The equilibrium state of silicate minerals with groundwater in an aquifer can be known from the saturation state obtained by speciation codes, like WATEQ4F, or from stability mineral diagrams. In the study of the ground water-rock interaction in an hydrogeological profile of Campo Arañuelo aquifer, the code WATEQ4F shows that kaolinite seems to be the unique clay mineral involved in dissolution reactions, while stability diagrams indicate smectites as the stable clay minerals in the solution. Nevertheless, after pH values, mineralization of water samples and H4SÍ04 activity, smectites seems to be the clay mineral that most probably will be formed by the arkoses-groundwater interaction in this aquifer