A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties. At its simplest, a mineral is a naturally-occurring solid chemical. Currently, there are over 5100 named and described minerals - about 200 of them are common and about 20 of them are very common. Mineral classification is based on anion chemistry. Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.

The silicates are the most abundant and chemically complex group of minerals. All silicates have silica as the basis for their chemistry. "Silica" refers to SiO2 chemistry. The fundamental molecular unit of silica is one small silicon atom surrounded by four large oxygen atoms in the shape of a triangular pyramid - this is the silica tetrahedron - SiO4. Each oxygen atom is shared by two silicon atoms, so only half of the four oxygens "belong" to each silicon. The resulting formula for silica is thus SiO2, not SiO4.

Biotite mica is a common phyllosilicate with the formula K(Mg,Fe)3(AlSi3O10)(OH)2 - potassium magnesium iron hydroxy-aluminosilicate. It has a nonmetallic luster, a hardness of about 2 to 3, forms hexagonal crystals, and has one perfect cleavage. Biotite mica can be peeled into ultrathin sheets, which is a consequence of its cleavage. Thin cleavage sheets are noticeably flexible (elastic). Thicker pieces of biotite are black-colored. Thin sheets are brownish-black to dark brown to brown.

Photo gallery of biotite mica: