Jacques Morel and Bas den Brok
Institut für Geowissenschaften, Johannes Gutenberg-Universität,
Becherweg 21, D-55099 Mainz, Germany.
Effect of stress on dissolution rate and on
optical dissolution microstructure of free faces of single crystals of
very soluble elastic-brittle salts.
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 µm/hr whereas the stressed hole grew at 5 µm/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 µm apart and several mm deep. In the stressed
samples, dissolution grooves are much larger in size, typically 20-30 µm
wide, 10-20 mm deep and oriented sub-parallel to <100> directions
and always perpendicular to the stress.