On the Kinetics of Elementary Processes of Pressure Solution

Pressure solution is a widespread deformation mechanism in crustal materials, in particular in polyphase aggregates. The kinetics of the elementary processes of pressure solution, that is, the convergence rate of two crystals separated by a liquid interlayer under non-hydrostatic stress, are fundamental for the prediction of strain rates. Based on the concept of disjoining pressure, the problem is solved for the film diffusion mechanism. The geometry is that of a sphere in contact with a plane interface. Solutions are obtained for two cases, higher solubility of the initially spherical crystal and higher solubility of the crystal with the initially planar interface. The problem is solved in a general form with expressions in closed form for two limiting cases, interface-controlled and diffusion-controlled kinetics. An assumption with respect to the rate-controlling step is not required for the calculation of the convergence rate. Only a characteristic parameter must be calculated, which includes the constants of the system. The value of this parameter indicates which step limits the rate of the process. The expression for the convergence rate includes the constant of disjoining pressure, but not the true thickness of the interlayer, which is not constant over the contact area and is difficult to determine experimentally. Using the equations derived in this work, the calculated convergence rates between an NaCl or KCl crystal and a less soluble solid (quartz) in a saturated salt solution are in satisfactory agreement with published results of experimental studies.