Ian C. Olson, Adam Z. Blonsky, Nobumichi Tamura, Martin Kunz, Pupa U. P. A. Gilbert
Nacre, the iridescent inner lining of many mollusk shells, interests materials scientists because of its unique brick-and-mortar periodic structure at the sub-micron scale and its remarkable resistance to fracture. However, it remains unclear how nacre forms. Here we present 20-nm, 2{\deg}-resolution Polarization-dependent Imaging Contrast (PIC) images of shells from 15 species, mapping nacre tablets and their orientation patterns, showing where crystals nucleate and how they grow in nacre. 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 gradual changes in orientation within a stack, as nacre grows.
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http://arxiv.org/abs/1301.6273
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