Repulsive hydration forces between calcite surfaces and their effect on the brittle strength of calcite-bearing rocks

The long-term brittle strength of rocks and other polycrystalline materials, such as cement and ceramics, is affected by environmental conditions such as humidity or pore fluid chemistry. Slow brittle failure takes place through the subcritical propagation of fractures in the material, but for most minerals, the mechanism of subcritical fracturing is still unknown.

Calcite is the major component in limestone and is an important mineral in several petroleum reservoirs and active fault zones. Although preferential dissolution at the fracture tip has been though to be a key mechanism, recent experiments have suggested subcritical fracturing in calcite is driven by a repulsive force between the fracture surfaces, caused by adsorption of water on the hydrophilic calcite surfaces.

In order to test this hypothesis, we used atomic force microscopy (AFM) to measure the nanometer-scale forces between two calcite surfaces submerged in mixtures of water and glycol. We found a strong repulsion between calcite surfaces in water, while increasing the glycol concentration made the surfaces adhere to each other. This supports the model for adsorption driven subcritical cracking in calcite. Also, the experimental setup presents a new opportunity for analysing the effects of fluid chemistry on the long-term brittle strength of other polycrystalline materials.