We study how corals form their skeletons in their natural environments or in aquaria.
We found that in all 6 species thus far analyzed aragonite (CaCO3) formation is preceded by particle attachment of amorphous calcium carbonate (ACC) precursors, hydrated and anhydrous (ACC-H2O and ACC). See papers 145 and 156. These are Mass et al. Procs Natl Acad Sci 2017, and Sun et al.Procs Natl Acad Sci 2020.
We found that in all 13 species thus far analyzed all mature coral skeletons are made of aragonite (CaCO3), and all crystals are arranged spherulitically, that is, as acicular crystals radiating from common centers, termed center of calcification (CoCs). First we gave a quantitative definition of spherulite (<35° angle spread of c-axes measured with PIC mapping in adjacent crystals), then used this definition to confirm that all 13 coral skeletons are spherulitic. We found that a subset of species also retains “sprinkles”. That is, randomly oriented crystals, interspersed with spherulites. Using diverse coral skeletons as a model system, we figured out how all spherulites form: sprinkles are nucleated first at the growth front of a spherulite, then coarsening makes radially oriented crystals get larger at the expense of sprinkles. This growth mechanism applies to inorganic and organic spherulites, from coral skeletons, to geologic minerals, metals, aspirin, and chocolate (both sucrose and cocoa butter form spherulites in chocolate).
Phase-field theory simulations by Lászlo Gránásy confirm this formation mechanism for spherulites. See papers 144, 159, 160. These are Sun et al. ACS Nano 2017, Lo et al. Procs Natl Acad Sci 2021, Sun et al. Acta Biomaterialia 2021.