Nacre has long served as a research model in the field of biomineralization and biomimetic materials. It is widely accepted that its basic components, aragonite biocrystals, namely, tablets, are formed by the nanoparticle-attachment pathway. However, the details of the nanoparticle morphology and arrangement in the tablets are still a matter of debate. Here, using field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM), we observed the nanostructure of the growing tablets at different growth stages and found that: (1) the first detectable tablet looked like a rod; (2) tablets consisted of subhedral nanoparticles (i.e., partly bounded by crystal facets and partly by irregular non-crystal facets) that were made of aragonite single crystals with a width of 160–180 nm; and (3) these nanoparticles were ordered in orientation but disordered in position, resulting in unique subhedral and jigsaw-like patterns from the top and side views, respectively. In short, we directly observed the growth of nacre biocrystals by the self-assembly of aragonite nanoparticles with a novel subhedral morphology.
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