Protonation–deprotonation of dioctahedral smectites

This paper reviews protonation–deprotonation reactions and loss of structural hydroxyls which change the total anionic charge of the layers of dioctahedral smectites. Protons for protonation of the layers may be derived from interlayer water or from organic material. Deprotonation is caused by proton acceptors. It may also occur on heating in the course of migration of small interlayer cations into the clay layers. Reduction reactions of structural Fe3+ in iron-rich smectites cause drastic changes in the anionic framework. Oxidation of structural Fe2+ is readily effected, but it has not been established whether this causes a change in the anionic composition of the layers. The ease with which changes in anionic composition of dioctahedral smectites can be brought about by fairly mild pretreatment is a warning against the tendency to regard this feature as a constant material characteristic.