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 i n situ fluid cell atomic force microscopy (AFM), we show that the movement of monomolecular steps comprising growth hillocks on {10(1) over bar 4} fac es during the growth of this anisotropic material is specific to crystallog raphic direction. By quantifying the sensitivity of step growth kinetics to supersaturation, we can produce spiral hillocks with unique geometries. Th ese forms are caused by a complex dependence of step migration rates, nu(s) and nu(s-), upon small differences in solution chemistry as they grow nor mal to the conventional fast ([(4) over bar 41](+) and [481](+)) and slow ( [(4) over bar 41](-_) and [48(1) over bar](-)) crystallographic directions. As solute activity, a, decreases, nu(s+) and nu(s-) converge and growth hi llocks express a pseudoisotropic form. At still lower supersaturations wher e a approaches its equilibrium value, a(e), an inversion in the rates of st ep advancement produces hillocks with unusual reversed geometries. Comparis ons 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 dem onstrate the control of crystallographic structure on the local-scale kinet ics of growth to stabilize the formation of unusual hillock morphologies at the near-equilibrium conditions found in natural environments. Copyright ( C) 1999 Elsevier Science Ltd.