New Structural Model Of Nacre And Its Photonic Iridescence

Nacre is a natural composite with excellent mechanical properties and bright iridescence, which consists of aragonite and organic matter. Here we focus on the nanosructure and iridescent color of nacre from bivalve Perna viridis and Mytilus edulis with Scanning electron microscopy (SEM), fiber optic spectrometry, Fourier transform infrared spectrum (FTIR), X-ray diffraction (XRD), and theoretical simulation, etc. In addition, we also investigate freshwater-cultured pearls with similar methods. We got the following results:(1) P. viridis shell consists of three aragonitic layers:a nacreous, prismatic, and transitional layer. Comparatively, M. edulis contains two layers:an aragoniteic nacreous layer and calcitic prismatic layer. Additionally, freshwater-cultured pearls are almost nacreous.(2) The aragonite platelets of nacre in P. viridis appear amazing dome shape which gradually changes into flat ones from region near prismatic layer to that near internal surface. While, the transitional layer shows plenty of irregularly shaped particles that form a coarse substrate, on which aragonite particles grow into dome-shaped platelets. In addition, surface tension and the organic matter also affect the formation of dome-shaped platelets. On the contrary, the aragonite platelets in M. edulis and freshwater-cultured pearls are all flat.(3) The reflection spectra of iridescent color of P. viridis, M. edilus, and freshwater-cultured pearls appear multi-order peaks that are consistent with the simulated spectra. We believe that the iridescent color is caused by the one-dimensional photonic crystal structure constructed by aragonite platelets in the nacre and pearls. Also, the changes of aragonite platelets thickness in difference regions lead to a red shift or blue shift in spectra. Furthermore, pigment in nacre strengthens the iridescent color.(4) The colours of nacre in shell and freshwater-cultured pearls are characterized with chromaticity coordinate for the first time. And it will provide theoretical base for quality determination of pearls.