Crystal nucleation and growth of spherulites demonstrated by coral skeletons and phase-field simulations

Spherulites are radial distributions of acicular crystals, common in biogenic, geologic, and synthetic systems, yet exactly how spherulitic crystals nucleate grow is still poorly understood. To investigate these processes more detail, we chose scleractinian corals as a model system, because they well known to form their skeletons from aragonite (CaCO3) spherulites, comparative study crystal structures across coral species has not been performed previously. We observed that all 12 diverse analyzed here exhibit plumose spherulites skeletons, with well-defined centers calcification (CoCs), crystalline fibers radiating them. In 7 the species, skeletal structural motif previously: randomly oriented, equant which termed “sprinkles”. Acropora pharaonis, sprinkles localized at CoCs, while 6 other either layered growth front (GF) or distributed. At nano- micro-scale, fill space much single aragonite. Based on observations, tentatively propose spherulite formation mechanism nucleation (GFN) oriented sprinkles, competition for space, coarsening produce rather than previously assumed slightly misoriented nucleations “non-crystallographic branching”. Phase-field simulations support this mechanism, and, using minimal set thermodynamic parameters, able reproduce microstructural variation experimentally investigated skeletons. Beyond aspirin semicrystalline polymers chocolate, may also according proposed here. Understanding fundamental mechanisms broad ranging applications fields metallurgy, polymers, food science, pharmaceutical production. Using reef-building system investigating processes, new can only explain micro-structural diversity distantly related but point universal wide range biologically technologically relevant materials systems.