Structural Characteristics of Amorphous Calcium Sulfate: Evidence to the Role of Water Molecules
Caiyun Jia, Luchao Wu, John L. Fulton, Xinran Liang, James J. De Yoreo, Baohong Guan
Abstract
Amorphous calcium sulfate (ACS) is intimately involved in the crystallization of calcium sulfate minerals, a major contributor to sulfur sequestration and a technological hindrance of scale formation, but its structural characteristics remain largely undiscovered. Here, we studied the structure of ACS nanoparticles captured under low CaSO4 concentration conditions. The ACS nanoparticles have a proto-gypsum property ranging from short-range to medium-range order, and water molecules are key structural constituents that are in a disordered state and 8-fold coordinated with Ca in the first coordination shell. More importantly, on a molecular scale, we give evidence concerning the energetic role of water molecules in promoting ions transport, molecule rearrangement, and development of a local coordination environment that facilitates structural evolution of ACS. This work reveals some important structural characteristics of ACS and has significance in providing a better understanding of the crystallization and biomineralization of water-bearing minerals.