Narrowly distributed crystal orientation in biomineral vaterite

TL;DR

This study reveals that biomineral vaterite crystals in tunicates grow with narrowly distributed orientations, suggesting a unique ion-by-ion growth mechanism from solution, unlike previously assumed aggregation-based growth.

Contribution

It provides the first fully quantitative PIC mapping data showing narrow orientation spreads and proposes a novel growth mechanism for biominerals involving constrained nucleation.

Findings

Narrow 0°-30° orientation spreads between adjacent crystals.

Absence of organic layers at crystal interfaces.

Smooth nanoscale fracture surfaces indicating solution-based growth.

Abstract

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 multi crystalline spicules from the solitary tunicate Herdmania momus, with Polarization dependent Imaging Contrast PIC mapping, scanning and aberration corrected transmission electron microscopies. The first fully quantitative PIC mapping data, presented here, measured 0{\deg} 30{\deg} angle spreads between immediately adjacent crystals. Such narrowly distributed crystal orientations demonstrate that crystallinity does not propagate from one crystal to another 0{\deg} angle spreads, nor that new crystals with random orientation 90{\deg} 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{\deg}. 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{\deg} 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.