Molecular insight into the mechanochemical and tribochemical processes at the confined calcite-solution interface

When two mineral surfaces are compressed against each other in aqueous environment, surface forces are responsible for the presence of a thin solution film that remains confined between the two surfaces. This thin film provides a pathway for the ions and water to diffuse into the confined space and react with the mineral surfaces. Several geophysical and geochemical phenomena occur at confined interfaces, which are central to many natural processes at or near the Earth’s surface. For instance, pressure solution is the major mechanism of ductile deformation of the upper Earth crust, while the frictional behavior of carbonate faults can dictate earthquake nucleation. In spite of the relevance of these processes, not much is known about the confined mineral-solution interfaces at the fundamental level, which has motivated this doctoral work. Carbonate-based rocks abound in lithosphere. Our aim is to advance the fundamental knowledge of the mechanisms underlying both mechanochemical (pressure solution) and tribochemical (friction and lubrication) processes on confined carbonate surfaces in aqueous environment. Single calcite crystals were selected in this work because of their controlled atomically flat cleavage plane. To reach this objective, the following specific scientific goals were accomplished: I. Scrutinize the interfacial composition of the nanoconfined calcite-solution interface; II. Investigate the effect of the interfacial composition on frictional characteristics of the confined interface and the mechanisms that dictate lubrication; III. Elucidate how the interfacial composition affects pressure solution of calcite. By performing surface force measurements by Atomic Force Microscopy (AFM), both DLVO forces and non-DLVO forces were scrutinized to reveal the electrochemical surface properties and the composition of the confined calcite-solution interface with nanoscale resolution. The comparison between two electrolytes, NaCl and CaCl2 solutions, revealed ion-specific effects on the interfacial composition, specifically, differences in the structure of the calcite’s hydration layer. By conducting single-asperity friction experiments using an AFM, the frictional behavior of single calcite crystals under increasing contact stresses was characterized: viscous shear of a lubricious solution film at low normal stresses; shear-promoted thermally-activated slip, similar to dry friction but influenced by the hydrated ions localized at the interface; and pressure-solution facilitated slip at sufficiently high stresses and slow sliding velocities leading to a prominent decrease in friction. While the friction is lower in NaCl solutions at low normal stress, the weakening of the friction force, when pressure solution is triggered at high normal stresses, is more prominent in CaCl2 solutions. This suggested ion-specific effects on the pressure solution of calcite, which was then confirmed by pressure solution measurements with an extended Surface Forces Apparatus (SFA). Furthermore, these results were shown to be consistent with our understanding of the interfacial composition and the distortion of the hydration structure of calcite. In summary, this research has employed nanoscale evidence to scrutinize the influence of the solution composition on the interfacial reactions occurring on calcite, which has advanced the fundamental knowledge of tribomechanical and mechanochemical processes. The results of our studies can be extrapolated to carbonate fault friction in the presence of infiltrated fluids, where pressure solution provides a weakening mechanism of the fault strength at the level of single-asperity contacts and the composition of the fluid plays a significant role. Furthermore, this work opens an avenue to leverage the high-resolution approaches typically used in surface science to advance the knowledge of geophysics and geochemistry, providing molecular insight into fundamental mechanisms. More broadly, the developed analysis of non-DLVO forces can be applied to other interfaces with a wide range of applications. The newly developed SFA technique in this work can be applied to other minerals, which notably expands the applications of SFA.U of I OnlyAuthor requested U of Illinois access only (OA after 2yrs) in Vireo ETD syste