Biotite - K(Mg,Fe2+)3[AlSi3O10(OH,F)2

Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula K(Mg,Fe)3AlSi3O10(F,OH)2. Biotite is a solid solution between the end members Phlogopite KMg3AlSi3O10(OH)2 and Annite KFe3AlSi3O10(OH)2, although pure Annite does not occur in nature. In addition, small amounts of Na, Rb, Cs, and Ba may substitute for K, and like in other minerals, F can substitute for OH and increase the stability of Biotite to higher temperatures and pressures.

Biotite was named by J.F.L. Hausmann in 1847 in honour of the French physicist Jean-Baptiste Biot, who, in 1816, researched the optical properties of mica, discovering many unique properties.

Biotite is a sheet silicate. Iron, magnesium, aluminium, silicon, oxygen, and hydrogen form sheets that are weakly bound together by potassium ions. It is sometimes called "iron mica" because it is more iron-rich than phlogopite. It is also sometimes called "black mica" as opposed to "white mica" (muscovite).

Biotite is the most ubiquitous ferromagnesian mineral occurring in most igneous and metamorphic rocks. Nearly pure phlogopite is found in hydrous ultrabasic rocks like kimberlite, and is also found in metamorphosed dolomites. Biotite, with more Fe-rich compositions is common in dacitic, rhyolitic, and trachytic volcanic rocks, granitic plutonic rocks, and a wide variety of metamorphic rocks.

Optical Properties:
Relief: Moderate
Color: Brown to black
Pleochroism: Yellow to brown to green colors
Interference colors: III order colors
Cleavage: Perfect {001} micaceous cleavage
Form: Pseudoexagonal basal section (001), rectangular (110) section


Bibliography



• Deer, W.A., Howie, R.A., Zussman, J. (1998) Rock-forming Minerals.
• Optical Mineralogy : The Nonopaque Minerals by Phillips / Griffen
• E. WM. Heinrich (1956): Microscopic Petrografy. Mcgraw-hill book company,inc
Stephen. A. Nelson


Photo
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Biotite crystals in a Granite. PPL image , 2x (Field of view = 7mm)
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Biotite crystals in a Granite. PPL image , 2x (Field of view = 7mm)
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Biotite crystals in a Granite. PPL image , 2x (Field of view = 7mm)
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Biotite crystals in a Granite. PPL image , 2x (Field of view = 7mm)
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Biotite crystals in a Granite. PPL image , 2x (Field of view = 7mm)
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Biotite crystals in a Granite. XPL image , 2x (Field of view = 7mm)
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Biotite crystals in a Granite. PPL image , 2x (Field of view = 7mm)
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Biotite crystals in a Granite. PPL image , 2x (Field of view = 7mm)
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Biotite crystals in a Granite. PPL image , 2x (Field of view = 7mm)
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Biotite crystals in a Granite. PPL image , 10x (Field of view = 2mm)
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Biotite crystal in a Granite. XPL image , 10x (Field of view = 2mm)
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Biotite crystals in a Granite. PPL image , 10x (Field of view = 2mm)
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Biotite crystals in a Granite. XPL image , 10x (Field of view = 2mm)
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Biotite crystals in a Granite. PPL image , 10x (Field of view = 2mm)
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Biotite crystals in a Granite. XPL image , 10x (Field of view = 2mm)
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Biotite crystals in a Granite. XPL image , 2x (Field of view = 7mm)
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Biotite crystals in a Granite. XPL image , 2x (Field of view = 7mm)