Chiral and Achiral Mechanisms of Regulation of Calcite Crystallization

The regulation of calcite mineralization by chiral biological molecules is one of the fundamental unresolved issues at the interface between biological, geological, and physical sciences. Here we address the role of chirality of l -aspartic acid ( l -Asp), a model additive, in the regulation of the growth of calcite crystals. We apply phase-shift interferometry to nonintrusively monitor in-situ the morphology of the surface and quantify the velocity of propagation of the edges of the unfinished crystal layers, the steps, during crystallization. We show that l -Asp leads to several-fold increase in the step velocity, in all directions, at low supersaturations, and several-fold decrease in the step velocity at high supersaturations. l -Asp also introduces asymmetry in the velocity of steps related by mirror symmetry, however, of ≤10%. To explain the complex effects of l -Asp and, likely, of other biological regulators of calcite crystallization, we show that prior to incorporation into steps, calcium and carbonate ions adsorb on the terraces and diffuse toward the steps. l -Asp accelerates the surface diffusion toward the steps, an achiral process, likely because of rearrangement by l -Asp of the structure of the water coating the calcite crystal surface. Importantly, l -Asp delays by ∼20× the incorporation of calcite ions into the steps, the only chiral process in the calcite crystallization mechanism, likely by blocking a significant fraction of the kinks. We show that the low asymmetry between the two chiral directions is due to suppression of chirality by the symmetric surface supply fields of the steps. The results and analyses presented here suggest that the chiral effects of biogenerated molecules on crystallization may be too weak to determine the biological or nonbiological origin of minerals.