Figure 7.

Schematic of spherulite growth mechanisms. All cases a, b, c share growth front nucleation (GFN), and, in the final spherulite, radial crystals oriented and thus colored similarly to their neighboring crystals. a. This is the spherulite formation mechanism proposed here: All newly nucleating crystals have random orientations (small hexagons with diverse colors, termed sprinkles), and because of competition for space they grow only if they are radially oriented. Coarsening makes larger crystals grow at the expense of smaller crystals, thus resulting in sprinkle-free spherulites. b. In a subset of coral skeletons the sprinkles are retained, because of incomplete or absent coarsening. These are the exceptional cases suggesting the formation mechanism for all spherulites, potentially. c. All newly nucleating crystals have orientation similar but not identical to the mother crystal. This previous assumption was termed non-crystallographic branching, and is ruled out by the alternative growth mechanism in a, inspired by coral skeletons.