VATERITE GROWTH AND DISSOLUTION IN AQUEOUS-SOLUTION .3. KINETICS OF TRANSFORMATION

The kinetics of transformation of vaterite to calcite in aqueous solut ion was studied at temperatures between 25 degrees C and 45 degrees C and ionic strengths between 15 and 415 mmol dm(-3). Both vaterite and calcite precipitated spontaneously, with vaterite being initially the only solid phase in the system. The progress of the transformation was followed by recording pH as a function of time. It was found that thi s transformation was solution-mediated, and that the growth of calcite was the rate-determining process. This conclusion was additionally su pported by a mathematical model made to predict changes of both the so lid phase and the solution during the overall process. The model is ba sed on the rate constants and the mechanism of the processes involved in the transformation, as well as on the experimentally determined num ber density of the crystals formed. The seeded growth kinetics of calc ite was determined in separate experiments, at temperatures between 10 degrees C and 55 degrees C, and the rate law was found to be paraboli c. The high activation energy obtained for the temperature dependent r ate constants E-a = 55.29 kJ mol(-1), supports the assumption that the calcite growth was controlled by processes at the crystal surface.