4. Structural Characteristics and the Occurrence of Polyamorphism in Amorphous Calcium Carbonate

Researchers have long aimed to understand the mechanistic origin of the unique material properties and the formation processes of biominerals. To meet these goals, many recent studies have focused on understanding the structural properties of amorphous calcium carbonate (ACC), a material known to be critical in the formation of biominerals and biomimetic materials. Here, we present what we believe to be the fundamental elements to an informed understanding of ACC, focused on its structural characteristics. First, we provide an overview of the static structure of ACC, largely founded on X-ray scattering studies. Then, we present the important roles of water—which is critical in the structure—and of impurities, including structural properties and their role in the amorphous to crystalline transformation. ACC impurities such as magnesium can influence the energetics/stability and structure. Finally, we highlight what we believe to be an exciting new area in the field, the study of polyamorphism. We present the latest work on advanced experimental and modeling techniques producing a model from which structural characteristics of the crystalline CaCO₃ polymorphs can be identified, resulting in a polyamorphic behavior.