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biotite
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biotite granite Bowen's reaction series
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Basalt sample returned by Apollo 15, from near a long sinous lunar valley called Hadley Rille. Measured at 3.3 years old.
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Adolf Hitler (Nazi, nazism, German leader).
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The assassination of President Abraham Lincoln at Ford's Theatre, Washington, D.C., April 14th, 1865; from a lithograph by Currier and Ives.
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Lead image for "A History of U.S. Presidential Elections in Maps" list
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Skyline of Detroit, Michigan. Photo approximately 2015-2019.
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Dr. Martin Luther King Jr. addresses marchers during his "I Have a Dream" speech at the Lincoln Memorial in Washington. August 28th 1963
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Nelson Mandela, undated photo.
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biotite

mineral
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Also known as: black mica
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biotite
biotite
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Also called:
black mica
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mica
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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).

Biotite is regarded as a mixture composed of variable proportions of four basic aluminosilicates of potassium, iron, magnesium, or aluminum: annite, K2Fe6(Si6Al2O20)(OH)4; siderophyllite, K2Fe5Al(Si5Al3O20)(OH)4; phlogopite, K2Mg6(Si6Al2O20)(OH)4; and eastonite, K2Mg5A1(Si5Al3O20)(OH)4. Biotite is arbitrarily designated as having a magnesium-to-iron ratio less than 2:1.

This article was most recently revised and updated by Amy Tikkanen.