Biotite
thin tabular Biotite aggregate (Image width: 2.5 mm)
General
Category	Dark Mica series
Chemical formula	K(Mg,Fe)3(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]

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 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, in some instances side-by-side.

Properties[link]

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.

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

Occurrence[link]

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 approximately 7 m² (75 sq ft) sheets found in Iveland, Norway.^[4]

Uses[link]

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[link]

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