Biotite

Biotite is a common phyllosilicate meeneral athin the mica group, wi the approximate chemical formula K(Mg,Fe)
₃AlSi
₃O
₁₀(F,OH)
₂. Mair generally, it refers tae the dark mica series, primarily a solid-solution series atween the iron-endmember annite, an the magnesium-endmember phlogopite; mair aluminous endmembers include siderophyllite. Biotite wis named bi J.F.L. Hausmann in 1847 in honour o the French pheesicist Jean-Baptiste Biot, who, in 1816, researched the optical properties o mica, discovering mony unique properties.^[4]

Biotite
Thin tabular biotite aggregate (Image width: 2.5 mm)
General
Category	Dark mica series
Formula (repeatin unit)	K(Mg,Fe) 3(AlSi 3O 10)(F,OH) 2
Creestal seestem	Monoclinic (2/m) Space Group: C 2/m
Identification
Formula mass	433.53 g
Colour	Mirk broun, greenish-broun, blackish-broun, yellae, white
Creestal habit	Massive tae platy
Twinnin	Common on the [310], less common on the {001}
Cleavage	Perfect on the {001}
Fractur	Micaceous
Tenacity	Brickle tae souple, elastic
Mohs scale haurdness	2.5–3.0
Skinkle	Vitreous tae pearly
Streak	White
Diaphaneity	Skyre tae leamin tae opaque
Speceefic 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 sheet silicate. Airn, magnesium, aluminium, silicon, oxygen, an hydrogen furm sheets that are waikly boond thegither bi potassium ions. It is whiles cried "iron mica" acause it is mair airn-rich nor phlogopite. It is an aa whiles cried "black mica" as opponed tae "white mica" (muscovite) – baith furm in some rocks, in some instances side-bi-side.

References

1. Biotite mineral information and data Mindat
2. Biotite Mineral Data Webmineral
3. Handbook of Mineralogy
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.)