These minerals have a complex chemical composition, and are so variable that it is difficult to give formulae for them; they are silicates of alumina, together with potash, lithia, magnesia, iron, or manganese. When crystallized, the micas all form six-sided prisms, which, though of hexagonal habit, are in reality mono-clinic. All varieties have a remarkably perfect cleavage, and split into thin, elastic, and flexible leaves, by which they may be readily recognized. They are quite soft, and most of them may be scratched with the finger-nail.

1. Muscovite

Muscovite may be selected as the most important and wide-spread of the numerous alkaline micas, with the general formula, K20, 3 A1203, 6 Si02, 2 H20. It is a lustrous, silvery-white mineral, usually transparent and colourless in thin leaves; it has a specific gravity of 2.76-3.1, and a hardness of 2.1-3.

Sericite is a silvery or pale green form of muscovite, which is an alteration product and often is derived from a felspar.

2. Biotite

Biotite is the most important and widely disseminated of the numerous dark-coloured, ferromagnesian micas. This mineral is black or dark green in mass, and smoky even in thin leaves;, chemically it is a silicate of potash, alumina, iron, and magnesia. In hardness and specific gravity it differs little from muscovite.