Clay is generally defined as any very fine grained, natural, earthy material, which is generally plastic at appropriate water contents and hardens when dried or fired. Despite the lack of a standard definition for clay among geologists, agronomists, engineers, and soil scientists, the term clay is generally understood by all who use it. As industrial minerals, clays are a complex group, encompassing diverse mineral commodities, each having somewhat different mineralogy, geologic occurrence, manufacturing technology, and uses. There are about 30 minerals included in the 3 clay groups. These types of clay minerals do not easily fit into the general 3-fold classification.
Most clay minerals are the result of weathering of some pre-existing rock.
The bulk of clay present in sedimentary rocks was derived by the weathering of silicate minerals composing igneous rocks. Illite Beidellite Dickite Montmorillonite Halloysite Nontronite Kaolinite Nacrite Classification by Use Clays are rarely present as pure minerals, but rather are mixtures of the different clay types, one group or type normally being dominant. The physical properties of clays, which determine their commercial value, depend on whether or not the various types are admixed, what types of clays are present and their percentages, the percentage and composition of non-clay materials present, and other factors. Utilization of clay as a resource material requires that the clay have a restricted range of properties for each industry. A lightweight aggregate manufacturer desires a clay which upon firing, bloats or swells to become a cellular frothy mass.