Abstract The Layer Charge (I.c.) of a 2:1 clay mineral is defined as the number (X+Y) of permanent charges per formula unit: (Table 1)*.The elementary (unit) cell contains two formula units. The permanent charges result from isomorphic substitutions and defects in the octahedral and tetrahedral sheet. A further contribution may arise from protons which are adsorbed by oxygen ions or desorbed from hydroxide ions. The formula given above considers the layers to be very large so that the chemistry at the edges may be ignored.