ABSTRACT
The terms "clay" and "clay mineral" are used in very different contexts. For example, a common interpretation of a clay substance is a material whose constituent particles are very small (i.e., "clay sized"). This is typically an engineering and sedimentological usage. Curiously, it is only very recently that clay mineralogists have agreed on meanings for the two terms at the head of this paragraph (Guggenheim and Martin, 1995). The term "clay" now refers to any material which exhibits a plastic behavior when mixed with water, while "clay mineral" refers to materials which have a layer structure or a structure substantially derived from or containing major features of such layer structures. Thus, clay minerals will be, e.g., kaolinite, illite, and smectite but not, e.g., micas, talc, and pyrophyllite which occur in much larger particle sizes. A complication inherent with clays is the occurrence of materials normally considered to be amorphous or having a considerable concentration of defects, such as stacking faults. Clay minerals typically form at low temperatures (by geological standards), at low pressures (ambient or slightly elevated) and in the presence of abundant water. Under these conditions, perfection in the organization of the crystal structure is unlikely. The details of the crystal structure of these materials is of great importance in understanding the physical and chemical properties of clays. This also is true of the highly disordered or amorphous materials where there still exists short range order.
