Biotite
Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula K(Mg, Fe)3AlSi3O10(F, OH)2. More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers include siderophyllite.
It is colored greenish to brown or black, and even yellow when weathered. It can be transparent to opaque, has a vitreous to pearly luster, and a grey-white streak. In its weathered yellow, sparkly form, it is one type of "fool’s gold." (Pyrite is the official "fool’s gold.") Large chunks of biotite are called "books" because they resemble books with many pages.
Etymology
Biotite was named by Johann Friedrich Ludwig Hausmann in 1847. He gave the name in honor of the French physicist Jean-Baptiste Biot who discovered optical properties of micas in 1816.
Occurrence
Biotite is found in a wide variety of igneous rocks and metamorphic rocks. For instance, biotite occurs in the lava of Mount Vesuvius and at Monzoni. It is an essential phenocryst in some varieties of lamprophyre. Biotite is occasionally found in large sheets, especially in pegmatite veins, as in New England, Virginia and North Carolina. Other notable occurrences include Bancroft and Sudbury, Ontario. It is an essential constituent of many metamorphic schists, and it forms in suitable compositions over a wide range of pressure and temperature. It is mined by quarrying or underground mining, depending on its depth of occurrence.
Characteristics
Biotite is a sheet silicate. Iron, magnesium, aluminum, silicon, oxygen, and hydrogen form sheets that are weakly bonded 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)—both form in some rocks, sometimes side-by-side.
Like other mica minerals, biotite has a highly perfect basal cleavage, and consists of flexible sheets, or lamellae, which easily flake off. It has a monoclinic crystal system, with tabular to prismatic crystals with an obvious pinacoid termination. It has four prism faces and two pinacoid faces to form a pseudohexagonal crystal. Although not easily seen because of the cleavage and sheets, fracture is uneven. It has a hardness of 2.5-3, a specific gravity of 2.7-3.1, and an average density of 3.09 g/cm³.
Uses
- Biotite has few practical uses. One application is in electrical devices.
- It is valued for collection purposes.
- Biotite is used extensively to constrain ages of rocks, by either potassium-argon dating or argon-argon dating. Because argon escapes readily from the biotite crystal structure at high temperatures, these methods may provide only minimum ages for many rocks. 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.
See also
ReferencesISBN links support NWE through referral fees
- Farndon, John. 2006. The Practical Encyclopedia of Rocks & Minerals: How to Find, Identify, Collect and Maintain the World's best Specimens, with over 1000 Photographs and Artworks. London: Lorenz Books. ISBN 0754815412.
- Klein, Cornelis, and Barbara Dutrow. 2007. Manual of Mineral Science. 23rd ed. New York: John Wiley. ISBN 978-0471721574.
- Pellant, Chris. 2002. Rocks and Minerals. Smithsonian Handbooks. New York: Dorling Kindersley. ISBN 0789491060.
- Shaffer, Paul R., Herbert S. Zim, and Raymond Perlman. 2001. Rocks, Gems and Minerals. Rev. ed. New York: St. Martin's Press. ISBN 1582381321.
- Mineral Gallery. 2006. The Mineral Biotite Amethyst Galleries. Retrieved May 14, 2007.
External links
- Biotite Mineral Data. Webmineral.com. Retrieved May 14, 2007.
- Biotite. library.thinkquest.org. Retrieved May 14, 2007.
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