A barium- and titanium-bearing biotite mica. Eastonite (of Winchell) A name for the Biotite end-member. PPL. c-axis parallel section of biotite, showing the perfect cleavage. A mineral may have good cleavage in one or two directions but fracture in another direction. Cleavage is the tendency of a mineral to break along smooth planes parallel to zones of weak bonding. Biotite or black mica is a member of the mica group within the sheet silicates. Manganophyllite: A Mn-rich variety of Biotite. Heterophyllite: Designation for a uniaxial biotite from Mangualde, Portugal. As a result of that, biotite readily cleaves into parallel and flexible sheets. Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula K(Mg,Fe)3 AlSi3O10(F,OH)2. It is also known as Black Mica. This crystal contains euhedral inclusions of apatite. Fish Canyon Tuff. Fracture is the tendency of a mineral to break along curved surfaces without a definite shape. Click here to go back to the main page. Chromium-bearing Biotite: A Cr-bearing variety of biotite. Biotite is a mineral that’s named after French physicist Jean Baptist who studied and researched about it for its optical properties. 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. It usually occurs in rust, golden brown, reddish brown, dark brown, and brown colors. Mg(T) Mica 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. That gives rise to the most characteristic feature of micas: perfect basal cleavage. PPL. Biotite. Biotite Properties. 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 pleochroism in biotite: the same crystal rotated through 90° to show the change in body colour. Cleavage and Fracture. Chrom-Biotit: A Cr-bearing variety of biotite. Like other mica minerals, biotite has a highly perfect basal cleavage…

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