AN IDEAL SOL ID SOLUTION MODEL FOR CALCULAT ING SOLUBIL ITY OF CLAY MINERALS

ABSTRACT: A method for estimating Gibbs free energies and stabilities of clay minerals i proposed for use with computer p ograms aimed at calculating the chemical evolution of natural water-rock systems. This is based on (i) a model for ideal solid solutions of a large number of end-member compositions and (ii) a data set of estimated solubility products from 36 end-members. The application ofthe method to the production of experimental or natural clay stabilities i discussed. In the evolution of natural water-rock systems, clay minerals appear as omnipresent reactants or products of chemical reactions. In an attempt to model such reactions by the use of computer programs, clay mineral compositions have to be considered essentially as variable. It is possible to take into account such chemical composition variations by considering clays (i) as a set of separate mineral phases and including in the models a large number of minerals of discrete compositions and their solubilities, or (ii) as solid solutions whose compositions change continuously during the chemical processes. The first method was followed by Fritz & Tardy (1974, 1976) and Fritz (1975). The results obtained were roughly in agreement with natural observations. Nevertheless, thi