Abstract: Dyestuffs and other reagents which exhibit characteristic colors when adsorbed on clay granules frequently have been employed as aids in clay identification. The colors generally are believed to result from pleochroic effects and acid-base or oxidation-reduction reaction mechanisms. Certain aniline dyes, solutions of which vary in color according to their hydrogen ion content, may be used to indicate the relative acidity of clay surfaces as well as to heighten the natural pleochroism of kaolin minerals. When a clay sample has been pretreated with acid, the colors assumed by the adsorbed dye depend on the characteristic base exchange capacities of the various clay mineral groups present.
Aromatic diamines, amino phenols, and other compounds which can be oxidized to colored semiquinone cations permit particularly sensitive and positive identification of members of the montmorillonoid family and certain other three-layer clay minerals. Theories concerning the nature of this apparently catalytic oxidation on clay surfaces are inadequate to explain the reactions. New concepts and additional experimental information are needed.