Abstract From an analysis of cation-exchange capacity (CEC), surface area, and structural data of talc-chlorite mixtures from Trimouns, France, ground in various manners, the talc component appears to have surface properties that can be predicted from its ideal structure. The CEC of 0.15 to 0.35 meq/100 g results only from the dissociation of silanol groups on lateral surfaces. The experimental equivalent size of an exchangeable site is 15 ± 8 Å compared with the theoretical value of 19.5 Å. The CEC of the chlorite component, ranging from 7 to 10 meq/100 g, has three different origins: (1) structural CEC of about 5.8 meq/100 g; (2) CEC resulting from Si-OH and IV Al-OH dissociation on lateral surfaces, where the equivalent size of an exchange site is 29.5 Å 2 ; and CEC resulting from hydroxide-sheet leaching. Estimates show that half of the Mg is leached from a basal face if it consists of a hydroxide sheet and that the contribution of Mg leaching from lateral surfaces leads to a CEC three times that of the tetrahedral border CEC These results suggest that the surface properties of chlorite cannot be understood if the mineral is assumed to be stable and in electrostatic equilibrium.