WATER FILMS AT GRAIN-GRAIN CONTACTS: DEBYE-HUCKEL, OSMOTIC MODEL OF STRESS, SALINITY, AND MINERALOGY DEPENDENCE

Abstract

Water film diffusion is one of the mechanisms proposed to explain the deformation of rocks by pressure-solution during geological processes in the upper crust. This mechanism assumes that matter is dissolved inside the contact between two grains. The resulting solutes are transported in the pore fluid through diffusion in an adsorbed water film.The main problem of this theory is that it requires the presence of a water film that is believed to be stable under large deviatoric stresses inside the contact between two grains. In this paper, we show that the electrically charged surface of a mineral can attract counter-ions from the pore and, by the related change of osmotic pressure, keep water within the contact. This is due to the counter ions in the water film that increase the salinity in the film relative to that in the pore. This lowers the free energy of water in the contact zone to a degree that balances the increase in free energy of water due to the elevated pressure in the film. These notions are made more precise by combining the theory of the Debye-Huckle double layer with equations of osmotic pressure. The resulting D-H/O theory predicts the dependence of the water film thickness on stress across the contact, composition of the pore fluid, and the identity of the minerals involved.