Stress induced grain boundary migration in very soluble brittle salt

Grain boundary migration (GBM) was studied in-situ at room temperature, atmospheric pressure and an applied diffmfwerential stress of ∼9.5 MPa under the optical microscope, in a wet aggregate of an elastic-brittle salt (sodium chlorate). The aggregate was previously deformed predominantly by a combination of grain boundary sliding, pressure solution and cataclastic solution creep. After deformation, but when the sample was still under differential stress, undeformed, fracture-free grains were observed to grow at the cost of deformed, intensely fractured grains. GMB rates typically fell in the range 2--10 μm/day. GBM took place only as long as the sample was under stress. Boundaries stopped to migrate as soon as stress was taken off and started to migrate again as soon as the sample was stressed again. Our observations suggest that GBM in this material and under the prevailing experimental conditions is a stress-induced process, i.e. driven by differences in elastic strain between fractured and unfractured grains.