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Biotite

A biotite slice
General
Category Silicate mineral
Chemical formula K(Mg,Fe)2(AlSi3O10)(F,OH)2
Identification
Molar mass 433.53 g
Color Dark brown, Greenish brown, Blackish brown, Yellow, White
Crystal habit massive to platy
Crystal system Monoclinic (2/m) Space Group: C 2/m
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 2.7–3.1
Density 2.8-3.4
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 [1][2][3]
Two fragments of Biotite

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 was named by J.F.L. Hausmann in 1847 in honour of the French physicist Jean-Baptiste Biot, who, in 1816, researched the optical properties of mica, discovering many unique properties.

Biotite is a sheet silicate. Iron, magnesium, aluminium, 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.

Contents

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 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 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.

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. It 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.

The largest documented single crystals of biotite were~7 m2 sheets found in Iveland, Norway.[4]

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.

Biotite is used in electrical devices, usually as a dielectric in capacitors and thermionic valves.

References

  • "The Mineral Biotite". 1995, 1996 Amethyst Gallery Inc. [1]
  • "Biotite". [2]
  • "BIOTITE". LoveToKnow 1911 Online Encyclopedia. 2003, 2004 LoveToKnow. [3]
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1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

BIOTITE, an important rock-forming mineral belonging to the group of micas. The name was given by J. F. L. Hausmann in 1847 in honour of the French physicist, J. B. Biot, who in 1816 found the magnesia-micas to be optically uniaxial or nearly so. The magnesia-micas are now referred to the species biotite and phlogopite, which differ in that the former contains a considerable but widely varying amount of iron. Biotite is an orthosilicate of aluminium, magnesium, ferrous and ferric iron, potassium and basic hydrogen, with small amounts of calcium, sodium, lithium, fluorine, titanium, &c., and ranges in composition between (H,K) 2 (Mg,Fe) 4 (A1,Fe) 2 (SiO 4) 4 and (H,K)2(Mg,Fe)2Al2(S104)3.

Like the other micas, it is monoclinic with pseudo-hexagonal symmetry (figs. 1, 2) and possesses a perfect cleavage in one direction (c). Biotite is, however, readily distinguished by its darker colour, strong pleochroism, and small optic axial angle.

FIG. I.

The colour is usually dark-green or brown; thick crystals are often deep-black and opaque. The absorption of light-rays vibrating parallel to the cleavage is much greater than of rays vibrating in a direction perpendicular thereto, and in darkcoloured crystals the former are almost completely absorbed. The angle between the optic axes is usually very small, the crystals being often practically uniaxial; an axial angle of 50° has, however, been recorded in a dark-coloured biotite. The specific gravity of biotite is, as a rule, higher than that of other micas, varying from 2.7 to 3.1 according to the amount of iron present. The hardness is 22 to 3.

Several varieties of biotite are distinguished. By G. Tschermak it is divided into two classes, meroxene and anomite; in the former the plane of the optic axis coincides with the plane of symmetry, whilst in the latter it is perpendicular thereto. Meroxene includes nearly all ordinary biotite, and is the name given by A. Breithaupt in 1841 to the Vesuvian crystals; on the other hand, anomite (named from iivo,uos, "contrary to law") is of rare occurrence. Haughtonite and siderophyllite are black varieties rich in ferrous iron, and lepidomelane (from X iris, a scale, and µEkas, black) is a variety rich in ferric iron. In barytobiotite and manganophyllite the magnesia is partly replaced by baryta and manganous oxide respectively. Rubellane, hydrobiotite, pseudobiotite, and others are altered forms of biotite, which is a mineral particularly liable to decomposition with the production of chlorites and vermiculites.

Biotite is a common constituent of igneous and crystalline rocks: in granite, gneiss and mica-schist it is often associated with muscovite (white mica), the two kinds having sometimes grown in parallel position. In volcanic rocks, and in nearly all other kinds of igneous rocks with the exception of granite, biotite occurs to the exclusion of the muscovite. In the dyke-rocks known as mica-traps or mica-lamprophyres biotite is especially abundant. It is also one of the most characteristic products of contact-metamorphism, being developed in sedimentary and other rocks at their contact with granite masses. In the ejected blocks of crystalline limestone of Monte Somma, Vesuvius, the most perfectly developed crystals of biotite (figs. I, 2), or indeed of any of the micas, are found in abundance, associated with brilliant crystals of augite, olivine, humite, &c.

Although biotite (black mica) is much more common and widely distributed than white mica, yet it is of far less economic importance. The small size of the sheets, their dark colour and want of transparency render the material of little value. Large, cleavable masses yielding fine smoky-black and green sheets, sufficiently elastic for industrial purposes, are, however, found in Renfrew county, Ontario. (L. J. S.)


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