biotite series

{{Infobox mineral | name = Biotite | category = Phyllosilicate minerals | group = Mica group, trioctahedral mica group, biotite subgroup | boxwidth = | boxbgcolor =#7b4118 | boxtextcolor = #fff | image = Biotite aggregate - Ochtendung, Eifel, Germany.jpg | imagesize = 260px | caption = Thin tabular biotite aggregate(Image width: 2.5 mm) | formula = | IMAsymbol=Bt | system = Monoclinic | class = Prismatic (2/m) (same H-M symbol) | symmetry = C2/m | color = Dark brown, greenish-brown, blackish-brown, yellow | 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 = 2.5–3.0 | luster = Vitreous to pearly | polish = | refractive = nα = 1.565–1.625 nβ = 1.605–1.675 nγ = 1.605–1.675 | opticalprop = Biaxial (-) | birefringence = δ = 0.03–0.07 | dispersion = r r > v weak (Mg rich) | pleochroism = Strong | fluorescence= None | absorption = | streak = White | gravity = 2.7–3.3 | melt = | fusibility = | diagnostic = | solubility = | diaphaneity = Transparent to translucent to opaque | other = | references = |var1=Manganophyllite|var1text=K(Fe,Mg,Mn)3AlSi3O10(OH)2}}

Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula . It is primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous end-members include siderophyllite and eastonite. Biotite was regarded as a mineral species by the International Mineralogical Association until 1998, when its status was changed to a mineral group. The term biotite is still used to describe unanalysed dark micas in the field. 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.