Abstract:
Coprecipitation experiments from aqueous solution at room temperature reveal that divalent Co, Zn, Cd, and Ba exhibit different preferences for incorporation among multiple surface sites present on the calcite {1014} face during spiral growth. Synchrotron X-ray fluorescence microanalysis reveals that Co and Cd are preferentially incorporated into the parallel arrays of <441>- steps that define one pair of symmetrically equivalent vicinal faces on polygonized growth spirals. In contrast, Zn and Ba are preferentially incorporated into <441>+ growth steps, which define a second pair of vicinal faces on the growth spirals but are symmetrically nonequivalent to the steps on the first pair. The step types differ in their direction and rate of migration on the {1014} face and in the detailed structural configuration of the kink sites that lie within them. In comparison with previous work showing that ion size (relative to host Ca) controls the preference of ions between surface sites of different size and geometry, the present observations suggest more complex influences. Zinc exhibits step-specific preferences similar to those of the larger ions Ba and Sr, which may reflect its strong association with ligands in solution or a particular interaction with surface sites.The inherent differences in step velocities between nonequivalent growth steps show a greater range of variability in the present work than previously. The step velocity anisotropy does not appear to be correlated with the differential, step-specific incorporation, although the dependence of impurity partitioning on step velocity may be an important influence superimposed on the structural control on trace element incorporation.The findings demonstrate how element-specific surface affinities and anisotropic kinetic behavior result from the existence of multiple expressions of a bulk site when exposed at a surface. This microscopic path dependence of surface processes is important for understanding the macroscopic chemical behavior of carbonates and other minerals.