Biotite
Biotite | ||
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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) | |
Ultraviolet fluorescence | None | |
References | [3][4][2] | |
Major varieties | ||
Manganophyllite | K(Fe,Mg,Mn)3AlSi3O10(OH)2 |
Biotite is a common group of
Members of the biotite group are
Properties
Like other
Biotite
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Flaky biotite sheets.
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Thick biotite sample featuring many sheets.
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Biotite crystal exhibiting pseudohexagonal shape.
Optical properties
In
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Biotite (in brown) and muscovite in anorthogneissthin section under plane-polarized light.
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Biotite in thin section under cross-polarized light.
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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
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.[14]
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.[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.
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View of tetrahedral sheet structure of biotite. The apical oxygen ions are tinted pink.
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View of trioctahedral sheet structure of biotite. The binding sites for apical oxygen are shown as white spheres. Red spheres are hydroxide ions.
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View of trioctahedral sheet structure of mica emphasizing magnesium or iron sites
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View of biotite structure looking at surface of a single layer
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View of biotite structure looking along sheets
Occurrence
Members of the biotite group are found in a wide variety of
An igneous rock composed almost entirely of dark mica (biotite or phlogopite) is known as a glimmerite or biotitite.[18]
Biotite may be found in association with its common alteration product chlorite.[12]
The largest documented single crystals of biotite were approximately 7 m2 (75 sq ft) sheets found in Iveland, Norway.[19]
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Biotite-bearing granite samples (small black minerals).
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Biotite-bearing gneiss sample.
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Gneiss sample bearing biotite and chlorite (green), a common alteration product of biotite.
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Glimmerite from Namibia.
Uses
Biotite is used extensively to constrain ages of rocks, by either
References
- S2CID 235729616.
- ^ a b Handbook of Mineralogy
- ^ Biotite mineral information and data Mindat
- ^ Biotite Mineral Data Webmineral
- ^ "The Biotite Mineral Group". Minerals.net. Retrieved 29 August 2019.
- ^ "Biotite".
- ^ 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.)
- .
- .
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- ^ Faithful, John (1998). "Identification Tables for Common Minerals in Thin Section" (PDF). Archived (PDF) from the original on 2022-10-09. Retrieved March 17, 2019.
- ^ a b 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.
- ^ a b Nesse 2000, p. 238.
- ^ Nesse 2000, p. 235.
- ^ Nesse 2000, pp. 235–237.
- ISBN 978-0-07-009987-6.
- .
- .
- ^ P. C. Rickwood (1981). "The largest crystals" (PDF). American Mineralogist. 66: 885–907.
Bibliography
- Nesse, William D. (2000). Introduction to mineralogy. New York: Oxford University Press. p. 238. ISBN 9780195106916.