Formation mechanism and morphology in precipitation of vaterite-nano-aggregation or crystal growth?

Spherical and polycrystalline particles of vaterite precipitated from solution has been shown to grow by crystal growth of a spherulitic type, and not by aggregation of nano-sized precursor crystals as often claimed in the literature. The mixing of concentrated solutions of calcium nitrate and sodium carbonate resulted in the formation of amorphous calcium carbonate which transforms within minutes to produce crystalline vaterite. Vaterite spheres nucleate and grow at a moderate relative supersaturation ratio of σ ≅ 5, determined by the dissolution of the amorphous compound. The size of the resulting vaterite spherulites is accounted for by the crystal growth rate determined in absence of nucleation, which disproves the nano-aggregation mechanism. The inner structure of fractured vaterite spheres exhibit the radiating pattern characteristic for spherulitic crystal growth. The final numbers of vaterite spherulites are typical for heterogeneous nucleation processes. It would take supersaturation values, orders of magnitude higher, to obtain the particle numbers required for the proposed nano-aggregation process.