Repulsive hydration forces between calcite surfaces and their effect on the brittle strength of calcite bearing rocks.
Published by Anja Røyne, K. N. Dalby and T. Hassenkam in journal of Geophysical research letters.
Experimental procedure. (a) Calcite particle resting on a cleaved calcite surface. A drop of UV-curing glue has been picked up by a tipless cantilever, which is gradually lowered onto the particle. (b) While in contact at a set deflection value, the glue is exposed to UV light and cured. (c) Experiments are performed in place by raising and lowering the particle. Fluid is added with the particle in the raised position. Not moving the cantilever-particle assembly ensures that the surfaces stay parallel. (d–g) SEM images of particles on cantilevers. The total area and RMS roughness of these surfaces are 3560 μm2 and 195 nm (Figure 2d); 317 μm2 and 73 nm (Figure 2e); 2090 μm2 and 586 nm (Figure 2f ); and 1660 μm2 and 113 nm (Figure 2g). In Figure 2d, the typical step height is on the order of 10 nm.
The long-term mechanical strength of calcite-bearing rocks is highly dependent on the presence and nature of pore fluids, and it has been suggested that the observed effects are due to changes in nanometer-scale surface forces near fracture tips and grain contacts. In this letter, we present measurements of forces between two calcite surfaces in air and water-glycol mixtures using the atomic force microscope. We show a time- and load-dependent adhesion at low water concentrations and a strong repulsion in the presence of water, which is most likely due to hydration of the strongly hydrophilic calcite surfaces. We argue that this hydration repulsion can explain the commonly observed water-induced decrease in strength in calcitic rocks and single calcite crystals. Furthermore, this relatively simple experimental setup may serve as a useful tool for analyzing surface forces in other mineral-fluid combinations.