Evolution of Amorphous Calcium Sulfate Nanoparticles into Crystalline Phases and Development of Short Range Order

The involvement of amorphous precursors in crystal nucleation remains an important but poorly understood phenomenon in materials science. In particular, the mechanisms by which amorphous precursor evolves to crystalline phase is typically unclear. By maintaining calcium sulfate at a low supersaturation to slow the nucleation rate, we captured amorphous calcium sulfate (ACS) nanoparticles of distinct sizes and shapes, and observed several stages of its evolution. The ACS nanoparticles grow by fusion accompanied by internal structural evolution and gradually develop a layered morphology. Aggregation of the growing ACS particles gives birth to bulk ACS material exhibiting a more compact structure within which crystalline domains form and develop into the gypsum structure. Structural characterization reveals the ACS nanoparticles have a proto-gypsum property ranging from short-range order to medium-range order and that the ACS nanoparticles are hydrous within which elimination of H2O molecules from the first to the second coordination shell of Ca facilitates ions transport and therefore formation of nanocrystalline domains within the amorphous precursors. This finding provides new insights into the evolution of amorphous precursors and their role in multi-stage crystallization.