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Biotite is a common phyllosilicate meeneral athin the mica group, wi the approximate chemical formula K(Mg,Fe)
. 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 aggregate - Ochtendung, Eifel, Germany.jpg
Thin tabular biotite aggregate
(Image width: 2.5 mm)
Category Dark mica series
(repeatin unit)
Creestal seestem Monoclinic (2/m)
Space Group: C 2/m
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.


  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.)