The mesoscale order of nacreous pearls

Abstract

A pearl's distinguished beauty and toughness are attributable to the periodic stacking of aragonite tablets known as nacre. Nacre has naturally occurring mesoscale periodicity that remarkably arises in the absence of discrete translational symmetry. Gleaning the inspiring biomineral design of a pearl requires quantifying its structural coherence and understanding the stochastic processes that influence formation. By characterizing the entire structure of pearls (∼3 mm) in a cross-section at high resolution, we show that nacre has medium-range mesoscale periodicity. Self-correcting growth mechanisms actively remedy disorder and topological defects of the tablets and act as a countervailing process to 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 [Formula: see text] behavior, suggesting that growth is coupled to external stochastic processes-a universality found across disparate natural phenomena, which now includes pearls.

Keywords: SEM; TEM; mesoscale; nacre; pearls.

Fig. 1.

Formation of non–bead-cultured Akoya “keshi” pearls produced in a P. i. fucata mollusk. (A) Optical overview of a nonbeaded keshi–cultured pearl showing iridescence due to the interplay of constructive interference at nacre tablet layers with light. (B) Cross-section showing CaCO₃ growth begins onto an organic center. (C) Mature nacre (purple box in B) showing the ordered state in their thickness and interface curvature. (D and E) Atomic-resolution ADF-STEM of mature nacre and its corresponding Fourier transform indicating highly crystalline nacre and a lattice constant that is consistent with aragonite. (F) Cross-sectional backscatter SEM at the center of the pearl (yellow box in B) showing transition from spherulitic aragonite structures to nacre. (G and H) Aggregated nanoparticles (at time of observation) form massive, structurally indistinct aragonite structure. (I) Formation of nacre begins directly on massive, structurally indistinct aragonite.

Fig. 2.

Quantification of mesoscale periodicity in nacre. (A) Cross-sectional backscatter SEM of the early, middle, and mature stages of nacre growth shows ordering through reduced variation in the interface curvature and tablet thickness. Fourier transforms (Right) of nacre (Left) imaged by BSE-SEM show that angular broadening decreases from ±15 to ±5°. (B) Pair-correlation functions of nacreous layers represent the probability of finding tablets spaced a given number of unit cells apart. Sharpening of peaks in later nacre indicates increasing long-range order. (C) Correlation of the thickness of tablets with nearest neighbors shows a negative correlation. If one tablet is thick, the next one tends to be thin. (D) Cumulative disorder in nacre described by a real paracrystalline model, demonstrating that nacre has order between that of a crystal and a paracrystal model.

Fig. 3.

Topological defects in nacre. (A) Schematic of topological defects (a screw dislocation) in nacre. (B) Forty-five-degree tilted backscatter electron microscopy showing defect. (C) Cross-section of the schematic perpendicular to the slip plane (along direction of blue arrow in A). (D and E) ADF-STEM showing the extra tablet generated due to a topological defect and the mineral bridge connected to the adjacent layer. (F and G) Thickness map of the extra tablet showing abrupt change of thickness at the point of defect. (H) Cross-section of the schematic along the slip plane (along the direction of red arrow in A). (I and J) ADF-STEM showing the extra organic interface split by the defect. (K and L) Thickness map of the extra organic interface showing an abrupt change of thickness at the point of defect.

Fig. 4.

Growth processes of nacre throughout the entire cross-section of the pearl. (A) Overview of the pearl cross-section spanning from center to edge. (B) Log–log plot of the spectral density of the tablet thickness profile showing nacre thickness variation described by Markov processes. (C) Thickness profile across the entire pearl cross-section. (D) Thickness map from the early to mature stage of nacre corresponding to early nacre. It shows an abrupt attenuation of disorder in thickness and interface curvature. (E) Thickness map of the mature nacre showing different length scales associated with tablet thickness fluctuations.