MICA.
The different varieties of mica are important as rock-forming minerals, but they are not as often met with in brick-earths as is generally supposed, except in insignificant quantity. Some of the purest clays, however, contain a great deal of mica, derived almost directly from the destruction of granite. The two commonest varieties of the mineral are biotite and muscovite.
Biotite Mica.—This mineral, usually known as ferro-magnesian mica, is composed of silicates of magnesia, alumina, iron, and alkalies in variable proportions. It occurs as six-sided plates or irregular scales, usually of a bronze-black colour. Biotite weathers with comparative facility, hence the reason why it is not more commonly met with in brown and other impure clays.
Muscovite Mica.—This is sometimes called potash- or alumino-alkaline mica, composed of the silicates of alumina, alkalies, iron, and magnesia; the proportion of silica ranges from 45 to 50 per cent. It may usually be distinguished at sight from biotite by its silvery white or light brown colour. When large enough, both the micas mentioned may be split up into thin plates, muscovite yielding large transparent sheets. Compared with all other constituents of brick-earth, the micas are bright and of semi-metallic lustre. Muscovite is more durable than biotite, and is much more frequently met with in brick-earths, especially in the sandy varieties.
The influence of mica in the kiln is not of much importance in ordinary brickmaking; in general its alkaline character renders it fusible, though a high temperature is necessary at all times to effect that. In china-clay mica is regarded as a nuisance, and in breaking down the material it is separated in the washing process by running water, the mineral collecting in depressions or basins, called “micas.” When muscovite contains much fluorine, as it frequently does, it is very undesirable in clays for high-class purposes. At the best of times the proportion of iron in mica is sufficient to mar the quality of the otherwise most excellent clays. In the kiln, or porcelain furnace, the presence of mica (more particularly biotite) is apt to create yellow and brown specks, or a species of mottling. It is highly satisfactory, therefore, to note that these little shiny flakes may be easily floated off by a moderate amount of care in washing, and thus separated from the other constituents of the clay.