Particle-size fractions of several Georgia kaolins, which were prepared by sedimentation procedures, were examined from the standpoint of crystallinity, cation-exchange capacity, and surface area. Crystallinity was studied using X-ray techniques, exchange capacities were measured using the manganese saturation method, and surface areas were determined using glycerol adsorption techniques. A linear relation was obtained between surface areas and exchange capacities when areas were increased by decreasing the particle size or by changing from wellcrystallized to poorly crystallized kaolins. In most cases, the empirically determined crystallinity ratios indicated a change in crystallinity with change in particle size, the crystallinity generally increasing with decreasing particle size in individual samples. Unlike, however, the relation noted for exchange, the crystallinity did not consistently correlate with area changes both among the various samples and within the various particle-size fractions of a single sample. These results suggest that the relatively high cation-exchange capacity of poorly ordered kaolins is more directly a result of high surface area with crystallinity playing, at most, a very minor role.

In connection with the study of certain properties of clay-water systems, an investigation was made of the exchange behavior-surface area relations of a series of controlled particle-size (monodisperse) fractions of a domestic kaolin. Surface areas were estimated by application of the Brunauer-Emmett-Teller theory to low-temperature nitrogen adsorption isotherms. Similar estimates were made based on room-temperature water adsorption isotherms. Exchange behavior was studied using direct quantitative techniques involving the exchange of divalent manganese. Good correlation was found to exist between the computed Brunauer-Emmett-Teller areas and the exchange capacities over the particle-size range 10 to 0.05 p, equivalent spherical diameter. These results suggest that the exchange behavior in kaolinites is primarily a surface phenomenon and does not depend on isomorphous substitutions.

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