Biotite is a common phyllosilicate mineral whose chemical formula is: K(Mg,Fe++)3AlSi3O10(F,OH)2 and has a molecular weight of 433.53g/mol. Nowadays, however, biotite more generally refers to the series of annite, eastonite, phlogopite and siderophyllite, that is, its a general name for dark micas.
Biotite is a sheet silicate. Iron magnesium aluminum silicate forms sheets, and weakly bond 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), which both form in similar region, often side-by-side. Biotite is found in granitic rocks, gneisses, and schists.
Like other mica minerals, biotite has a highly perfect basal cleavage, and consists of flexible sheets, or lamellar, which easily flake off. . It has a monoclinic crystal system, with tabular to prismatic crystals with a obvious pinacoid termination. It has four prism face 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 , and average density of 3.09g/cm^3. It is colored greenish to brown or black, and even yellow when weathered. It can be transparent to opaque, has a vitreous to pearly lustre, and a grey-white streak. In its weathered yellow, sparkly form, it is often considered a common type of “fool’s Gold”, even if Pyrite is the official “fool’s Gold”. Biotite is occasionally found in large sheets, especially in pegmatite veins, and also occurs as a contact metamorphic rock or the product of the alteration of hornblende, augite, wernerite, and similar minerals. When biotite is found in large chunks, they are called “books” because it resembles a book with pages of many sheets.
Biotite occurs in the lava of Mount Vesuvius, at Monzoni, and many other European locations, such as Sicily and Russia. In the United States, it is found in the pegmatites of New England, Virginia and North Carolina, as well as in the granite of Pikes Peak, Colorado. In Canada, it is found at Bancroft and Sudbury, Ontario. It is not industrially useful, but it is mined using quarrying and underground mining (depending on the depth of the biotite) for collection purposes. Its age could be determined using Potassium-Argon radioactive dating.
Biotite was named by J.F.L. Hausmann in 1847 in honour of the French physicist Jean-Baptiste Biot, who, in 1816, discovered optical properties of micas.