Biotite
General Information
Biotite is a common rock forming mineral, being present in most igneous and both regional and contact metamorphic rocks. The typical black to brown color of biotite is characteristic although it is difficult to distinguish brown biotite from dark brown phlogopite. The two are actually end members in a series that is dependent on the percentage of iron. Phlogopite is iron poor and biotite is iron rich. The darker color and density increase with the iron content. Biotite tends to form in a wider range of conditions than phlogopite, which is limited mostly to ultramafic rocks and magnesium rich marbles and pegmatites. Biotite, like other micas, has a layered structure of iron magnesium aluminum silicate sheets weakly bonded together by layers of potassium ions. These potassium ion layers produce the perfect cleavage. Biotite is rarely considered a valuable mineral specimen, though it can accompany other minerals and compliment them. In Bancroft, Ontario, Biotite forms large crystals with green apatite and hornblende. Single large plates or "books" of biotite can grow to considerable size and can make impressive mineral specimens.
Biotite is named for J.B. Biot (1774-1862). He was a French physicist and astronomer, as well as a mathmatician |
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Chemistry: K(Mg, Fe+3)3AlSi3O10(F, OH)2, Potassium iron magnesium aluminum silicate hydroxide fluoride |
Class: Silicates
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Subclass: Phyllosilicates, Mica group
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Color: Black, Brownish Black, Greenish Black, Dark Green,
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Streak: Colorless |
Luster: Pearly
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Transparency: Transparent to Translucent
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Crystal System: Monoclinic
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Hardness: 2.5-3
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Cleavage: Perfect in one direction
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Fracture: Uneven
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Specific Gravity: 2.7-3.4
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Crystal Habits: Usually tabular, sometimes barrel-shaped; plates, scales
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Uses: Heat insulator for industrial purposes
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Associated Minerals: Quartz, Feldspars, Apatite, Calcite, Hornblende, Garnets, Schorl
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Other Characteristics: Cleavage sheets flexible and elastic
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Notable Occurrences: Russia; Greenland; Scandinavia; Brazil; Italy; Alaska, United States
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Best Field Indicators: Dark Color and Small Crystals
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