Biotite is black magnesium/iron-based mica of low commercial value. It appears in the form of thin sheets which generally range from 0.003 mm to 0.1 mm in thickness.
|Vitreous, may be pearly
The high iron content, which imparts dark colors, rules out biotite from many industrial uses. However, because biotite is abundant in granitic rocks, pegmatites, and schists that are also associated with the more desirable muscovites, the mineral is important for exploration.
Biotite is mica of little commercial value. Other micas with less commercial value include zinnwaldite, a lithium/iron–based mica that is gray to brown; and roscoelite, a vanadium/potassium/magnesium mica that can be green or brown.