Relationship between surface structure, growth mechanism, and trace element incorporation in calcite

Abstract

Crystal growth and coprecipitation experiments demonstrate the manner in which surface structure and, in turn, crystal structure influence growth mechanism and trace element incorporation in calcite. Dominant {10 1 4} faces grow by the spiral mechanism, producing asymmetric polygonized growth hillocks comprised of two pairs of nonequivalent vicinal faces. Trace elements Mg, Mn, and Sr are differentially incorporated into structurally distinct growth steps that comprise the nonequivalent vicinal faces. The resulting trace element distributions represent intrasectoral zoning patterns and are found to be consistent with face symmetry. Lateral spreading rates are also different for nonequivalent growth steps at a given degree of supersaturation, and the rate anisotropy is dependent on the Ca 2+:CO 3 2− ratio in the growth solution. A rounding transition associated with changes in kink site density occurs preferentially on only one pair of equivalent growth steps, further demonstrating the importance of step-specific kinetics and affinities on {10 1 4} faces. Growth on other forms, including {01 1 2}, {11 2 0}, and {0001}, does not result in differential partitioning of trace elements and intrasectoral zoning, which is consistent with each of their surface symmetries and allowed growth mechanisms. However, sectoral zoning of Mg, Mn, and Sr occurs between these nonequivalent sectors, as well as {10 1 4}. We present a model detailing the geometry and coordination of elementary kink sites to explain both the differential incorporation and the rate anisotropy between nonequivalent growth steps on individual {10 1 4} faces. The model, constrained by face symmetry, accounts for affinities of different trace elements among four structurally distinct kink sites at which incorporation is preferred. The model also explains the observed differences in growth step velocity, as well as the in situ observations of growth step velocities by atomic force microscopy. Trace element incorporation on the dominant {10 1 4} faces of calcite is controlled by the detailed structure of the interface, which varies spatially on a face, as well as with external conditions. Consequently, our observed trace element distributions violate equilibrium partitioning, and it is likely that many trace element distributions in natural carbonates also may not reflect equilibrium.