Translucent to opaque. Crystal System. It has a monoclinic crystal system, with tabular to prismatic crystals with an obvious pinacoid termination. Biotite was named by J.F.L. [12] Biotite is an essential phenocryst in some varieties of lamprophyre. Biotite (in brown) and muscovite in an orthogneiss thin section under plane-polarized light. Usually strongly pleochroic so grains are darker when the trace of cleavage is parallel to the lower polarizer. The specific gravity (G) is greater with increasing iron content, while the color in hand sample is darker with an increase in iron. It appears in the form of thin sheets which generally range from 0.003 mm to 0.1 mm in thickness. The color of biotite is usually black and the mineral has a hardness of 2.5–3 on the Mohs scale of mineral hardness. Micaceous. It can be transparent to opaque, has a vitreous to pearly luster, and a grey-white streak. Like other mica minerals, biotite has a highly perfect basal cleavage, and consists of flexible sheets, or lamellae, which easily flake off. Biotite-bearing granite samples (small black minerals). Individual group member minerals such as Phlogopite and Eastonite can be in lighter colors. Biotite is used extensively to constrain ages of rocks, by either potassium-argon dating or argon–argon dating. Color is the observable color of a mineral specimen in natural light. The color of a specific mineral specimen may be different than its streak color. Biotite is a member of the mica branch of the silicate mineral group.It is common as a rock-forming mineral and is found associated with muscovite, feldspar, and quartz. Color/Pleochroism. Muscovite is also called “white mica” and biotite is sometimes called “brown mica” or “black mica”. It has four prism faces and two pinacoid faces to form a pseudohexagonal crystal. It is an essential constituent of many metamorphic schists, and it forms in suitable compositions over a wide range of pressure and temperature. Biotite dissolves in both acid and alkaline aqueous solutions, with the highest dissolution rates at low pH. 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. It has a widespread occurance and is present in all three rock types igneous, metamorphic, and sedimentary. Vitreous, may be pearly. Biotite is also useful in assessing temperature histories of metamorphic rocks, because the partitioning of iron and magnesium between biotite and garnet is sensitive to temperature. Gneiss sample bearing biotite and chlorite (green), a common alteration product of biotite. Basal sections of biotite in thin section are typically approximately hexagonal in shape and usually appear isotropic under cross-polarized light.[11]. The term "iron mica" is sometimes used for iron-rich biotite, but the term also refers to a flaky micaceous form of haematite, and the field term Lepidomelane for unanalysed iron-rich Biotite avoids this ambiguity. Monoclinic. [10] Under cross-polarized light, biotite exhibits extinction approximately parallel to cleavage lines, and can have characteristic bird's eye extinction, a mottled appearance caused by the distortion of the mineral's flexible lamellae during grinding of the thin section. Biotite is black magnesium/iron-based mica of low commercial value. Both can be separated into thin translucent sheets, although phlogopite sheets will transmit light more readily than biotite, and less readily than muscovite. Members of the biotite group are sheet silicates. Colors are X = colorless, light tan, pale greenish brown, pale green; Y~Z = brown, olive brown, dark green, dark red-brown. Biotite is also sometimes called "black mica" as opposed to "white mica" (muscovite) – both form in the same rocks, and in some instances side-by-side. Thick biotite sample featuring many sheets. Description. 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 can be transparent to opaque, has a vitreous to pearly luster, and a grey-white streak. Streak: White: Hardness: 2.5 - … It appears greenish to brown or black, and even yellow when weathered. Its streak is white or gray, and it has a vitreous luster. Mohs Hardness. It has four prism faces and two pinacoid faces to form a pseudohexagonal crystal. © Copyright 1997 - 2020 Hershel Friedman and Minerals.net, all rights reserved. The color of biotite in hand sample is brown to black (sometimes greenish). Although not easily seen because of the cleavage and sheets, fracture is uneven. [4][5] The term biotite is still used to describe unanalysed dark micas in the field. Density: 2.8 - 3.4, Average = 3.09 : Diaphaneity: Transparent to translucent to opaque : Fracture: Uneven - Flat surfaces (not cleavage) fractured in an uneven pattern. Biotite is a common rock-forming mineral, and is especially noted in, Biotite is found in abundance worldwide, but collectors specimens are not common and come from few localities. Cleavage: Basal, perfect: Mohs Hardness: 2.5 to 3: Specific Gravity: 2.7 to 3.4: Diagnostic Properties: Dark color, perfect cleavage [13], An igneous rock composed almost entirely of dark mica (biotite or phlogopite) is known as a glimmerite or biotitite. Physical Properties of Biotite: Chemical Classification: Dark mica: Color: Black, dark green, dark brown: Streak: White to gray, flakes often produced: Luster: Vitreous: Diaphaneity: Thin sheets are transparent to translucent, books are opaque. It has been estimated that biotite comprises up to 7% of the exposed continental crust. It appears greenish to brown or black, and even yellow when weathered. Our Recommended Book Selection at Amazon.com, Financial Donations and Collection Acquisitions. Habit: Noteworthy localities include Mabubas, Bengo Province, Angola; Mendig, Eifel Mts, Germany; and Serra da Freita, Arouca, Portugal, where it comes in rounded. It has a monoclinic crystal system, with tabular to prismatic crystals with an obvious pinacoid termination. [14], Biotite may be found in association with its common alteration product chlorite. Structural groups mainly; based on rruff.info/ima, modified, Biotite mineral information and data Mindat, "The effect of pH, grain size, and organic ligands on biotite weathering rates", "Identification Tables for Common Minerals in Thin Section", https://en.wikipedia.org/w/index.php?title=Biotite&oldid=988046073, Short description is different from Wikidata, Creative Commons Attribution-ShareAlike License, Dark brown, greenish-brown, blackish-brown, yellow, white, This page was last edited on 10 November 2020, at 18:52. Color: Black, dark brown, dark green, reddish black. It is an aluminosilicate mineral that forms sheets or flakes. For instance, biotite occurs in the lava of Mount Vesuvius and in the Monzoni intrusive complex of the western Dolomites. In thin section, biotite exhibits moderate relief and a pale to deep greenish brown or brown color, with moderate to strong pleochroism. Because argon escapes readily from the biotite crystal structure at high temperatures, these methods may provide only minimum ages for many rocks. Biotite occurs with other types of mica, even within a single rock. Members of the biotite group are found in a wide variety of igneous and metamorphic rocks. Biotite has a high birefringence which can be partially masked by its deep intrinsic color. Biotite was regarded as a mineral species by the International Mineralogical Association until 1998, when its status was changed to a mineral group. Biotite is a dark-colored mica. Its streak is white or gray, and it has a vitreous luster. When biotite crystals are found in large chunks, they are called "books" because they resemble books with pages of many sheets. If you feel any of the content is incorrect, or if you feel we are missing vital locality information, please fill out the form below so we can update the site. Color. These two minerals can range in color from dark brown or black, often with a greenish tint. Like other mica minerals, biotite has a highly perfect basal cleavage, and consists of flexible sheets, or lamellae, which easily flake off. Biotite is occasionally found in large cleavable crystals, especially in pegmatite veins, as in New England, Virginia and North Carolina USA. Iron, magnesium, aluminium, silicon, oxygen, and hydrogen form sheets that are weakly bound together by potassium ions. Biotite is typically black to brown in color, but it can be difficult to distinguish it from other similarly formed metamorphic rock such as phlogopite, another brown, silicate mica. The color of biotite is usually black and the mineral has a hardness of 2.5–3 on the Mohs scale of mineral hardness. Although biotite is sometimes called black mica, it occurs in other colors, including brown, greenish-brown, yellow, and even white. We strive for accurate content and locality information. 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[ 11 ] Book Selection at Amazon.com Financial... Thin sheets which generally range from 0.003 mm to 0.1 mm in thickness Bancroft and Sudbury, Canada. Appears in the Monzoni intrusive complex of the biotite found in its volcanic equivalent rhyolite! Observable color of biotite group member minerals such as Phlogopite and Eastonite can transparent. Biotite has a monoclinic crystal system, with moderate to strong pleochroism it has a vitreous.... Amazon.Com, Financial Donations and Collection Acquisitions 1998, when its status was to... Or gray, and it has four prism faces and two pinacoid faces to form a crystal! Of mineral hardness or black, dark brown and even white sheets that are weakly bound by!

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