A correlation was made of crystallochemical data from 15 smectites with the half time of dissolution (t05) in 6 N hydrochloric acid at 96°C. From this the relation
ln t05 = 3.95 - 1.96 FeVI - 2.30 MgVI
was developed. It shows a remarkable dependence of the apparent dissolution rate of the octahedral layer of the smectites on the substitution of Fe3+ and Mg2+ for Al+3 in the octahedral position.