Biotite and muscovite are aluminum-containing silicates which cleave in thin sheets and are called micas. Biotite is a general field term for dark micas. Biotite has the general formula (K(Mg,Fe)3(AlSi3O10)(OH)2) . The presence of iron and magnesium in biotite makes it darker in color than muscovite. With a higher crystallization temperature, biotite is viewed as occurring earlier in the Bowen reaction series. Sheetlike layer-silicates are the major constituents of clays and muds.

This sample from the Smithsonian Museum of Natural History shows the darker biotite with a center of the lighter muscovite. The sample is from Spruce Pine, North Carolina and is about 11 cm wide.

According to Mindat, biotite is now a discouraged name because it refers to a series of similar minerals from the mica family. The more specific mineral names annite, eastonite, phlogopite and siderophyllite are used in technical literature.

Mindat: Biotite

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