The Mesoscale Crystallinity of Nacreous Pearls

Abstract

A pearl’s distinguished beauty and toughness is attributable to the periodic stacking of aragonite tablets known as nacre. Nacre is a naturally occurring mesocrystal that remarkably arises in the absence of translational symmetry. Gleaning the inspiring biomineral design of a pearl requires quantifying its structural coherence and understanding the stochastic processes that govern formation. By characterizing the entire structure of pearls (~3 mm) in cross-section at high resolution, we show nacre is a medium-range mesocrystal formed through nanoparticle assembly processes. Self-correcting growth mechanisms actively remedy disorder and topological defects of the tablets and act as a countervailing force to paracrystallinity (i.e. long-range disorder). Nacre has a correlation length of roughly 16 tablets (~5.5 µm) despite persistent fluctuations and topological defects. For longer distances (> 25 tablets, ~8.5 µm), the frequency spectrum of nacre tablets follows f-1.5 behavior suggesting growth is coupled to external stochastic processes—a universality found across disparate natural phenomena which now includes pearls.