ABSTRACT Morel, J., and den Brok, S.W.J. (1999) Effect of stress on dissolution rate and on optical dissolution microstructure of free faces of single crystals of very soluble elastic-brittle salts. Abstract volume of the International conference on "Deformation Mechanisms, Rheology and Microstructures" held in Neustadt an der Weinstrasse, Germany, 22-24 March 1999, page ....

Effect of stress on dissolution rate and on optical dissolution microstructure of free faces of single crystals of very soluble elastic-brittle salts.

Jacques Morel and Bas den Brok

Institut für Geowissenschaften, Johannes Gutenberg-Universität, Becherweg 21, D-55099 Mainz, Germany.
Geologisches Institut ETH, Sonneggstr 5, CH-8092 Zürich, Switzerland.(morel@mail.uni-mainz.de; denbrok@erdw.ethz.ch)

Elastic strains are commonly regarded to have a negligible effect on growth and dissolution rate, especially compared to crystal-plastic strain (crystal defects). Recent experimental work by Ristic et al. (1997) on K-alum and sodium chlorate has shown, however, that (tensile) elastic strain may have a very strong effect on growth and dissolution rate. For example, an increase in the tensile stress by a factor ~2 caused a decrease in the growth rate by a factor ~2. We studied the effect of compressive elastic strain on the dissolution rate and on the (optical) dissolution microstructure of free-faces of single crystals of different elastic-brittle salts. To this end, solution-grown single crystals of three different elastic-brittle salts (K-alum, sodium chlorate and potassium dihydrogen phosphate) were elastically strained in their solution undersaturated to 0-1.5 degrees and held under stress for 1 to 5 days. The solution was continuously stirred and the temperature controlled to 0.1° in the range 18-30°C. Samples were right-angled with sides of 10, 6, and 4 mm long. Stress was applied parallel to the long side and fell in the range 5-20 MPa. In all experiments, one stressed and one stress-free sample were put next to each other ~5 cm apart, so that the dissolution features in the stressed and stress-free material could be directly compared. A 2 mm diameter hole was drilled in the middle of all samples, perpendicular to the applied stress. Hole diameter was measured before and after each experiment. After experiments, hole diameter was larger in the stressed than in the stress-free samples and elliptical, with long axis perpendicular to stress. Dissolution rate was roughly three times larger in the stressed than in stress-free material. For example, for K-alum at 28°C and an undersaturation of 0.25° the stress free hole grew at 1.5 Ám/hr whereas the stressed hole grew at 5 Ám/hr (9 MPa applied stress). Note, that this increase dissolution rate is much too large to be explained by the theoretic increase in driving force by elastically stored energy. Optical microscopy of the dissolved surface of the stress-free sample show fine dissolution grooves parallel to <100> directions, ~5 Ám apart and several mm deep. In the stressed samples, dissolution grooves are much larger in size, typically 20-30 Ám wide, 10-20 mm deep and oriented sub-parallel to <100> directions and always perpendicular to the stress.