Crystal nucleation and near-epitaxial growth in nacre

TL;DR

This study uses high-resolution imaging to reveal that nacre formation involves near-epitaxial aragonite crystal growth with mineral bridges facilitating orientation changes, advancing understanding of its layered structure and growth mechanisms.

Contribution

It provides detailed imaging evidence supporting near-epitaxial crystal growth and introduces the bridge-tilting mechanism as a way to explain orientation variations in nacre.

Findings

Nacre exhibits stacks of co-oriented aragonite tablets arranged in columns or diagonally.

Crystal growth in nacre is near-epitaxial, with orientation changes near the prismatic boundary.

Mineral bridges likely facilitate orientation continuity and changes during nacre growth.

Abstract

Nacre is a layered, iridescent lining found inside many mollusk shells, with a unique brick-and-mortar periodic structure at the sub-micron scale, and remarkable resistance to fracture. Despite extensive studies, it remains unclear how nacre forms. Here we present 20-nm, 2{\deg}-resolution Polarization-dependent Imaging Contrast (PIC) images of shells from 15 mollusk shell species, mapping nacre tablets and their orientation patterns, showing where new crystal orientations appear and how they propagate across organic sheets as nacre grows. In all shells we found stacks of co-oriented aragonite (CaCO3) tablets arranged into vertical columns or staggered diagonally. Only near the nacre-prismatic boundary are disordered crystals nucleated, as spherulitic aragonite. Overgrowing nacre tablet crystals are most frequently co-oriented with the underlying spherulitic aragonite or with another tablet, connected by mineral bridges. Therefore aragonite crystal growth in nacre is epitaxial or near-epitaxial, with abrupt or gradual changes in orientation, with c-axes within 20{\deg}. Based on these data, we propose that there is one mineral bridge per tablet, and that "bridge-tilting" is a possible mechanism to introduce small, gradual or abrupt changes in the orientation of crystals within a stack of tablets as nacre grows.