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Pyrophyllite and talc sorb stearic acid on edge surfaces. The grinding of clay-stearic acid associations in the presence of alkali halides converts some of the acid into the ionic form, this occurring more readily with talc than with pyrophyllite. Heating in a closed or semi-closed system causes dissociation of stearic acid adsorbed on talc, but not on pyrophyllite. The infrared absorption frequencies of the adsorbed ionic form vary with the clay mineral and the amount of water present. The thermal stabilities of the clay-stearic acid associations depend on the rate of escape of the acid, which is determined by the strength of bonding to the clay and the nature of the system, and on the degree of dissociation of the acid on the clay surfaces.

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