The mechanism of dissolution of minerals in acidic and alkaline solutions: Part IV equilibrium and near-equilibrium behaviour

• The approach to equilibrium of dissolution reactions is not always described by the overall equilibrium expression.
• The proposed mechanism envisages the formation of anions and cations from the surface in parallel reactions.
• One parallel reaction might reach equilibrium before the other, giving rise to partial equilibrium.
• Examples for calcite, muscovite, K-feldspar and kyanite are given to illustrate the application of the theory.

The retarding effect of reaction products on the rate of dissolution of minerals is explored in this paper. It is shown that this retarding effect is not correctly described by methods based on the equilibrium for the overall reaction, such as those of chemical affinity. Such models frequently fail to describe the kinetics of the reverse reaction. Instead, in line with the mechanism of dissolution proposed in earlier papers in this series, it is proposed that the removal of surface species to form anions and cations in solution occurs in parallel partial reactions. An implication of the parallel nature of these partial reactions is that either of the partial reactions might reach equilibrium before that of the other partial reaction. This gives rise to the novel concept of partial equilibrium for dissolution reactions. The equations for this partial equilibrium are derived for both acidic and alkaline solutions. The application of the proposed theory to the dissolution of calcite, muscovite, feldspar and kyanite are given as examples to illustrate the concepts of full and partial equilibrium.