Biotite mica 2 (31739438210).jpg
   HOME

TheInfoList



OR:

Biotite is a common group of phyllosilicate minerals within the
mica Micas ( ) are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into extremely thin elastic plates. This characteristic is described as perfect basal cleavage. Mica is ...
group, with the approximate chemical formula . It is primarily a solid-solution series between the iron- endmember annite, and the magnesium-endmember phlogopite; more aluminous end-members include
siderophyllite Siderophyllite is a rare member of the mica group of silicate minerals with formula KFe2+2Al(Al2Si2)O10(F,OH)2. The mineral occurs in nepheline syenite pegmatites and granite and aplite greisens. It is associated with microcline and astrophyllite ...
and eastonite. Biotite was regarded as a mineral ''species'' by the
International Mineralogical Association Founded in 1958, the International Mineralogical Association (IMA) is an international group of 40 national societies. The goal is to promote the science of mineralogy and to standardize the nomenclature of the 5000 plus known mineral species. Th ...
until 1998, when its status was changed to a mineral ''group''. 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 Micas ( ) are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into extremely thin elastic plates. This characteristic is described as perfect basal cleavage. Mica is ...
. 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 Hematite (), also spelled as haematite, is a common iron oxide compound with the formula, Fe2O3 and is widely found in rocks and soils. Hematite crystals belong to the rhombohedral lattice system which is designated the alpha polymorph of . ...
, 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 In geology, rock (or stone) is any naturally occurring solid mass or aggregate of minerals or mineraloid matter. It is categorized by the minerals included, its chemical composition, and the way in which it is formed. Rocks form the Earth's ...
, and in some instances side by side.


Properties

Like other
mica Micas ( ) are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into extremely thin elastic plates. This characteristic is described as perfect basal cleavage. Mica is ...
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 Table may refer to: * Table (furniture), a piece of furniture with a flat surface and one or more legs * Table (landform), a flat area of land * Table (information), a data arrangement with rows and columns * Table (database), how the table data ...
to
prismatic An optical prism is a transparent optical element with flat, polished surfaces that are designed to refract light. At least one surface must be angled — elements with two parallel surfaces are ''not'' prisms. The most familiar type of optical ...
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 ''Weathered'' is the third studio album by American rock band Creed, released on November 20, 2001. It was the last Creed album to be released until '' Full Circle'' came out in October 2009, with Creed disbanding in June 2004. It is the only Cr ...
. 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 In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a sequ ...
and
alkaline In chemistry, an alkali (; from ar, القلوي, al-qaly, lit=ashes of the saltwort) is a base (chemistry), basic, ionic compound, ionic salt (chemistry), salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as ...
aqueous solution An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, or sodium chloride (NaCl), in water would be re ...
s, with the highest dissolution rates at low pH. However, biotite dissolution is highly
anisotropic Anisotropy () is the property of a material which allows it to change or assume different properties in different directions, as opposed to isotropy. It can be defined as a difference, when measured along different axes, in a material's physic ...
with crystal edge surfaces ( ''h k''0) reacting 45 to 132 times faster than basal surfaces (
001 001, O01, or OO1 may refer to: *1 (number), a number, a numeral *001, fictional British agent, see 00 Agent *001, former emergency telephone number for the Norwegian fire brigade (until 1986) *AM-RB 001, the code-name for the Aston Martin Valkyrie ...
). File:Biotite mica 2 (31739438210).jpg, Flaky biotite sheets. File:BiotitaEZ.jpg, Thick biotite sample featuring many sheets. File:Biotite1.jpg, Biotite crystal exhibiting pseudohexagonal shape.


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 Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. These optically anisotropic materials are said to be birefringent (or birefractive). The birefring ...
which can be partially masked by its deep intrinsic color. Under
cross-polarized light Polarized light microscopy can mean any of a number of optical microscopy techniques involving polarized light. Simple techniques include illumination of the sample with polarized light. Directly transmitted light can, optionally, be blocked with ...
, biotite exhibits extinction approximately parallel to cleavage lines, and can have characteristic bird's eye maple 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 Isotropy is uniformity in all orientations; it is derived . Precise definitions depend on the subject area. Exceptions, or inequalities, are frequently indicated by the prefix ' or ', hence ''anisotropy''. ''Anisotropy'' is also used to describe ...
under cross-polarized light. File:Muscovite and Biotite2a.jpg, Biotite (in brown) and muscovite in an orthogneiss thin section under plane-polarized light. File:Thin Section of Biotite (test) (cropped to Biotite).jpg, Biotite in thin section under cross-polarized light. File:Sagenitic biotite.JPG, Basal section of biotite, with needle-like rutile inclusions, in thin section under plane-polarized light.


Structure

Like other micas, biotite has a crystal structure described as ''TOT-c'', meaning that it is composed of parallel ''TOT'' layers weakly bonded to each other by
cation An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
s (''c''). The ''TOT'' layers in turn consist of two tetrahedral sheets (''T'') strongly bonded to the two faces of a single octahedral sheet (''O''). It is the relatively weak ionic bonding between ''TOT'' layers that gives biotite its perfect basal cleavage. The tetrahedral sheets consist of silica tetrahedra, which are silicon ions surrounded by four oxygen ions. In biotite, one in four silicon ions is replaced by an aluminium ion. The tetrahedra each share three of their four oxygen ions with neighboring tetrahedra to produce a hexagonal sheet. The remaining oxygen ion (the ''apical'' oxygen ion) is available to bond with the octahedral sheet. The octahedral sheet in biotite is a trioctahedral sheet having the structure of a sheet of the mineral brucite, with magnesium or ferrous iron being the usual cations. Apical oxygens take the place of some of the hydroxyl ions that would be present in a brucite sheet, bonding the tetrahedral sheets tightly to the octahedral sheet. Tetrahedral sheets have a strong negative charge, since their bulk composition is AlSi3O105-. The trioctahedral sheet has a positive charge, since its bulk composition is M3(OH)24+ (M represents a divalent ion such as ferrous iron or magnesium) The combined TOT layer has a residual negative charge, since its bulk composition is M3(AlSi3O10)(OH)2. The remaining negative charge of the TOT layer is neutralized by the interlayer potassium ions. Because the hexagons in the T and O sheets are slightly different in size, the sheets are slightly distorted when they bond into a TOT layer. This breaks the hexagonal symmetry and reduces it to monoclinic symmetry. However, the original hexahedral symmetry is discernible in the pseudohexagonal character of biotite crystals. File:Mica T.png, View of tetrahedral sheet structure of biotite. The apical oxygen ions are tinted pink. File:Mica tO.png, View of trioctahedral sheet structure of biotite. The binding sites for apical oxygen are shown as white spheres. Red spheres are hydroxide ions. File:Mica tOs.png, View of trioctahedral sheet structure of mica emphasizing magnesium or iron sites File:Mica tri.png, View of biotite structure looking at surface of a single layer File:Mica tri side.png, View of biotite structure looking along sheets


Occurrence

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 Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained groundmass. The mineral ...
. Biotite is an essential phenocryst in some varieties of lamprophyre. Biotite is occasionally found in large cleavable crystals, especially in
pegmatite A pegmatite is an igneous rock showing a very coarse texture, with large interlocking crystals usually greater in size than and sometimes greater than . Most pegmatites are composed of quartz, feldspar, and mica, having a similar silicic com ...
veins, as in New England, Virginia and North Carolina USA. Other notable occurrences include Bancroft and
Sudbury Sudbury may refer to: Places Australia * Sudbury Reef, Queensland Canada * Greater Sudbury, Ontario (official name; the city continues to be known simply as Sudbury for most purposes) ** Sudbury (electoral district), one of the city's federal e ...
, 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. An igneous rock composed almost entirely of dark mica (biotite or phlogopite) is known as a ''
glimmerite Glimmerite is an igneous rock consisting almost entirely of dark mica ( biotite or phlogopite). Glimmerite has also been referred to as biotitite, though the use of this term to describe phlogopite-rich rocks has been criticized. Glimmerite may c ...
'' or ''biotitite''. Biotite may be found in association with its common alteration product
chlorite The chlorite ion, or chlorine dioxide anion, is the halite with the chemical formula of . A chlorite (compound) is a compound that contains this group, with chlorine in the oxidation state of +3. Chlorites are also known as salts of chlorous ac ...
. The largest documented single crystals of biotite were approximately sheets found in Iveland, Norway. File:Biotite Granite student sample.jpg, Biotite-bearing granite samples (small black minerals). File:Biotite Gneiss.JPG, Biotite-bearing gneiss sample. File:Biotite and chlorite gneiss mg 7971.jpg, Gneiss sample bearing biotite and chlorite (green), a common alteration product of biotite. File:Sheet mica, Namibia.jpg, Glimmerite from Namibia.


Uses

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.


References

{{Phyllosilicates Magnesium minerals Iron(II) minerals Potassium minerals Aluminium minerals Phyllosilicates Monoclinic minerals Minerals in space group 12 Mica group