Spherulitic Growth of Coral Skeletons and Synthetic Aragonite: Nature’s Three-Dimensional Printing

Coral skeletons were
long assumed to have a spherulitic structure,
that is, a radial distribution of acicular aragonite (CaCO3) crystals with their c-axes radiating from series
of points, termed centers of calcification (CoCs). This assumption
was based on morphology alone, not on crystallography. Here we measure
the orientation of crystals and nanocrystals and confirm that corals
grow their skeletons in bundles of aragonite crystals, with their c-axes and long axes oriented radially and at an angle from
the CoCs, thus precisely as expected for feather-like or “plumose”
spherulites. Furthermore, we find that in both synthetic and coral
aragonite spherulites at the nanoscale adjacent crystals have similar
but not identical orientations, thus demonstrating by direct observation
that even at nanoscale the mechanism of spherulite formation is non-crystallographic
branching (NCB), as predicted by theory. Finally, synthetic aragonite
spherulites and coral skeletons have similar angle spreads, and angular
distances of adjacent crystals, further confirming that coral skeletons
are spherulites. This is important because aragonite grows anisotropically,
10 times faster along the c-axis than along the a-axis direction, and spherulites fill space with crystals
growing almost exclusively along the c-axis, thus
they can fill space faster than any other aragonite growth geometry,
and create isotropic materials from anisotropic crystals. Greater
space filling rate and isotropic mechanical behavior are key to the
skeleton’s supporting function and therefore to its evolutionary
success. In this sense, spherulitic growth is Nature’s 3D printing.