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This report demonstrates how typical particle morphologies documented in biomimetic mineralization studies of calcium carbonate will precipitate also from solutions without adding modulating additives, at supersaturation levels below the solubility level of amorphous calcium carbonate (ACC). In the literature, hexagonal plates and flower structures of vaterite, as well as dumbbell structures of aragonite are explained by non-classical aggregation mechanisms from precursor crystals, assisted and stabilized by biomolecules, ions and templates, or by transformation from ACC. By performing experiments at both depleting and constant low supersaturation ratios for a range of temperatures, we show that the vaterite morphology changes from hexagonal monocrystalline plates expressing the basal (001) faces, to dendritic flower shapes and finally to spherulites, as a function of increasing supersaturation. Aragonite goes through a similar transition from monocrystalline elongated structures to polycrystalline dumbbells and spherical structures. We conclude that the key to understand the shape development is quantification of the activity based supersaturation and the realization that calcium carbonate forms along classical crystallization pathways. The higher number of crystals required for aggregation based growth is not favoured at these low supersaturation values. Dislocation and surface nucleation driven crystal growth is responsible for faceted morphologies at moderate supersaturation, whereas dendritic and spherulitic growth patterns appear due to interface instability at higher driving forces.
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