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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.

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Structural biocomposites found in nature often have a well-defined organization on the nanometer scale. For mineralized materials, interactions between organic and inorganic phases are important for controlling crystal size, morphology, and spatial arrangement, which is a requirement when structural biomaterials are designed. In this paper, we studied influence of low concentrations of alginate on calcium carbonate crystallization by seeded and unseeded experiments, at controlled activity-based supersaturations. Crystal growth and nucleation were characterized by scanning electron microscopy (SEM), calcium concentration measurements, and crystal volume distribution measurements through the crystallization experiments. Alginate concentrations as low as 10 ppm were found to have a significant effect on growth of vaterite seeds, resulting in decreased growth rates and extensive agglomeration, compared to the case without alginate. For increased alginate concentrations (100 and 200 ppm), vaterite seed growth rates were decreased further. The decreased growth rates were probably caused by adsorption of alginate onto the active growth sites of the crystal surface. Alginate with 65% G-units (HighG) reduced the growth rate more than alginate with 43% G-units (LowG), which may be accounted for by the greater G-block length, and thus higher affinity to calcium, in HighG alginate. The unseeded experiments showed that mainly small vaterite crystals nucleated with 100 ppm alginate present, after an induction time of 50-80 min, while large calcite crystals were formed after some time by transformation from vaterite. The decreased crystal growth rates and higher nucleation rates caused by increased concentrations of alginate explain how small size mineral particles can be formed in alginate gel networks to form nanostructured composite materials.

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