Improved method for estimating the standard free energies of formation (ΔGf,298.150) of smectites

Two alternative empirical methods were developed for estimating the standard free energies of formation of layer silicate minerals with cation exchange capacity due to isomorphous substitutions. These methods involve the summation of either hydroxide or oxide components to form the desired smectite mineral similarly to a method developed by Nriagu, but differ in that (1) no ad hoc ‘calibrations’ are necessary for the $\text{ΔG}{}_{\mathrm{f,298.15}}{}^0$ values of alkali hydroxides and (2) a three-parameter equation is used for the correction factor, with the charge on the mineral due to isomorphous substitutions and the ionic radius and charge on the exchangeable cation taken as independent variables.A comparison between experimental and estimated $\text{ΔG}{}_{\mathrm{f,298.15}}{}^0$ values indicated that one of the methods (that involving the summation of hydroxide components) was significantly better than all other methods in predicting $\text{ΔG}{}_{\mathrm{f,298.15}}{}^0$ values of smectites while at the same time requiring no ad hoc thermochemical assumptions.