| Biotite | |
|---|---|
 Thin tabular biotite aggregate (Image width: 2.5 mm) | |
| General | |
| Category | Phyllosilicate minerals |
| Group | Mica group, trioctahedral mica group, biotite subgroup |
| Formula | K(Mg,Fe)3(AlSi3O10)(F,OH)2 |
| IMA symbol | Bt[1] |
| Crystal system | Monoclinic |
| Crystal class | Prismatic (2/m) (sameH-M symbol) |
| Space group | C2/m |
| Identification | |
| Color | Dark brown, greenish-brown, blackish-brown, yellow |
| 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 hardness | 2.5–3.0 |
| Luster | Vitreous to pearly |
| Streak | White |
| Diaphaneity | Transparent to translucent to opaque |
| Specific gravity | 2.7–3.3[2] |
| Optical properties | Biaxial (-) |
| 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) |
| Ultravioletfluorescence | None |
| References | [3][4][2] |
| Major varieties | |
| Manganophyllite | K(Fe,Mg,Mn)3AlSi3O10(OH)2 |
Biotite is a common group ofphyllosilicateminerals within themica group, with the approximatechemical formulaK(Mg,Fe)3AlSi3O10(F,OH)2. It is primarily asolid-solution series between theiron-endmemberannite, and themagnesium-endmemberphlogopite; morealuminous end-members includesiderophyllite andeastonite. Biotite was regarded as a mineralspecies by theInternational Mineralogical Association until 1998, when its status was changed to a mineralgroup.[5][6] The termbiotite is still used to describe unanalysed dark micasin the field. Biotite was named byJ.F.L. Hausmann in 1847 in honor of the FrenchphysicistJean-Baptiste Biot, who performed early research into the manyoptical properties ofmica.[7]
Members of the biotite group aresheet silicates.Iron,magnesium,aluminium,silicon,oxygen, andhydrogen form sheets that are weakly bound together bypotassiumions. The term "iron mica" is sometimes used for iron-rich biotite, but the term also refers to a flaky micaceous form ofhaematite, 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 may form in the samerocks, and in some instances side by side.
Like othermica minerals, biotite has a highly perfectbasal cleavage, and consists of flexible sheets, orlamellae, which easily flake off. It has amonoclinic crystal system, withtabular toprismatic crystals with an obviouspinacoid termination. It has four prism faces and two pinacoid faces to form apseudohexagonal crystal. Although not easily seen because of the cleavage and sheets, fracture is uneven. It appears greenish to brown or black, and even yellow whenweathered. It can be transparent to opaque, has a vitreous to pearlyluster, and a grey-whitestreak. 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 theMohs scale of mineral hardness.
Biotitedissolves in bothacid andalkalineaqueous solutions, with the highestdissolution rates at lowpH.[8] However, biotite dissolution is highlyanisotropic with crystal edge surfaces (h k0) reacting 45 to 132 times faster than basal surfaces (001).[9][10]
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Inthin section, biotite exhibits moderaterelief and a pale to deep greenish brown or brown color, with moderate to strongpleochroism. Biotite has a highbirefringence which can be partially masked by its deep intrinsic color.[11] Undercross-polarized light, biotite exhibits extinction approximately parallel to cleavage lines, and can have characteristicbird'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 appearisotropic under cross-polarized light.[12]
_(cropped_to_Biotite).jpg)
Like other micas, biotite has a crystal structure described asTOT-c, meaning that it is composed of parallelTOT layers weakly bonded to each other bycations (c). TheTOT 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 betweenTOT layers that gives biotite its perfect basal cleavage.[13]
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 (theapical oxygen ion) is available to bond with the octahedral sheet.[14]
The octahedral sheet in biotite is a trioctahedral sheet having the structure of a sheet of the mineralbrucite, 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.[15]
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.[13]
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.
Members of the biotite group are found in a wide variety ofigneous andmetamorphic rocks. For instance, biotite occurs in thelava ofMount Vesuvius and in the Monzoni intrusive complex of the westernDolomites. Biotite ingranite tends to be poorer in magnesium than the biotite found in its volcanic equivalent,rhyolite.[16] Biotite is an essentialphenocryst in some varieties oflamprophyre. Biotite is occasionally found in large cleavable crystals, especially inpegmatite veins, as inNew England,Virginia andNorth Carolina USA. Other notable occurrences includeBancroft andSudbury,Ontario Canada. It is an essential constituent of many metamorphicschists, and it forms in suitable compositions over a wide range ofpressure andtemperature. It has been estimated that biotite comprises up to 7% of the exposed continental crust.[17]
An igneous rock composed almost entirely of dark mica (biotite or phlogopite) is known as aglimmerite orbiotitite.[18]
Biotite may be found in association with its common alteration productchlorite.[12]
The largest documented single crystals of biotite were approximately 7 m2 (75 sq ft) sheets found inIveland, Norway.[19]
Biotite is used extensively to constrain ages of rocks, by eitherpotassium–argon dating orargon–argon dating. Becauseargon 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 ofiron andmagnesium between biotite andgarnet is sensitive to temperature.
bird's eye extinction thin section grinding.
