Water-Induced Reconstruction that Affects Mobile Ions on the Surface of Calcite

Time-sequenced contact-force micrographs show that the (101̄4) calcite cleavage surface reconstructs in humid air through pit formation and film growth. After 8 h at 80% relative humidity (RH), 50% to 80% of the surface is covered by islands that are flat-topped and 1-nm high. The lateral growth rates of individual islands are 4.2 ± 0.4 nm min^-1 in the 4̄41 direction and 1.8 ± 0.4 nm min^-1 in the 481̄ direction, resulting in islands having distinct major and minor axes. On some samples, a contiguous, 1.5-nm-high film rapidly grows between the islands and the pits. The areal expansion rate of the film is 500 times faster than that of the islands. Gaps between the contiguous film and the islands expand and contract, which suggests that mass is exchanged between them and that both are loosely bound. Complementing the topographic images, polarization heights are simultaneously measured by polarization-force microscopy. The polarization heights of the islands and the contiguous film are −6 to −10 nm and −4 to −5 nm, respectively, compared to their respective topographic heights of +1.0 and +1.5 nm. Under our experimental conditions, the polarization heights are a surrogate for the local dielectric constant of the sample ε and arise from a convolution of the mobility and the density of surface ions. The polarization heights imply that ε_substrate > ε_film > ε_island. Changes in topographic and polarization heights at 20% and 50% RH suggest that the structures of the islands are in dynamic equilibrium with the adsorbed water. Our evidence suggests that the islands contain loosely bound water and may therefore be a hydrated calcium carbonate phase stabilized by the calcite surface.