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Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula K (Mg,Fe)3AlSi3O10(F,OH)2. It is primarily a solid-solution series between the iron - endmember annite, and the magnesium -endmember phlogopite; more aluminous end-members include siderophyllite and eastonite.
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The biotite Mineral Group. Biotite is a very common form of mica. It is named in honor Jean Baptiste Biot (1774 - 1862), a French physicist, mathematician, and astronomer who researched the mica minerals for their optical properties.
Biotite is a group of black mica minerals found in igneous and metamorphic rocks. Learn about its chemical composition, physical properties, diagnostic features, and commercial applications.
- Dark mica
- Vitreous
- Black, dark green, dark brown
- White to gray, flakes often produced
biotite, a silicate mineral in the common mica group. It is abundant in metamorphic rocks (both regional and contact), in pegmatites, and also in granites and other intrusive igneous rocks. For chemical formula and detailed physical properties, see mica (table).
- The Editors of Encyclopaedia Britannica
Phlogopite is the magnesium endmember of the biotite solid solution series, with the chemical formula KMg 3 AlSi 3 O 10 (F,OH) 2. Iron substitutes for magnesium in variable amounts leading to the more common biotite with higher iron content.
Biotite and amphiboles often in the form of hornblende are more abundant in granodiorite than in granite, giving it a more distinct two-toned or overall darker appearance. Mica may be present in well-formed hexagonal crystals, and hornblende may appear as needle-like crystals.
A common, widespread, rock-forming mineral, biotite is a significant mineral in granites, diorites, and igneous pegmatites. In pegmatites, biotite crystals can be very large. It also occurs in schists, gneisses, and hornfels that resulted from either regional or contact metamorphism.
