Reversed calcite morphologies induced by microscopic growth kinetics: insight into biomineralization

This experimental investigation of calcite growth quantifies relationships between solution supersaturation and the rates of step advancement. Using in situ fluid cell atomic force microscopy (AFM), we show that the movement of monomolecular steps comprising growth hillocks on {1014} faces during the growth of this anisotropic material is specific to crystallographic direction. By quantifying the sensitivity of step growth kinetics to supersaturation, we can produce spiral hillocks with unique geometries. These forms are caused by a complex dependence of step migration rates, vs+ and vs−, upon small differences in solution chemistry as they grow normal to the conventional fast ([441]+ and [481]+) and slow ([441]− and [481]−) crystallographic directions. As solute activity, a, decreases, vs+ and vs− converge and growth hillocks express a pseudo-isotropic form. At still lower supersaturations where a approaches its equilibrium value, ae, an inversion in the rates of step advancement produces hillocks with unusual reversed geometries. Comparisons of the kinetic data with classical theoretical models suggest that the observed behavior may be due to minute impurities that impact the kinetics of growth through blocking and incorporation mechanisms. These findings demonstrate the control of crystallographic structure on the local-scale kinetics of growth to stabilize the formation of unusual hillock morphologies at the near-equilibrium conditions found in natural environments.