Spherulitic Growth of Calcium Carbonate

The onset of crystal nucleation was studied for seeded semibatch experiments in aqueous solutions in which either the feed rate, the seed size, or seed amount was varied. The results of these experiments show that the nucleation behavior of polycrystalline vaterite features striking similarities with monocrystalline particles in general. Investigations of the nucleation rate of vaterite during spontaneous precipitation experiments have revealed that it would take orders of magnitude higher supersaturation values to obtain the particle numbers required for a proposed nanoaggregation process. The subunit “size” of polycrystalline vaterite is markedly different from particle to particle in seeded semibatch experiments in which nucleation occurred. Differences in the subunit “size” of particles formed under the same process conditions can hardly be explained by aggregation of precursor particles, as nanoaggregation would lead to a uniform distribution of nanoparticles among all particles. Crystal growth, on the other hand, can explain this phenomenon as it may depend on the underlying crystal surface and on the spherulite size. This points at spherulitic growth as the underlying particle enlargement mechanism. The same could be shown for spherulites of calcite for which the particle growth mechanism has been found to be dependent on the crystal surface structure. The current study suggests furthermore the performance of further studies concerning other substances forming polycrystalline particles to establish the correct particle enlargement mechanism.