Resolving Point Defects in the Hydration Structure of Calcite (10.4) with Three-Dimensional Atomic Force Microscopy

It seems natural to assume that defects at mineral surfaces critically influence interfacial processes such as the dissolution and growth of minerals in water. The experimental verification of this claim, however, is challenging and requires real-space methods with utmost spatial resolution, such as atomic force microscopy (AFM). While defects at mineral-water interfaces have been resolved in 2D AFM images before, the perturbation of the surrounding hydration structure has not yet been analyzed experimentally. In this Letter, we demonstrate that point defects on the most stable and naturally abundant calcite (10.4) surface can be resolved using high-resolution 3D AFM—even within the fifth hydration layer. Our analysis of the hydration structure surrounding the point defect shows a perturbation of the hydration with a lateral extent of approximately one unit cell. These experimental results are corroborated by molecular dynamics simulations.

Figure 1

AFM images of the atomically resolved calcite (10.4) surface with defects. Panels (a) and (b) show two AFM images at the same sample position recorded with a time difference of 2 min. Two red circles shown at equivalent positions in both images serve as a guide to the eye by showing the same defects, illustrating that the locations of the defects are fixed. The data range in both images is 1–6 kHz. All images are trace-down images, the $[42\overline{1}]$ direction points to the lower right corner.

Figure 2

Lateral slices of a 3D excitation frequency shift dataset showing a defect. The number in each panel indicates the hydration layer, in which the lateral slice was extracted (the corresponding $z$ piezodisplacement is shown in Fig. 3). The color scale ranges from black (low) over orange to white (high). The lower right panel shows the atomic structure of the (10.4) surface unit cell along with a scale bar that applies to all panels.

Figure 3

Excitation frequency shift profiles extracted above calcium ions (yellow) and carbonate groups (brown) at different sites. The extraction sites are shown in the inset. Profiles extracted at the defect site (marked with “$D$” in the inset) are drawn as thick lines; all other profiles are drawn as thin lines. The vertical lines (with numbers 1–5) indicate the tip-sample distance, at which the slices presented in Fig. 2 were extracted.

Figure 4

Defect-induced perturbation of the calcite hydration layer structure from simulation. Lateral slices through the time averaged water oxygen density at heights corresponding to the first to fourth hydration layers (HL1–HL4) over surfaces with ${\mathrm{Ca}}^{2+}$ and ${\mathrm{CO}}_3^{2-}$ vacancies as well as ${\mathrm{Mg}}^{2+}/{\mathrm{Fe}}^{2+}$, ${\mathrm{Sr}}^{2+}$, and ${[\mathrm{FeOH}]}^{2+}$ substitutions. A calcite surface unit cell defined by the calcium ions is indicated by the orange rectangle. The position of the defect is at the center of the dashed green circles. The oxygen density peak originating from the hydroxide ion in HL1 over the Fe(III) ion is indicated by a dashed red circle. The $[42\overline{1}]$ direction points to the right, the [010] direction points up.