The potentiality of clay minerals as useful indicators of the environment of their formation is summarized from the published literature on the origin of clay minerals. Caution must be observed when drawing geologic inferences from a rock or mineral concerning its origin because the rock or mineral may not have been formed where it now is found (occurs). Clay minerals are products of parent materials responding to the energy present in the reaction under which they were formed. In the genesis of some minerals the effect of parent material is dominant; in others the controlling factor is environment.

The environment of clay mineral formation may be described in part, or vaguely and broadly, in terms of climate as used in the usual sense. The environment is more clearly defined as a chemical micro-climate in terms of concentration of active ions, oxidation potential, and temperature of the reacting systems. This type of environmental description may be related to various geologic domains, such as in the ground-water zone, and under paludal, lacustrine, and marine conditions.

The kaolin group of minerals is formed where aluminum silicate rocks react so that the Al to Si ratio is relatively high, H ions are gained, and monovalent and divalent ions are lost. The montmorillonite and illite groups of minerals are formed from aluminum silicate rocks where a relatively high concentration of appropriate metal cations and relatively lower concentration of H ions are maintained.

A geologic illustration is cited where clay mineral facies in rocks of Pennsylvanian age in Missouri have been used to infer the geologic environments prevailing during the deposition of the clay minerals.