Evaluation of clay mineral equilibria in some clay soils (usterts) of the brigalow lands

Abstract

A model of the thermodynamic stability of the montmorillonite-kaolinitequartz system is constructed on the basis of published values of standard free energies of formation. The effect of differences in stability of montrnorillonite is estimated by considering different montmorillonites. Compositions of soil solutions from soil samples from the brigalow lands of south-east Queensland with a montmorillonite-kaolinite-quartz mineralology are plotted against equations for the montmorillonite-kaolinite, montmorillonite-quartz, and montmorillonite-kaolinite-quartz equilibria. The effect of different assumptions regarding the relation between the composition of saturation extracts and soil solutions is shown. A shallow surface zone is established with a chemical environment favourable to the formation of montmorillonite. In the deeper layers the soil solution is much closer to equilibrium with the mineral constituents, or quartz may be favoured instead of montmorillonite. Kaolinite cannot form in the present chemical environment. The high kaolinite content, therefore, is suggested to be a relict from an earlier phase of soil formation. It is further suggested that the strong gilgai, and the acidity of the deeper soil layers, may be explained by the hypothesis that montmorillonite has formed in originally kaolinitic materials, because the kaolinite to montmorillonite conversion is accompanied by volume increase, and release of H+ ions.