Ion specific effects on the pressure solution of calcite single crystals

Pressure solution of carbonate-based rocks participates in many geophysical and geochemical processes occurring at or near the Earth's surface, but fundamental knowledge is still lacking. Here, the pressure solution of calcite single crystals in water, NaCl and CaCl₂ brines was investigated with an extended surface forces apparatus (SFA). This highly mechanically and thermally stable measuring technique enables to measure the deformation of two single calcite crystals undergoing pressure solution and the deformation rates with high precision as a function of load and fluid composition. The measurements in water reveal both diffusion- and dissolution-limited scenarios. Further, the pressure solution of calcite decelerates with a small concentration of NaCl compared to CaCO₃-saturated solution, while the rate is promoted with an increase to 100 mM NaCl, reflecting the competition of at least two different mechanisms. In contrast, the addition of small concentrations of CaCl₂ already speeds up calcite's pressure solution. These experimental results cannot be solely explained via the increase in solubility of calcite with ionic strength, as generally proposed. Instead, they are consistent with the ion-specific effects of the interfacial composition and the distortion of the hydration structure of calcite under an applied pressure smaller than ∼25 MPa. This work advances the fundamental knowledge of pressure solution of calcite. More broadly, the novel technique developed here can be applied to other brittle and unexfoliable minerals, which notably expands the applications of SFA.