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) (same H-M symbol)
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
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

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 performed early research into the many optical properties of mica.

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.