(17a) Chemo-Mechanical Coupling in Single-Crystal Calcite {100}: An SFA and Diffusion–Reaction Model Investigation

Pressure solution, a fundamental chemo-mechanical process involving stress-enhanced dissolution at grain contacts and diffusive mass transport, governs the long-term evolution of carbonate rocks. Despite its geological significance, quantitative understanding of the underlying interfacial mechanisms, particularly for specific crystallographic orientations like calcite {100} and under asymmetric boundary conditions, remains limited.

This study employs a Surface Force Apparatus (SFA) integrated with interferometry to directly measure in situ nanometer-scale dissolution kinetics of single-crystal calcite {100} interfaces under controlled normal stresses (5.8-16.2 MPa) and aqueous chemical environments (pH 7). Force-distance profiles probe operative intermolecular forces, including electrostatic double-layer interactions (influenced by surface charge and counterion distribution) and short-range forces such as hydration repulsion and van der Waals attraction, while simultaneously tracking thinning rates indicative of dissolution. Experiments comparing symmetric calcite{100}-calcite{100} (Ca-Ca) and asymmetric calcite{100}-mica (Ca-Mica) contacts reveal distinct kinetics. The asymmetric Ca-Mica interface, characterized by differing surface potentials and potentially dissimilar interfacial water structuring and surface complexation affinities for dissolved ions, exhibits significantly accelerated dissolution rates (over double in 30 mM CaCl₂) compared to the symmetric Ca-Ca system. Introduction of CaCl2 electrolyte alters the interfacial environment by screening electrostatic forces (reducing Debye length), modifying Ca2+/ CO32- activities near the interface via the common ion effect, and influencing counterion binding, thereby modulating dissolution kinetics. A coupled diffusion-reaction model, incorporating stress-dependent dissolution kinetics (implicitly related to activation energy barriers modified by mechanical work) and Fickian diffusion of ionic species (Ca2+, HCO3-/CO32-) through the confined interfacial fluid film, successfully replicates the observed initial dissolution phases, particularly the near-equilibrium conditions under high saturation states. The model reinforces a diffusion-limited mechanism but requires refinement for long-term Ca-Ca behavior in pure water, where observed creep rates deviate from predictions, potentially indicating unmodeled effects like evolving contact geometry, asperity creep, precipitate formation obstructing diffusion pathways, or altered hydration layer dynamics under sustained stress. The illusion of a stress heterogeneity factor indicates the non-uniform stress distribution at nanoscale contacts.

These findings provide quantitative, mechanism-based insights into how interfacial asymmetry, electrolyte chemistry (impacting counterion screening and surface complexation), and the interplay of intermolecular forces (hydration, electrostatic, van der Waals) govern pressure solution kinetics, critical for advancing models of diagenesis, fault zone weakening, and CO2 sequestration integrity.