Biotite Properties. Click here to go back to the main page. Eastonite (of Winchell) A name for the Biotite end-member. A mineral may have good cleavage in one or two directions but fracture in another direction. Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula K(Mg,Fe)3 AlSi3O10(F,OH)2. Fracture is the tendency of a mineral to break along curved surfaces without a definite shape. Chromium-bearing Biotite: A Cr-bearing variety of biotite. Heterophyllite: Designation for a uniaxial biotite from Mangualde, Portugal. It usually occurs in rust, golden brown, reddish brown, dark brown, and brown colors. Biotite is a mineral that’s named after French physicist Jean Baptist who studied and researched about it for its optical properties. c-axis parallel section of biotite, showing the perfect cleavage. Cleavage is the tendency of a mineral to break along smooth planes parallel to zones of weak bonding. Like other mica minerals, biotite has a highly perfect basal cleavage… Cleavage and Fracture. This property together with the softness (2.5-3 on the Mohs scale) and good reflectance makes it one of the most easily identifiable common minerals. Chrom-Biotit: A Cr-bearing variety of biotite. PPL. That gives rise to the most characteristic feature of micas: perfect basal cleavage. It is also known as Black Mica. Fish Canyon Tuff. As a result of that, biotite readily cleaves into parallel and flexible sheets. Biotite. This crystal contains euhedral inclusions of apatite. Its general formula is K(Mg,Fe) 3 (AlSi 3 O 10)(OH) 2.Biotite is usually a solid solution of several minerals, mostly Fe-rich annite, KFe 3 (AlSi 3 O 10)(OH) 2, and Mg-rich phlogopite, KMg 3 (AlSi 3 O 10)(OH) 2; the exact composition of a sample is variable and has to be determined through a thorough analysis. Mg(T) Mica PPL. Biotite Crystallography: Axial Ratios: a:b:c =0.5771:1:1.1046 : Cell Dimensions: a = 5.343, b = 9.258, c = 10.227, Z = 2; beta = 100.26° V = 497.79 Den(Calc)= 2.89 Manganophyllite: A Mn-rich variety of Biotite. pleochroism in biotite: the same crystal rotated through 90° to show the change in body colour. Biotite or black mica is a member of the mica group within the sheet silicates. A barium- and titanium-bearing biotite mica.