Dictionary:

biotite

  (bī'ə-tīt')

A dark-brown or dark-green to black mica, K(Mg,Fe)₃(Al,Fe)Si₃O₁₀(OH)₂, found in igneous and metamorphic rocks.

[After Jean Baptiste Biot (1774–1862), French physicist.]

An iron-magnesium-rich layer silicate; it is also known as black mica. Biotite is the most abundant of the mica group of minerals. The name is derived from that of the French chemist J. Biot. The formula for the ideal end member, phlogopite, is KMg₃AlSi₃O₁₀(OH)₂. The more general formula is AX₃Y₄O₁₂(Z)₂, where A (interlayer cation) = K, Na, Ca, Ba, or vacancies; X (octahedral cations) = Li, Mg, Fe²⁺, Fe³⁺, Al, Ti, or vacancies; and Y (tetrahedral cation) = Fe³⁺, Al, Si; Z = (OH), F, Cl, O²⁻. This formula is more indicative of the wide range of compositions known for this mineral. Biotite has no commercial value, but vermiculite, an alteration product of magnesium-rich biotite, is used as insulation, as packing material, and as an ingredient for potting soils. See also Vermiculite.

Biotites are found commonly in igneous and metamorphic rocks. They are the common ferromagnesian phase in most granitic rocks, and are also found in some siliceous and intermediate volcanic rocks. In basaltic rocks biotite sometimes occurs in the crystalline groundmass, and is a common late interstitial phase in gabbroic rocks. It has been recognized in samples of the Earth's mantle found as inclusions in volcanic rocks. Biotites are not stable at the surface of the Earth, as they decompose by both hydrolysis and oxidation when exposed to the Earth's atmosphere. They alter to vermiculite, chlorite, and iron oxides, and thus are uncommon in sedimentary rocks. Biotites are important constituents of metamorphic rocks such as schist and gneiss, and the first appearance of biotite is an important marker in metamorphism. Biotite persists to very high grades of metamorphism, where it reacts with quartz to form granulites made up of potassium feldspar and orthopyroxene, garnet, or cordierite, in addition to quartz and plagioclase. Under conditions of ultrametamorphism, biotite reacts with quartz, plagioclase, and alkali feldspar to form siliceous melts. Biotite is also a common gangue mineral in ore deposits. The mineral has been used as an indicator of H₂O, HF, O₂, and S₂ activities in both rock- and ore-forming processes. See also Igneous rocks; Metamorphic rocks; Metamorphism; Mica; Silicate minerals.

(click to enlarge)
Biotite mica from the district of Juchitán, Oaxaca, Mex. (credit: Courtesy of the Field Museum of Natural History, Chicago; photograph, John H. Gerard-EB Inc.)

Silicate mineral in the common mica group. It is abundant in metamorphic rocks, in pegmatites, and in granites and other igneous rocks. Biotite is a layer silicate structure in which aluminum and silicon occur in infinitely extending Si-Al-O sheets that alternate with potassium-rich and magnesium- (and iron-) containing sheets.

For more information on biotite, visit Britannica.com.

Environment

Like muscovite, one of the rock-making minerals of igneous and metamorphic rocks, but rarer than muscovite and associated with less alkalic pegmatites; no show specimens.

Good crystals common in pegmatites and in some metamorphosed, impure limestones; usually tabular, sometimes somewhat barrel-shaped. Most often in embedded grains, sometimes intergrown with muscovite, but it does not form platy groups like muscovite.

Dark brown to black, rarely (in Vesuvian bombs) light yellow. Luster glassy; hardness 2Ɖ-3; specific gravity 2.8-3.4; cleavage perfect basal. Peels, yielding thin flexible and elastic sheets; very opaque as a rule.

Alkaline potassium, magnesium, iron, aluminum silicate (averaging about 8.5% K ₂ O, 21.0% MgO, 13.0% FeO plus Fe2O3, 16.0% Al ₂ O ₃ , 38.0% SiO ₂ , and 3.5% H ₂ O).

Fuses easily on thin edges to a dull, black magnetic glass.

Distinguished from the other micas by the dark color; in its rare (Vesuvian) light phases, by its sulfuric acid reaction (forming milky solution on boiling in strong acid).

A common mineral of pegmatites, often taking the place of muscovite in pegmatites bearing rare-earth minerals; hence, a useful hint of that type of pegmatite. Associated feldspar likely to be brick red. Good crystals are found in New England and elsewhere. Commonly found in sections of the same occurrences as muscovite. Small, complex, light-colored crystals occur in cavities in the Vesuvius limestone blocks. Often in dark volcanic rocks as larger crystals (porphyry phenocrysts).

Two fragments of Biotite

A Biotite slice

Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula K(Mg, Fe)₃AlSi₃O₁₀(F, OH)₂. More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers include siderophyllite.

Biotite is a sheet silicate. Iron, magnesium, aluminum, silicon, oxygen, and hydrogen form sheets that are weakly bond together by potassium ions. It is sometimes called "iron mica" because it is more iron-rich than phlogopite. It is also sometimes called "black mica" as opposed to "white mica" (muscovite) -- both form in some rocks, 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 has a hardness of 2.5 - 3, a specific gravity of 2.7 - 3.1, and an average density of 3.09 g/cm³. It is colored greenish to brown or black, and even yellow when weathered. It can be transparent to opaque, has a vitreous to pearly lustre, and a grey-white streak. In its weathered yellow, sparkly form, it is a common type of “fool’s Gold” (Pyrite is the official “fool’s Gold”). When biotite is found in large chunks, they are called “books” because it resembles a book with pages of many sheets.

Biotite is found in a wide variety of igneous rocks and metamorphic rocks. For instance, biotite occurs in the lava of Mount Vesuvius and at Monzoni. It is an essential phenocryst in some varieties of lamprophyre. Biotite is occasionally found in large sheets, especially in pegmatite veins, as in New England, Virginia and North Carolina. Other notable occurrences include Bancroft and Sudbury, Ontario. It is an essential constituent of many metamorphic schists, and it forms in suitable compositions over a wide range of pressure and temperature.

It is not industrially useful, but it is mined using quarrying and underground mining (depending on the depth of the biotite) for collection purposes.

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.

Biotite is used in electrical devices.

Biotite was named by J.F.L. Hausmann in 1847 in honour of the French physicist Jean-Baptiste Biot, who, in 1816, discovered optical properties of micas.

See also

• List of minerals

References

"The Mineral Biotite". 1995,1996 Amethyst Gallery Inc. [1]

"Biotite Mineral Data". [2]

"Biotite". [3]

"BIOTITE". LoveToKnow 1911 Online Encyclopedia. 2003, 2004 LoveToKnow. [4]

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