Inorganic ions regulate amorphous-to-crystal shape preservation in biomineralization

Calcified minerals in biogenic materials often play a utilitarian role, such as structural supports in bone, teeth, and shells, where the crystals are arranged in well-ordered arrays (1, 2). The ability of organisms to produce single crystalline scaffolds and hierarchical architectures with unique features, such as bent or spheroidal shapes, has long fascinated scientists. The mechanisms of biomineral nucleation and growth are complex and not fully understood, while the ability to mimic these processes in vitro has proven challenging. Much attention has been given to studying the skeletal sections of sea urchins and other organisms (e.g., mollusks, echinoderms, calcisponges, and corals) that contain curved surfaces and various convoluted shapes (3). From numerous studies of these materials, which are predominantly composed of calcium carbonate minerals, it has become evident that crystallization originates from the initial formation of amorphous calcium carbonate (ACC) (4, 5). The pathways by which ACC transforms into crystalline CaCO₃ polymorphs has been a topic of ongoing investigation, although many fundamental details remain elusive. In PNAS, Liu et al. (6) report the time-resolved evolution of ACC to crystalline calcium carbonate using in situ transmission electron microscopy to show that the presence of inorganic ions leads to the direct transformation of the amorphous phase to calcite, all the while preserving the morphology of the original precursor.

ACC has become a recognized and frequently investigated form of calcium carbonate owing to its importance in biomineralization. Six additional crystalline forms of calcium carbonate (1) include calcite, aragonite, vaterite, …

↵¹Email: jrimer{at}central.uh.edu.