Calcite Kinks Grow via a Multistep Mechanism

Abstract

The classical model of crystal growth assumes that kinks grow via a sequence of independent adsorption events where each solute transitions from the solution directly to the crystal lattice site. Here, we challenge this view by showing that some calcite kinks grow via a multistep mechanism where the solute adsorbs to an intermediate site and only transitions to the lattice site upon the adsorption of a second solute. We compute the free energy curves for Ca and CO₃ ions adsorbing to a large selection of kink types, and we identify kinks terminated both by Ca ions and by CO₃ ions that grow in this multistep way.

Figure 1

Free energy profiles for Ca and CO₃ ions adsorbing to various kink types. Letters a–d denote the kink geometry (as shown in the schematic) and the labels (i) and (ii) represent the distinct kinks due to the alternating CO₃ orientations along each step. A value of zero along the x-axis corresponds to the adsorbate residing along the same {10.4} plane as the upper terrace, i.e., residing in the lattice site of the kink. The highlighted green and yellow regions correspond to the lattice and bidentate configurations (see Figure 2).

Figure 2

Side views of cross sections of calcite along the step, demonstrating an example of lattice and bidentate configurations. The outline of the steps on which kinks nucleate and propagate are traced with dashed lines. Ca ions are shown in green, C in gray, and O in red. Water molecules are shown in blue and the terminating Ca ion is shown in gold.

Figure 3

(a) Four snapshots (A–D) illustrate the multistep growth mechanism. Here, Ca atoms in the upper terrace are shown in pink. The two terminating ions are shown in gold. The perspective of the snapshots is one which directly faces the step, which runs horizontally. The kinks grow from the left side. (b) Schematic depicting the perspective of the snapshots and the direction of growth of the kink. (c) Free energy as a function of the position of the CO₃ and Ca ions adsorbing to the d(i) kink. A third reaction coordinate that accounts for dehydration has been integrated out. The minimum free energy pathway is traced with a dashed black line.