Biotite

Biotite is a common silicate mineral that contains potassium, magnesium, iron and aluminium. It is sometimes called "iron mica" and is found in granitic rocks, gneisses, schists, etc. Like others of the mica group of minerals, biotite has a highly perfect basal cleavage. It has a hardness of 2.5-3, a specific gravity of 2.7-3.1, is colored greenish to brown or black, and can be transparent to opaque. 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.

Biotite occurs in the lava of Mount Vesuvius, at Monzoni, and many other European locations. In the United States it is found in the pegmatites of New England and of Virginia and North Carolina, and the granite of Pike's Peak, Colorado.

Biotite was named in honor of the French physicist Jean Baptiste Biot.