Unveiling the critical process in which organic molecules control the polymorphism of magnesium-containing calcium carbonate: the early nucleation of amorphous precursors or the subsequent amorphous to crystalline transformations?

Abstract

Mineralization can be divided into the nucleation of amorphous precursors and the subsequent amorphous to crystalline transformation. Although the biomimetic transformations from amorphous precursors to different crystalline phases have been reported in a variety of settings, the issue on what determines the polymorph selection and morphology remains elusive. The present work studied the roles of organic molecules with different functional groups added in the nucleation process and added in the transformation process in controlling the polymorph and morphology of magnesium-containing calcium carbonate. We find that polymorph selection is controlled by the functional groups of organic molecules added into the starting amorphous precursor reactant rather than added in the transformation process. Specifically, when added in the first process, hydroxyl and amine groups induced a preferential transformation from amorphous to the thermodynamically metastable aragonite and carboxyl groups to the thermodynamically most stable calcite, whereas little difference was effected when these functional groups were introduced in the second process. Our work has important implications in in vivo polymorph selections of aragonite and calcite from amorphous precursors by functional groups of the omnipresent biomolecules such as proteins and polysaccharides.