KINK DENSITY AND RATE OF STEP MOVEMENT DURING GROWTH AND DISSOLUTION OF AN AB CRYSTAL IN A NONSTOICHIOMETRIC SOLUTION

Despite the fact that the growth and dissolution of sparingly soluble minerals in aquatic and biological systems usually occur under nonstoi chiometric conditions, little attention has been paid to the effect of solution ionic ratio on the rates. Equations for kink density along a n atomic step of an AB crystal with NaCl lattice in super-and undersat urated solutions have been derived using a kinetic approach previously developed for a Kossel crystal. The equations reduce to that obtained thermodynamically at solubility equilibrium. The rate of step movemen t can subsequently be calculated. For growth and dissolution controlle d by integration and detachment, respectively, it is shown that the ra te cannot be defined solely in terms of the ionic activity product; it also depends upon the activity ratio and the relative integration fre quencies of the lattice ions. At a given thermodynamic driving force, a maximum rate occurs at a certain ratio of lattice ion activities,The present theory is expected to be applicable to sparingly soluble salt s, which are characterized by a low kink density, The effect of soluti on stoichiometry is also discussed for diffusion-controlled crystal gr owth and dissolution. (C) 1998 Academic Press.