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Soluble additives are widely used to control crystallization processes, modifying the morphologies, sizes, polymorphs, and physical properties of the product crystals. Here, a simple and versatile strategy is shown to significantly enhance the potency of soluble additives, ranging from ions and amino acids to large dye molecules, enabling them to be effective even at low concentrations. Addition of small amounts of miscible organic co-solvents to an aqueous crystallization solution can yield enhanced morphological changes and an order of magnitude increase of additive incorporation within single crystals─a level that cannot be achieved in pure aqueous solutions at any additive concentration. The generality of this strategy is demonstrated by application to crystals of calcium carbonate, manganese carbonate, and strontium sulfate, with a more pronounced effect observed for co-solvents with lower dielectric constants and polarities, indicating a general underlying mechanism that alters water activity. This work increases the understanding of additive/crystal interactions and may see great application in industrial-scale crystal synthesis.

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 ⟨1120⟩ 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 ...