Geology

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. There is a difference of opinion regarding the crystalline system to which the micas should be referred. When crystallized, they all form six-sided prisms, but there are reasons for believing that this is a false symmetry. Usually certain micas are referred to the hexagonal, and others to the orthorhombic.

These minerals are very closely allied to the felspars in chemical composition, but differ from them in crystalline form and systems, but some authorities regard them all as monoclinic. 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 may be selected as the most important and wide-spread of the numerous alkaline micas, it being a hydrated potash-mica, with the general formula, K₂0, 3 AI₂0″ 6 Si0₂, 2 H₂0. It is a lustrous, silvery-white mineral, usually transparent and colorless 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. Lepidolite is a mica in which part of the potash has been replaced by lithia.
3. Biotite is the most important and widely disseminated of the numerous dark-coloured, ferro magnesian micas. This mineral is black or dark green in mass, and smoky even in thin leaves; chemically it is a hydrated silicate of potash, alumina, iron, and magnesia. In hardness and specific gravity it differs little from muscovite.