The kinetic behaviour of an ideal solid solution is modelled by two competing reactions: the stoichiometric dissolution of the existing solid and the precipitation of the least soluble compound, i.e. that with respect to which the oversaturation of the fluid is maximum; both reaction rates are expressed as a function of the corresponding departure from equilibrium, with a pH-dependent kinetic constant. Within this ideal model, the saturation curves and the fluid evolution of a simple reacting system may be displayed conveniently in chemical potential diagrams. The simulation of a geochemical system, constituted of a glauconitic sandstone aquifer infiltrated through by a sea water enriched in carbonates, allows to analyze the impact of taking into account two ferro-magnesian solid solutions, a calcite and a chlorite, on Fe-Mg exchanges during carbonates dissolution. The model accounts well for the tendency of solid solutions to dissolve congruently before precipitation starts and drives the fluid composition towards equilibrium.