Much of the latest research on CaCO3 polymorphs precipitated with and without additives discusses the processes of formation of the precipitates. Two prevalent growth processes have been proposed in this connection: classical and non-classical crystal growth. The classical theory is dependent on supersaturation and describes an ion-by-ion or single-molecule attachment to a critical crystal nucleus that only grows when the threshold value for the free-energy is larger than the critical radius and owth (Everett, 1988; Söhnel; 1992). The newer non-classical theory describes a nanoparticle mediated crystallization of primary nanoparticles. The aggregation of the nanoparticles can take several routes. In a pure system the nanoparticles are thought to evolve into an iso-oriented crystal through oriented attachment (Colfen and Mann, 2003). If polymers or other additives are involved, the process of nanoparticle aggregation is argued to occur via a mesoscale assembly into a single crystalline mesocrystal (Wohlrab et al., 2005). Aggregation of nanoparticles can also result in polycrystalline crystals if the aggregation is nonoriented (Kulak et al., 2007). The formation processes of crystals are difficult to address, however the non-classical theory is often claimed based on line broadening in a X-ray diffraction pattern or a polymorphic crystal. As a consequence, vaterite crystals with similar morphologies have been assigned to various formation processes.
In my studies I have not found any evidence in favor of the non-classical pathway and all my resulting growth rates and morphologies can be explained through the classical nucleation pathway. Both in pure systems and when additives are present. The below work represents some of my early work in the CaCO3 system.
Specifically I find that the aragonite morphology can be explained by a twinning processes and not as a result of nano aggregation. Both the branching aragonite structure and vaterite can be explained by classical spherulitic growth process.