Cation exchange in clays and mudrocks by methylene blue

The nature of methylene blue (hydrochloride) adsorption is reviewed because the dye has recently been adopted for determining the presence of clay in quarry aggregates. Although it is demonstrated that methylene blue is not an ideal exchange reagent, details are given for overcoming most of the shortcomings, with particular reference to the aggregates test. It is demonstrated that methylene blue commonly underestimates the cation exchange capacity (CEC) of untreated expandable smectites. In contrast, it provides a fair reflection of the change in ratio of 7 Å/10 Å clay minerals in British mudrocks and overconsolidated clays. Plasticity indices of the latter show a high level of positive correlation with measured CEC.