Pervasive pressure solution transfer in a quartz sand

Abstract

Abstract. The kinetics of deformation of a quartz aggregate by pervasive pressure solution can be, under certain conditions of temperature and grain size, strongly dependent upon the diffusivity of silica into the grain-to-grain contacts. An analysis of the factors affecting this key parameter (and less well constrained in the analysis of the problem of rock deformation by pressure solution) is presented. This analysis is based on recent advances on studying silica surfaces and particularly on the existence of a silica gel layer on the silica surfaces undergoing dissolution. By reinvestigating the electroviscous effect occurring at the grain-to-grain contact the present analysis shows that the diffusivity of silica at the grain-to-grain contacts is likely to be relatively similar to that in the bulk pore water (maybe 1 order of magnitude smaller but not more). This contradicts the previous work by Rutter [ 1976], which has been the key reference used in many subsequent papers to justify an extremely low value for the diffusivity of silica at the grain-to-grain contacts (5 orders of magnitude smaller than the diffusivity of silica in free water). This finding has dramatic implications concerning the deformation rate of quartz sands and sandstones by pressure solution in sedimentary basins with regard to (1) the limiting step affecting the kinetics of the process (diffusion of the solute or dissolution/precipitation chemistry) and (2) the existence of a thermodynamic equilibrium state when deformation by pressure solution occurs over geological timescales. A poroviscoplastic model is used to describe deformation associated with pervasive pressure solution transfer in quartz sands. This model is shown to be consistent with the current state of knowledge of the surface chemistry of silica. In addition, the comparison between this model and both laboratory and field data is rather good.

Structure and properties of loaded silica contacts during pressure solution: impedance spectroscopy measurements under hydrothermal conditions

“…These values are still lower than the values reported here, though. As a result of our high mean fluid film thickness and assumed bulk diffusion coefficients, our estimated values for Dd av (and therefore Z) are between a half and four orders of magnitude higher than generally derived in quartz pressure solution models that assume diffusion to be rate-limiting (Gratier and Guiguet 1986;Shimizu 1995;Revil 2001;Revil et al 2006).…”

“…This and the similarly limited decrease in thin film diffusion rates reported recently for grain boundary diffusion in ''pressure dissolving '' halite (e.g., De Meer et al 2002Van Noort et al 2006), contrast strongly with the decrease of up to 5 orders of magnitude predicted in early theoretical studies (e.g., Rutter 1976Rutter , 1983. Agreement is much better, however, with more recent models in which values for D, in fluid films a few nanometers thick, are estimated to be up to 1 or 2 orders of magnitude lower than in bulk water (e.g., Revil 2001).…”

“…This assumes the contact to be overwhelmingly the highest resistance in the cell circuit, which simple tests, performed at room temperature and atmospheric pressure, have shown to be the case. Furthermore, additional effects have to be taken into account, when interpreting electrical conductivity measurements through narrow contacts, such as contributions to the conductivity from conduction through the diffuse layer, Stern layer conduction and from conduction by protons in the hydration layer (e.g., see Revil andGlover 1997, 1998;Revil et al 1999). These contributions stem mainly from increased concentrations of ions near the surface and can be significant in for instance (micro-)porous materials (Revil et al 1999;Bolève et al 2007).…”

“…Of particular importance amongst these grain boundary parameters, and highly controversial for many years, is the question of the effective thickness and diffusivity of wetted grain boundaries, expressed in terms of the grain boundary diffusivity product, defined Z ¼ Dd av C Ã , where D is the diffusion coefficient of the dissolved solid in the grain boundary fluid, d av is the average thickness of the grain boundary fluid film and C Ã is the concentration of solid dissolved in the grain boundary fluid phase in m 3 m -3 . This product Z controls the rate of grain boundary diffusion controlled pressure solution in materials with wetted grain boundaries, but is very poorly constrained (e.g., Rutter 1976Rutter , 1983Revil 2001).…”

Influence of surface conductivity on the apparent zeta potential of calcite

“…a shear plane several nanometers away from the beginning of the diffuse layer, to reconcile high electrical potentials at the beginning of the diffuse layer computed by their surface complexation model to low measured apparent zeta potentials (the magnitude of the computed electrical potential decreases with the distance from the surface). Furthermore, on the contrary to silica where protruding polysilicic acid groups may increase the distance between the beginning of the diffuse layer and the shear plane [23,24], there is no physical reason explaining the presence of a stagnant diffuse layer at the calcite surface. Heberling et al [9,10] also assumed that the thickness of the stagnant diffuse layer decreases with increasing salinity.…”