That gives rise to the most characteristic feature of micas: perfect basal cleavage. Biotite is a mineral that’s named after French physicist Jean Baptist who studied and researched about it for its optical properties. Chromium-bearing Biotite: A Cr-bearing variety of biotite. It is also known as Black Mica. Biotite. Click here to go back to the main page. This crystal contains euhedral inclusions of apatite. Mg(T) Mica 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 Biotite Properties. A barium- and titanium-bearing biotite mica. c-axis parallel section of biotite, showing the perfect cleavage. Manganophyllite: A Mn-rich variety of Biotite. Fish Canyon Tuff. Biotite or black mica is a member of the mica group within the sheet silicates. Chrom-Biotit: A Cr-bearing variety of biotite. It usually occurs in rust, golden brown, reddish brown, dark brown, and brown colors. Eastonite (of Winchell) A name for the Biotite end-member. Cleavage is the tendency of a mineral to break along smooth planes parallel to zones of weak bonding. Cleavage and Fracture. Like other mica minerals, biotite has a highly perfect basal cleavage… 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. Heterophyllite: Designation for a uniaxial biotite from Mangualde, Portugal. pleochroism in biotite: the same crystal rotated through 90° to show the change in body colour. PPL. A mineral may have good cleavage in one or two directions but fracture in another direction. As a result of that, biotite readily cleaves into parallel and flexible sheets. PPL. 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. 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.

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