Role of electrochemical reactions in pressure solution

Abstract

Using a Surface Forces Apparatus we have measured changes in the electrical potential difference between quartz and mica surfaces that correlate with the changing quartz dissolution rate when surfaces are pressed together at relatively low pressures (2–3 atm) in aqueous electrolyte solutions of 30 mM CaCl 2 at 25 °C. No detectable dissolution or voltage potential difference is measured in symmetrical systems (e.g. mica–mica or quartz–quartz) or between dry surfaces subjected to similar pressures, indicating that the dissolution can not be attributed to a simple pressure effect, slow aging (creep), or plastic deformation of the quartz surface. In quartz–mica systems brought together under pressure or to close proximity in electrolyte solution, the onset of quartz dissolution is marked by a sudden, rapid decrease in the quartz thickness at initial rates in the range from 1 to 4 nm/min, which after several hours settles into a constant rate of approximately 0.01 nm/min (∼5 μm/yr). Concomitantly, the potential drops to a constant value once the dissolution rate has stabilized. The decrease in the decay rate is interpreted as being due to saturation of the confined aqueous film and/or to the buildup of a Stern layer on the quartz surface, and the constant rate as being due to the steady-state chemical dissolution and diffusion of the dissolving silica into the surrounding reservoir. The dissolution is ‘non-uniform’: the surfaces become rough as dissolution proceeds, with the appearance of pits in a manner analogous to corrosion. On occasions, the process of rapid dissolution followed by a gradual transition to steady dissolution repeats itself, suggesting that the pit structure and Stern layer are fragile and subject to collapse and/or expulsion from the gap. Preliminary experiments on the dissolution of multi-faceted milled quartz particles (∼1.0 μm diameter) compressed between two muscovite surfaces suggest an asymmetry in the dissolution rates at different crystallographic planes. The origin of the electrical potential is interpreted as arising from the overlapping of the electric double-layers of two dissimilar surfaces when they are forced into close proximity. This electrical potential difference, for as yet unknown reasons, appears to be the driving force for the dissolution, rather than pressure.