Narrowly Distributed Crystal Orientation in Biomineral Vaterite

Biominerals formed by animals provide skeletal support and many other functions. They were previously shown to grow by aggregation of amorphous nanoparticles but never to grow ion-by-ion from solution, which is a common growth mechanism for abiotic crystals. We analyze vaterite (CaCO3) multicrystalline spicules from the solitary tunicate Herdmania momus, with polarization-dependent imaging contrast (PIC)-mapping and scanning and aberration-corrected transmission electron microscopies. The first fully quantitative PIC-mapping data, presented here, measured 0-30 degrees angle spreads between immediately adjacent crystals. Such narrowly distributed crystal orientations demonstrate that crystallinity does not propagate from one crystal to another (0 degrees angle spreads), nor that new crystals with random orientation (90 degrees) nucleate. There are no organic layers at the interface between crystals; hence, a new, unknown growth mechanism must be invoked, with crystal nucleation constrained within 30 degrees. Two observations are consistent with crystal growth from solution: vaterite microcrystals express crystal faces and are smooth at the nanoscale after cryo-fracture. The observation of 30 degrees angle spreads, lack of interfacial organic layers, and smooth fracture figures broadens the range of known biomineralization mechanisms and may inspire novel synthetic crystal growth strategies. Spherulitic growth from solution is one possible mechanism consistent with all these observations.