Calcite (101¯4) surface in humid environments

In this study, alternating current (AC) mode Atomic Force Microscopy (AFM) height images combined with force measurements and phase imaging were used to investigate the surface reconstruction and chemistry of the lowest energy surface, (101¯4) plane, of calcite, a stable form of calcium carbonate (CaCO3), in the presence of relative humidity at different temperatures. At 296 K and 70% RH, calcite (101¯4) undergoes rapid restructuring during hydration forming regions on the surface that are most likely characterized as an amorphous hydrate layer similar to what forms in solution under high [Ca2+] supersaturation conditions. This hydrate layer in turn serves as a substrate for the crystallization of another layer that possesses structural properties which differ from hydrate layer. Phase imaging reveals that these different layer structures formed in the process of water adsorption and surface reconstruction have very different energy dissipation modes. The origin of the different dissipation modes are likely due to differences in water content and hydrophobicity of these regions. The newly formed layer on top of the hydration layer is proposed to be vaterite, another polymorph of calcium carbonate. At 278 K the formation mechanism of the vaterite layer changes due to nucleation of a more crystalline hydrate layer, similar to calcium carbonate hexahydrate, instead of the amorphous hydrate layer that forms at 296 K. Force measurements corroborate the assignment of the speciation of different regions on the surface. Importantly, the AFM data show that the surface of calcite is highly inhomogeneous with regions that vary in water content. This suggests that the reactivity of calcite in humid environments will be highly spatially dependent.