Fig. 4.

Polymorph-specific nucleation and oriented, directional growth. The balance between the local supersaturation and lattice mismatch can be controlled by the influx of CO₂ such that nucleation of a BaCO₃/SiO₂ microstructure first occurs at aragonite (green arrow) and subsequently grows toward the carbonate source released from the nearby dissolving vaterite (red arrow) while leaving nearby calcite unaffected (blue arrow). The dashed arrow in the schematic on the Left represents the direction of growth.