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Biotite aggregate - Ochtendung, Eifel, Germany.jpg
Thin tabular biotite aggregate
(Image width: 2.5 mm)
Category Phyllosilicate
(repeating unit)
Crystal system Monoclinic
Crystal class Prismatic (2/m)
(same H-M symbol)
Space group C2/m
ColorDark brown, greenish-brown, blackish-brown, yellow, white
Crystal habit Massive to platy
Twinning Common on the [310],
less common on the {001}
Cleavage Perfect on the {001}
Fracture Micaceous
Tenacity Brittle to flexible, elastic
Mohs scale hardness2.5–3.0
Luster Vitreous to pearly
Streak White
Diaphaneity Transparent to translucent to opaque
Specific gravity 2.7–3.3 [1]
Optical propertiesBiaxial (-)
Refractive index nα = 1.565–1.625
nβ = 1.605–1.675
nγ = 1.605–1.675
Birefringence δ = 0.03–0.07
Pleochroism Strong
Dispersion r < v (Fe rich);
r > v weak (Mg rich)
Ultraviolet fluorescence None
References [2] [3] [1]

Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula K(Mg,Fe)
. It is primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous end-members include siderophyllite and eastonite. Biotite was regarded as a mineral species by the International Mineralogical Association until 1998, when its status was changed to a mineral group. [4] [5] The term biotite is still used to describe unanalysed dark micas in the field. Biotite was named by J.F.L. Hausmann in 1847 in honor of the French physicist Jean-Baptiste Biot, who performed early research into the many optical properties of mica. [6]


Members of the biotite group are sheet silicates. Iron, magnesium, aluminium, silicon, oxygen, and hydrogen form sheets that are weakly bound together by potassium ions. The term "iron mica" is sometimes used for iron-rich biotite, but the term also refers to a flaky micaceous form of haematite, and the field term Lepidomelane for unanalysed iron-rich Biotite avoids this ambiguity. Biotite is also sometimes called "black mica" as opposed to "white mica" (muscovite) – both form in the same rocks, and in some instances side-by-side.


Like other mica minerals, biotite has a highly perfect basal cleavage, and consists of flexible sheets, or lamellae, which easily flake off. It has a monoclinic crystal system, with tabular to prismatic crystals with an obvious pinacoid termination. It has four prism faces and two pinacoid faces to form a pseudohexagonal crystal. Although not easily seen because of the cleavage and sheets, fracture is uneven. It appears greenish to brown or black, and even yellow when weathered. It can be transparent to opaque, has a vitreous to pearly luster, and a grey-white streak. When biotite crystals are found in large chunks, they are called "books" because they resemble books with pages of many sheets. The color of biotite is usually black and the mineral has a hardness of 2.5–3 on the Mohs scale of mineral hardness.

Biotite dissolves in both acid and alkaline aqueous solutions, with the highest dissolution rates at low pH. [7] However, biotite dissolution is highly anisotropic with crystal edge surfaces (h k0) reacting 45 to 132 times faster than basal surfaces (001). [8] [9]

Optical properties

In thin section, biotite exhibits moderate relief and a pale to deep greenish brown or brown color, with moderate to strong pleochroism. Biotite has a high birefringence which can be partially masked by its deep intrinsic color. [10] Under cross-polarized light, biotite exhibits extinction approximately parallel to cleavage lines, and can have characteristic bird's eye extinction, a mottled appearance caused by the distortion of the mineral's flexible lamellae during grinding of the thin section. Basal sections of biotite in thin section are typically approximately hexagonal in shape and usually appear isotropic under cross-polarized light. [11]


Members of the biotite group are found in a wide variety of igneous and metamorphic rocks. For instance, biotite occurs in the lava of Mount Vesuvius and in the Monzoni intrusive complex of the western Dolomites. Biotite in granite tends to be poorer in magnesium than the biotite found in its volcanic equivalent, rhyolite. [12] Biotite is an essential phenocryst in some varieties of lamprophyre. Biotite is occasionally found in large cleavable crystals, especially in pegmatite veins, as in New England, Virginia and North Carolina USA. Other notable occurrences include Bancroft and Sudbury, Ontario Canada. It is an essential constituent of many metamorphic schists, and it forms in suitable compositions over a wide range of pressure and temperature. It has been estimated that biotite comprises up to 7% of the exposed continental crust. [13]

An igneous rock composed almost entirely of dark mica (biotite or phlogopite) is known as a glimmerite or biotitite. [14]

Biotite may be found in association with its common alteration product chlorite. [11]

The largest documented single crystals of biotite were approximately 7 m2 (75 sq ft) sheets found in Iveland, Norway. [15]


Biotite is used extensively to constrain ages of rocks, by either potassium-argon dating or argon–argon dating. Because argon escapes readily from the biotite crystal structure at high temperatures, these methods may provide only minimum ages for many rocks. Biotite is also useful in assessing temperature histories of metamorphic rocks, because the partitioning of iron and magnesium between biotite and garnet is sensitive to temperature.

Related Research Articles

Gneiss A common high-grade metamorphic rock

Gneiss is a common and widely distributed type of metamorphic rock. Gneiss is formed by high temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. Orthogneiss is gneiss derived from igneous rock. Paragneiss is gneiss derived from sedimentary rock. Gneiss forms at higher temperatures and pressures than schist. Gneiss nearly always shows a banded texture characterized by alternating darker and lighter colored bands and without a distinct foliation.

Mica phyllosilicate minerals

The mica group of sheet silicate (phyllosilicate) minerals includes several closely related materials having nearly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition. The nearly perfect cleavage, which is the most prominent characteristic of mica, is explained by the hexagonal sheet-like arrangement of its atoms.

Schist Medium grade metamorphic rock with lamellar grain

Schist is a medium-grade metamorphic rock formed from mudstone or shale. Schist has medium to large, flat, sheet-like grains in a preferred orientation. It is defined by having more than 50% platy and elongated minerals, often finely interleaved with quartz and feldspar. These lamellar minerals include micas, chlorite, talc, hornblende, graphite, and others. Quartz often occurs in drawn-out grains to such an extent that a particular form called quartz schist is produced. Schist is often garnetiferous. Schist forms at a higher temperature and has larger grains than phyllite. Geological foliation with medium to large grained flakes in a preferred sheetlike orientation is called schistosity.

Metamorphic rock Rock which was subjected to heat and pressure causing profound physical or chemical change

Metamorphic rocks arise from the transformation of existing rock types, in a process called metamorphism, which means "change in form". The original rock (protolith) is subjected to heat and pressure, causing profound physical or chemical change. The protolith may be a sedimentary, igneous, or existing metamorphic rock.

Chromite spinel, oxide mineral

Chromite is a mineral that is an iron chromium oxide. It has a chemical formula of FeCr2O4. It is an oxide mineral belonging to the spinel group. The element magnesium can substitute for iron in variable amounts as it forms a solid solution with magnesiochromite(MgCr2O4). A substitution of the element aluminium can also occur, leading to hercynite (FeAl2O4). Chromite today is mined particularly to make stainless steel through the production of ferrochrome (FeCr), which is an iron-chromium alloy.

Phlogopite true mica, phyllosilicate mineral

Phlogopite is a yellow, greenish, or reddish-brown member of the mica family of phyllosilicates. It is also known as magnesium mica.

Petrography is a branch of petrology that focuses on detailed descriptions of rocks. Someone who studies petrography is called a petrographer. The mineral content and the textural relationships within the rock are described in detail. The classification of rocks is based on the information acquired during the petrographic analysis. Petrographic descriptions start with the field notes at the outcrop and include macroscopic description of hand specimens. However, the most important tool for the petrographer is the petrographic microscope. The detailed analysis of minerals by optical mineralogy in thin section and the micro-texture and structure are critical to understanding the origin of the rock. Electron microprobe or atom probe tomography analysis of individual grains as well as whole rock chemical analysis by atomic absorption, X-ray fluorescence, and laser-induced breakdown spectroscopy are used in a modern petrographic lab. Individual mineral grains from a rock sample may also be analyzed by X-ray diffraction when optical means are insufficient. Analysis of microscopic fluid inclusions within mineral grains with a heating stage on a petrographic microscope provides clues to the temperature and pressure conditions existent during the mineral formation.

Metasomatism The chemical alteration of a rock by hydrothermal and other fluids

Metasomatism is the chemical alteration of a rock by hydrothermal and other fluids. It is the replacement of one rock by another of different mineralogical and chemical composition. The minerals which compose the rocks are dissolved and new mineral formations are deposited in their place. Dissolution and deposition occur simultaneously and the rock remains solid.

Scapolite marialite-meionite solid solution series

The scapolites (Gr. σκάπος, rod, and λίθος, stone) are a group of rock-forming silicate minerals composed of aluminium, calcium, and sodium silicate with chlorine, carbonate and sulfate. The two endmembers are meionite (Ca4Al6Si6O24CO3) and marialite (Na4Al3Si9O24Cl). Silvialite (Ca,Na)4Al6Si6O24(SO4,CO3) is also a recognized member of the group.

Hornfels A series of contact metamorphic rocks that have been baked and indurated by the heat of intrusive igneous masses

Hornfels is the group name for a set of contact metamorphic rocks that have been baked and hardened by the heat of intrusive igneous masses and have been rendered massive, hard, splintery, and in some cases exceedingly tough and durable. These properties are due to fine grained non-aligned crystals with platy or prismatic habits. The term is derived from the German word Hornfels, meaning "hornstone", because of its exceptional toughness and texture both reminiscent of animal horns. These rocks were referred to by miners in northern England as whetstones.

Lamprophyre igneous rock

Lamprophyres are uncommon, small volume ultrapotassic igneous rocks primarily occurring as dikes, lopoliths, laccoliths, stocks and small intrusions. They are alkaline silica-undersaturated mafic or ultramafic rocks with high magnesium oxide, >3% potassium oxide, high sodium oxide and high nickel and chromium.

Melilite åkermanite-gehlenite solid solution

Melilite refers to a mineral of the melilite group. Minerals of the group are solid solutions of several endmembers, the most important of which are gehlenite and åkermanite. A generalized formula for common melilite is (Ca,Na)2(Al,Mg,Fe2+)[(Al,Si)SiO7]. Discovered in 1793 near Rome, it has a yellowish, greenish-brown color. The name derives from the Greek words meli (μέλι) "honey" and lithos (λίθους) "stone".

Foliation (geology) repetitive layering in metamorphic rocks

Foliation in geology refers to repetitive layering in metamorphic rocks. Each layer can be as thin as a sheet of paper, or over a meter in thickness. The word comes from the Latin folium, meaning "leaf", and refers to the sheet-like planar structure. It is caused by shearing forces, or differential pressure. The layers form parallel to the direction of the shear, or perpendicular to the direction of higher pressure. Nonfoliated metamorphic rocks are typically formed in the absence of significant differential pressure or shear. Foliation is common in rocks affected by the regional metamorphic compression typical of areas of mountain belt formation.

This glossary of geology is a list of definitions of terms and concepts relevant to geology, its sub-disciplines, and related fields. For other terms related to the Earth sciences, see Glossary of geography terms.

Birds eye extinction

Bird's eye extinction, or bird's eye maple, is a specific type of extinction exhibited by minerals of the mica group under cross polarized light of the petrographic microscope. It gives the mineral a pebbly appearance as it passes into extinction. This is caused when the grinding tools used to create petrographic thin sections of precise thickness alter the alignment of the previously perfect basal cleavage planes which split micas up into its characteristic thin sheets. The resulting, slightly roughened surface alters the extinction angle of various parts of the crystal lattice, leading to this type of extinction. Since it is not a natural feature of the mineral, bird's eye extinction is not observed in all mica crystals, nor from all angles, but it is quite common, and is used as a diagnostic feature for micas.

Annite mica, phyllosilicate mineral

Annite is a phyllosilicate mineral in the mica family. It has a chemical formula of KFe32+AlSi3O10(OH)2. Annite is the iron end member of the biotite mica group, the iron rich analogue of magnesium rich phlogopite. Annite is monoclinic and contains tabular crystals and cleavage fragments with pseudohexagonal outlines. There are contact twins with composition surface {001} and twin axis {310}.

Subduction zone metamorphism Changes of rock due to pressure and heat near a subduction zone

A subduction zone is a region of the earth's crust where one tectonic plate moves under another tectonic plate; oceanic crust gets recycled back into the mantle and continental crust gets created by the formation of arc magmas. Arc magmas account for more than 20% of terrestrially produced magmas and are produced by the dehydration of minerals within the subducting slab as it descends into the mantle and are accreted onto the base of the overriding continental plate. Subduction zones host a unique variety of rock types created by the high-pressure, low-temperature conditions a subducting slab encounters during its descent. The metamorphic conditions the slab passes through in this process creates and destroys water bearing (hydrous) mineral phases, releasing water into the mantle. This water lowers the melting point of mantle rock, initiating melting. Understanding the timing and conditions in which these dehydration reactions occur, is key to interpreting mantle melting, volcanic arc magmatism, and the formation of continental crust.

S-type granites are a category of granites first proposed by Chappell & White. They are recognized by a specific set of mineralogical, geochemical, textural, and isotopic characteristics. S-type granites are over-saturated in aluminium, with an ASI index greater than 1.1 where ASI = Al2O3 / (CaO + Na2O +K2O) in mol percent; petrographic features are representative of the chemical composition of the initial magma as originally put forth by Chappell and White are summarized in their table 1.


Glimmerite is an igneous rock consisting almost entirely of dark mica. Glimmerite has also been referred to as biotitite, though the use of this term to describe phlogopite-rich rocks has been criticized. Glimmerite may contain minor rutile and ilmenite, and variants of glimmerite bearing graphite, spinel, ankerite, pyrite, apatite, and the carbonate minerals calcite and dolomite have been described.

Siilinjärvi carbonatite

The Siilinjärvi carbonatite complex is located in central Finland close to the city of Kuopio. It is named after the nearby village of Siilinjärvi, located approximately 5 km west of the southern extension of the complex. Siilinjärvi is the second largest carbonatite complex in Finland after the Sokli formation, and one of the oldest carbonatites on Earth at 2610±4 Ma. The carbonatite complex consists of a roughly 16 km long steeply dipping lenticular body surrounded by granite gneiss. The maximum width of the body is 1.5 km and the surface area is 14.7 km2. The complex was discovered in 1950 by the Geological Survey of Finland with help of local mineral collectors. The exploration drilling began in 1958 by Lohjan Kalkkitehdas Oy. Typpi Oy continued drilling between years 1964 and 1967, and Apatiitti Oy drilled from 1967 to 1968. After the drillings, the laboratory and pilot plant work were made. The mine was opened by Kemira Oyj in 1979 as an open pit. The operation was sold to Yara in 2007.


  1. 1 2 Handbook of Mineralogy
  2. Biotite mineral information and data Mindat
  3. Biotite Mineral Data Webmineral
  4. "The Biotite Mineral Group". Retrieved 29 August 2019.
  6. Johann Friedrich Ludwig Hausmann (1828). Handbuch der Mineralogie. Vandenhoeck und Ruprecht. p. 674. "Zur Bezeichnung des sogenannten einachsigen Glimmers ist hier der Name Biotit gewählt worden, um daran zu erinnern, daß Biot es war, der zuerst auf die optische Verschiedenheit der Glimmerarten aufmerksam machte." (For the designation of so-called uniaxial mica, the name "biotite" has been chosen in order to recall that it was Biot who first called attention to the optical differences between types of mica.)
  7. Malmström, Maria; Banwart, Steven (July 1997). "Biotite dissolution at 25°C: The pH dependence of dissolution rate and stoichiometry". Geochimica et Cosmochimica Acta. 61 (14): 2779–2799. doi:10.1016/S0016-7037(97)00093-8.
  8. Hodson, Mark E. (April 2006). "Does reactive surface area depend on grain size? Results from pH 3, 25°C far-from-equilibrium flow-through dissolution experiments on anorthite and biotite". Geochimica et Cosmochimica Acta. 70 (7): 1655–1667. doi:10.1016/j.gca.2006.01.001.
  9. Bray, Andrew W.; Oelkers, Eric H.; Bonneville, Steeve; Wolff-Boenisch, Domenik; Potts, Nicola J.; Fones, Gary; Benning, Liane G. (September 2015). "The effect of pH, grain size, and organic ligands on biotite weathering rates". Geochimica et Cosmochimica Acta. 164: 127–145. doi: 10.1016/j.gca.2015.04.048 .
  10. Faithful, John (1998). "Identification Tables for Common Minerals in Thin Section" (PDF). Retrieved March 17, 2019.
  11. 1 2 Luquer, Lea McIlvaine (1913). Minerals in Rock Sections: The Practical Methods of Identifying Minerals in Rock Sections with the Microscope (4 ed.). New York: D. Van Nostrand Company. p.  91. bird's eye extinction thin section grinding.
  12. Carmichael, I.S.; Turner, F.J.; Verhoogen, J. (1974). Igneous Petrology. New York: McGraw-Hill. p. 250. ISBN   978-0-07-009987-6.
  13. Nesbitt, H.W; Young, G.M (July 1984). "Prediction of some weathering trends of plutonic and volcanic rocks based on thermodynamic and kinetic considerations". Geochimica et Cosmochimica Acta. 48 (7): 1523–1534. doi:10.1016/0016-7037(84)90408-3.
  14. Morel, S. W. (1988). "Malawi glimmerites". Journal of African Earth Sciences. 7 (7/8): 987–997. doi:10.1016/0899-5362(88)90012-7.
  15. P. C. Rickwood (1981). "The largest crystals" (PDF). American Mineralogist. 66: 885–907.