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 color of gley horizons, and exerts the main control on Fe dynamics. A previous EXAFS 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(II) by Mg(II), which leads to the general formula of the mineral: [Fe1-x2+Fex3+Mgy(OH)(2+2y)](x+)[xOH(-.)mH(2)O](x-). 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(II) and Fe(II), the chemical potential of any Fe(II)-Fe(III)-Mg(II) hydroxy-hydroxide is obtained as: mu = X(1)mu(1)degrees + X(2)mu(2)degrees + X(3)mu(3)degrees + RT[X(1)lnX(1) + X(2)lnX(2) + X(3)lnX(3)] + A(12)X(2)(1 - X-2). All experimental data show that the mole ratio X-2 = Fe(III)/[Fe-total + Mg] is constrained (1) structurally and (2) geochemically. Structurally, Fe(III) ions cannot neighbor each other, which leads to the inequality X-2 less than or equal to 1/3. Geochemically, Fe(III) cannot be too remote from each other for GR to form as Fe(OH)(2) and Mg(OH)(2) are very soluble, so X-2 greater than or equal to 1/4. A linear relationship is obtained between the Gibbs free energy of formation of GR, normalized to one Fe atom, and the electronegativity chi of the interlayer anion, as: mudegrees/n = -76.887chi - 491.5206 (r(2) = 0.9985, N = 4), from which the chemical potential of the mineral fougerite mu is obtained in the limiting case X-3 = 0, and knowing mu(1)degrees = -489.8 kJmol(-1) for Fe(OH)(2), and mu(3)degrees = -832.16 kJmol(-1) for Mg(OH)(2), the two unknown thermodynamic parameters of the solid-solution model are determined as: mu(2)degrees = +119.18 kJmol(-1) for Fe(OH)(3) (virtual), and A(12) = -1456.28 kJmol(-1) (non-ideality parameter). From Mossbauer in situ measurements and our model, the chemical composition of the GR mineral is constrained into a narrow range and the soil solutions-mineral equilibria computed. Soil solutions appear to be largely overstaurated with respect to the two forms observed.