Biotite

Biotite

Thin tabular biotite aggregate
(Image width: 2.5 mm)
General
Category Dark mica series
Formula
(repeating unit)		 K(Mg,Fe)
3(AlSi
3O
10)(F,OH)
2
Crystal system Monoclinic
Crystal class Prismatic (2/m)
H-M symbol: (2/m)
Space group C2/m
Identification
Color Dark 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 hardness 2.5–3.0
Luster Vitreous to pearly
Streak White
Diaphaneity Transparent to translucent to opaque
Specific gravity 2.7–3.3[1]
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 [2][3][1]

Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula K(Mg,Fe)
3AlSi
3O
10(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 end-members include siderophyllite. Biotite was named by J.F.L. Hausmann in 1847 in honor of the French physicist Jean-Baptiste Biot, who, in 1816, researched the optical properties of mica, discovering many properties.[4]

Biotite is a sheet silicate. Iron, magnesium, aluminium, silicon, oxygen, and hydrogen form sheets that are weakly bound 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, and in some instances side-by-side.

Properties

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 is found in large chunks, they are called “books” because it resembles a book 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.[5] However, biotite dissolution is highly anisotropic with crystal edge surfaces (h k0) reacting 45 to 132 times faster than basal surfaces (001).[6][7]

Under cross-polarized light biotite can generally be identified by the gnarled bird's eye extinction.

Occurrence

Biotite is 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.[8] 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. 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 has been estimated that biotite comprises up to 7% of the exposed continental crust.[9]

The largest documented single crystals of biotite were approximately 7 m2 (75 sq ft) sheets found in Iveland, Norway.[10]

Uses

biotite: Topotype deposit

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

  1. 1 2 Handbook of Mineralogy
  2. ↑ Biotite mineral information and data Mindat
  3. ↑ Biotite Mineral Data Webmineral
  4. ↑ 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.)
  5. ↑ 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.
  6. ↑ 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.
  7. ↑ 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.
  8. ↑ Carmichael, I.S.; Turner, F.J.; Verhoogen, J. (1974). Igneous Petrology. New York: McGraw-Hill. p. 250. ISBN 0-07-009987-1.
  9. ↑ 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.
  10. ↑ P. C. Rickwood (1981). "The largest crystals" (PDF). American Mineralogist. 66: 885–907.
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