Fabrication and growth mechanism of pumpkin-shaped vaterite hierarchical structures

CaCO3-based biominerals usually possess sophisticated hierarchical structures and promising mechanical properties. Recent researches imply that vaterite may play an important role in the formation of CaCO3-based biominerals. However, as a less common polymorph of CaCO3, the growth mechanism of vaterite remains unclear. Here we report the growth of a pumpkin-shaped vaterite hierarchical structure with a sixfold symmetrical axis and lamellar microstructure. We demonstrate that the growth is controlled by supersaturation and the intrinsic crystallographic anisotropy of vaterite. For the scenario of high supersaturation, the nucleation rate is higher than the lateral extension rate, favoring the "double-leaf" spherulitic growth. Meanwhile, nucleation occurs preferentially in as determined by the crystalline structure of vaterite, modulating the grown products with a hexagonal symmetry. The results are beneficial for an in-depth understanding of the biomineralization of CaCO3. The growth mechanism may also be applicable to interpreting the formation of similar hierarchical structures of other materials.