Negative Surface Charges of Ground Dickite and Kaolinite

An effect of grinding on the negative charges of the clay minerals has been studied, paying attention to the distribution of the charges on basal and edge surfaces. The negative charges on the basal surface resulting from the isomorphous substitution were assessed from the C. E. C. values at pH 4, whereas those on the edge surfaces which vary with pH values of the ambient aqueous media were computed from the difference in C. E. C. values at pH 10 and pH 4. Dickte from Hiraki-mine and kaolinite (Soft Georgia kaolin), chosen as samples, were dry ground or wet milled. Dry grinding produced particles on which the edge surfaces occupied a relatively greater proportion of the total surface area. The obtained conclusions are as follows. 1. The negative charge densities on edge-faces of the ground samples were larger than those on basal surfaces. 2. The negative charge densities decreased gradually as grinding proceeded, which may suggest that the isomorphous substitution diminishes from the outermost surfaces toward the interior of the unground particles.