DIRECTION-SPECIFIC BEHAVIOR OF STEPS ON CALCITE AT NEAR EQUILIBRIUM CONDITIONS: IMPACT ON UNDERSTANDING SOLUBILITY CONSTANTS

Direction-specific behavior is a fundamental property of anisotropic materials. A number of previous studies have documented the direction-dependent kinetics and energetics, as well as the zoned incorporation of impurities along the <441>₊ (obtuse) and <441>_- (acute) steps on calcite, a well known anisotropic mineral. This study investigates the behavior of the two steps at near equilibrium conditions. Experiments were conducted on spiral hillocks and the step morphology and motion were monitored by in situ fluid cell atomic force microscopy.  Experimental results reveal that while step growth occurs in the two steps when the ionic activity product (IAP) of Ca²⁺ and CO₃^2- in the solution is greater than 10^-8.41, dissolution starts to develop in both directions when IAP reduces to 10^-8.51.  Furthermore, when 10^-8.41 < IAP < 10^-8.47, dissolution is seen in the obtuse steps, but not in the acute ones: whereas the <441>₊ steps dissolve and become serrated, the <441>_- steps maintain straight, either staying immobile or advancing at a measurable rate.   These observations are consistent with the thermodynamic theory that proposes direction-specific solubility for anisotropic materials.  Thus, our results may have determined the first crystallographic direction-specific solubility constants for calcite.