KINETICS OF RHODOCHROSITE CRYSTAL GROWTH AT 25°C: THE ROLE OF SURFACE SPECIATION

Abstract

The rate of rhodochrosite (MnCO3) crystal growth in NaCl solutions has been measured, far from equilibrium and at 25°C, using a constant-composition method. Rates are not simply related to supersaturation (Ω) but also depend on PCO2 and the (CO32-)/(Mn2+) ratio, or pH, in solution. It has previously been proposed that the surface speciation of rhodochrosite is dependent on PCO2, Mn2+, and pH (Van Cappellen et al., 1993), where high PCO2 governs the formation of surface complexes # MnHCO30 and # MnCO-3. All experimental rate data can be explained by assuming that crystal growth is rate-limited by adsorption of lattice ions on these two dehydrated surface complexes. Dissolution is described as hydrolysis of these surface complexes, thus fulfilling the law of microscopic reversibility. Comparison with crystal growth rates of calcite is not straight forward because calcite growth has not been fit to this equation. However, at Ω = 5 and 50 crystal growth of rhodochrosite is about 4000 and 500 times slower, respectively, relative to calcite. These slow rates cannot be explained solely by the differences in solubility products, but suggest different kinetic control for rhodochrosite and calcite formation. The formation of calcite-rhodochrosite solid solutions is briefly discussed.