg, Fe). It can be found in massive crystal layers weighing several hundred pounds. It is abundant in metamorphic rocks (both regional and contact), pegmatites, and also in granites and other invasive magmatic rocks. Biotite usually occurs in brown to black, dark green variety.

It is a name used for a range of black mica minerals with different chemical compositions but with very similar physical properties. These minerals are usually indistinguishable from each other without laboratory analysis. There is a small list of biotite minerals that were down.

Crystallography: Monoclinic; prismatic. In tabular or short prismatic crystals with prominent basal planes. Crystals rare, frequently pseudorhombohedral. Usually in irregular foliated masses; often in disseminated scales or in scaly aggregates.

Composition: A potassium magnesium-iron-aluminum silicate, essentially K(Mg,Fe)3AlSi3Oio(OH)2

Diagnostic Features: Characterized by its micaceous cleavage and dark color

Name: In honor of the French physicist, J. B. Biot.

Similar Species: Glauconite, commonly found in green pellets in sedimentary deposits, is similar in composition to biotite.

MineralChemical Composition

Occurrence of Biotite

Biotite is found in a wide variety of magmatic and metamorphic rocks. It is common in various magmatic and metamorphic rocks. In igneous rocks, more commonly in silicic and alkaline rocks, e.g. granites, diorites, gabbros and peridotites. Schists are important in metamorphic rocks including gneisses, filites and horns. It is also found in immature sedimentary rocks, but changes the clay minerals when worn. The biotite in the granite tends to be weaker than the biotite present in the riolite in the volcanic equivalent of magnesium. It is sometimes found in large degradable crystals in pegmatite veins, as is sometimes the case in New England, Virginia and North Carolina USA. It is a fundamental component of many metamorphic shingers and forms in suitable compositions in a wide range of pressure and temperature. Biotite is estimated to be up to 7% of the exposed continental crust.

Biotite Physical Properties

Chemical ClassificationDark mica
ColorBlack, dark green, dark brown
StreakWhite to gray, flakes often produced
DiaphaneityThin sheets are transparent to translucent, books are opaque.
CleavageBasal, perfect
Mohs Hardness2.5 to 3
Specific Gravity2.7 to 3.4
Diagnostic PropertiesDark color, perfect cleavage
Chemical CompositionK(Mg,Fe)2-3Al1-2Si2-3O10(OH,F)2
Crystal SystemMonoclinic
UsesVery little industrial use

Biotite Optical Properties

Biotite under the microscope PPL and XPL
Crystal SystemMonoclinic (2/m)
Crystal HabitPseudo-hexagonal prisms or lamellar plates without crystal outline.
Physical PropertiesH = 2.5 – 3
G = 2.7 – 3.3The color of biotite in hand sample is brown to black (sometimes greenish). Its streak is white or gray, and it has a vitreous luster.
Cleavage(001) perfect
Color/PleochroismTypically brown, brownish green or reddish brown
Optic SignBiaxial (-)
Optic OrientationY=b
Z^a = 0 – 9o
X^c = 0 – 9o
optic plane (010)
Refractive Indices
alpha =
beta =
gamma =
Max Birefringence0.03-0.07
Extinction Parallel or close to parallel
Dispersionv > r (weak)

Biotite Uses

  • The modified biotite, produced by hydrothermal or abrasion processes, produces vermiculite, a porous material used as an insulating material in gypsum wall panels and other heat insulation applications. When heated, the vermiculite will expand to most of its initial volume at an accordion-like expansion of its layered structure. This makes it a very effective sealant.
  • In addition, soil is added to soil to improve the quality of the soil and to improve drainage as it easily absorbs liquids, fertilizers, herbicides and pesticides.
  • Biotite is commonly used to limit the age of rocks through potassium-argon escort or argon-argon escort.
  • Biotite is also useful for evaluating the temperature histories of metamorphic rocks, because the division of iron and magnesium between biotite and garnet is temperature sensitive.


• Bonewitz, R. (2012). Rocks and minerals. 2nd ed. London: DK Publishing.
• Dana, J. D. (1864). Manual of Mineralogy… Wiley.
• Mindat.org. (2019): Mineral information, data and localities.. [online] Available at: https://www.mindat.org/ [Accessed. 2019].
• Smith.edu. (2019). Geosciences | Smith College. [online] Available at: https://www.smith.edu/academics/geosciences [Accessed 15 Mar. 2019].