Nacre

Nacre is prized by jewelry makers. The iridescent, organically produced material has been used for centuries to make necklaces, rings, and earrings. Some artists bind it to ceramics and other polymers to create unique works. Researchers are also looking into using artificially produced nacre in the construction and manufacturing industries.

Nacre is composed of hexagonal platelets of aragonite (a form of calcium carbonate) 10–20 µm wide and 0.5 µm thick arranged in a continuous parallel lamina. Depending on the species, the shape of the tablets differ; in Pinna, the tablets are rectangular, with symmetric sectors more or less soluble. Whatever the shape of the tablets, the smallest units they contain are irregular rounded granules. These layers are separated by sheets of organic matrix (interfaces) composed of elastic biopolymers (such as chitin, lustrin and silk-like proteins). This mixture of brittle platelets and the thin layers of elastic biopolymers makes the material strong and resilient, with a Young's modulus of 70 GPa and a yield stress of roughly 70 MPa (when dry). Strength and resilience are also likely to be due to adhesion by the "brickwork" arrangement of the platelets, which inhibits transverse crack propagation. This structure, spanning multiple length sizes, greatly increases its toughness, making it almost as strong as silicon. The statistical variation of the platelets has a negative effect on the mechanical performance (stiffness, strength, and energy absorption) because statistical variation precipitates localization of deformation. However, the negative effects of statistical variations can be offset by interfaces with large strain at failure accompanied by strain hardening. On the other hand, the fracture toughness of nacre increases with moderate statistical variations which creates tough regions where the crack gets pinned. But, higher statistical variations generates very weak regions which allows the crack to propagate without much resistance causing the fracture toughness decreases.Studies have shown that this weak structural defects act as dissipative topological defects coupled by an elastic distortion. Nacre appears iridescent because the thickness of the aragonite platelets is close to the wavelength of visible light. These structures interfere constructively and destructively with different wavelengths of light at different viewing angles, creating structural colours. The crystallographic c-axis points approximately perpendicular to the shell wall, but the direction of the other axes varies between groups. Adjacent tablets have been shown to have dramatically different c-axis orientation, generally randomly oriented within ~20° of vertical. In bivalves and cephalopods, the b-axis points in the direction of shell growth, whereas in the monoplacophora it is the a-axis that is this way inclined. The interlocking of bricks of nacre has large impact on both the deformation mechanism as well as its toughness. In addition, the mineral–organic interface results in enhanced resilience and strength of the organic interlayers.

Nacre formation is not fully understood. The initial onset assembly, as observed in Pinna nobilis, is driven by the aggregation of nanoparticles (~50–80 nm) within an organic matrix that arrange in fibre-like polycrystalline configurations. The particle number increases successively and, when critical packing is reached, they merge into early-nacre platelets. Nacre growth is mediated by organics, controlling the onset, duration and form of crystal growth. Individual aragonite "bricks" are believed to quickly grow to the full height of the nacreous layer, and expand until they abut adjacent bricks. This produces the hexagonal close-packing characteristic of nacre. Bricks may nucleate on randomly dispersed elements within the organic layer, well-defined arrangements of proteins, or may grow epitaxially from mineral bridges extending from the underlying tablet. Nacre differs from fibrous aragonite – a brittle mineral of the same form – in that the growth in the c-axis (i.e., approximately perpendicular to the shell, in nacre) is slow in nacre, and fast in fibrous aragonite.