Clays, are secondary minerals, i.e. they are formed by the decomposition under certain conditions of some minerals, notably the felspars. They form at a later stage than the parent minerals from which they come.
The most common clay minerals belong to the kaolin, montmorillonite and hydromica groups. They all have definite crystalline structures, but are very fine-grained. Clays are basically hydrated silicates of aluminum, with iron, magnesium, calcium and potassium present in rather small proportions in some cases. Kaolinite (hydrated silicate of aluminum) is the chief clay mineral and the main constituent of ordinary clay.
It appears that clays are formed by the action of meteoric waters containing carbon dioxide on the felspars (e.g. Orthoclase) of igneous rocks acting over a period of time. It is possible that acidic vapors, remaining at a late stage of crystallization of an igneous rock such as granite, may play a considerable part in the attack on the felspars. In the breakdown of felspars to give clays, the alkalis—sodium and potas sium—are removed, together with some of the silica. Consequently clays are very rich in aluminium.
Clay is characterized by an extra ordinary property called plasticity, which is not yet thoroughly understood. Not all clays are equally plastic; the kaolins or china clays are but slightly so, and are called "lean", while what are called ball clays are highly plastic or "fat". Clays are much more plastic when wet than when dry, and are practically always impermeable to water. They are recognizable by the smell given off when moistened.
In time the erosive agencies such as rain, frost, wind, etc, transport the clays. They make their way down rivers and are deposited in plains, deltas and sea, where they are consolidated and most of the water is driven out to give well-cleaved shales or mud-stones, e.g. Wiannamatta Shale, Sydney. Shales and mudstones are by no means pure clay, for impurities such as quartz and pyrite creep in during deposition.
Boulder clay is a fine clay sediment laid down in layers in places where glaciers or ice-sheets have retreated. It is soft and through it are scattered all grades of pebbles and boulders. Tillite is hardened boulder clay and has the same origin. The Permian Age tillites of Bacchus Marsh (Vic.) and Wynyard (Tas.) are good examples, and point to a great ice sheet in S.E. Australia at that time. Finely laminated clays known as varved clays are often associated with tillites.
Marl is a clay containing considerable quantities of calcium carbonate, e.g. Balcombian Marl of Balcombe Bay, Vic.
Pottery clay is very plastic, highly aluminous and almost free from iron. Brick clays contain fluxes which fuse all the particles together and so give a coherent brick on burning.
Fireclays are very low in fluxes to enable them to withstand high temperatures without melting. Such clays must contain only small amounts of free silica, and negligible amounts of alkalies, iron and magnesium. Fireclay is used for refractory bricks for furnace linings.