Crystallization of CaCO3 in Water-Alcohol Mixtures: Spherulitic Growth, Polymorph Stabilization, and Morphology Change

The presence of alcohol in binary alcohol water mixtures can affect the precipitation pathways of anhydrous crystalline CaCO3 polymorphs and their morphology. We explored the formation pathways and the effects of several parameters on calcite, vaterite, and aragonite: concentration of simple alcohols, time, and shaking speed, and we derived a multiparameter model for predicting what phase is preferred. We found that shaking speed and alcohol concentration are the most important parameters for affecting the stability of vaterite and aragonite and for changing vaterite morphology, from cauliflower-shaped, spherical aggregates, to dendritic, flatter structures. In all our experiments, the precipitated aragonite was twinned, and both the vaterite and aragonite can be interpreted to form through spherulitic growth. Classical growth theory fully describes their formation; there is no need to invoke the popular hypothesis for nonclassical growth by self-assembly of nanocrystals. These studies, and future work with solutions of low water activity, are paving the way to a better understanding of how organisms select their preferred polymorph and engineer CaCO3 morphology during biomineralization.