A solid-solution model for Fe(II)-Fe(III)-Mg(II) green rusts and fougerite and estimation of their Gibbs free energies of formation

Fe(II)-Fe(III) green rust identified in soil as a natural mineral is responsible for the blue-green colour of gley horizons, and exerts the main control on Fe dynamics. A previous study of the structure of the mineral confirmed that the mineral belongs to the group of green rusts (GR), but showed that there is a partial substitution of Fe(Il) by Mg(ll), which leads to a general formula of the mineral: . The regular binary solid-solution model proposed previously must be extended to ternary, with provision for incorporation of Mg in the mineral. Assuming ideal substitution between Mg(Il) and Fe(II), the chemical potential of any Fe(II)-Fe(III)-Mg(II) hydroxy-hydroxide is obtained. All experimental data show that the mole ratio X (sub 2) = Fe(lII)/[Fe (sub total) + Mg] is constrained structurally and geochemically. From Moessbauer in situ measurements and a model, the chemical composition of the GR mineral is constrained into a narrow range and the soil solutions-mineral equilibria computed.