The growth of nacre in the abalone shell.

The process of mineral formation following periods of growth interruption (growth bands) is described. Flat pearl implantation as well as a new trepanning method are used to observe the transitory phases of calcium carbonate which nucleate and grow during this process. An initial random nucleation of the aragonite polymorph is observed followed by a transition towards spherulitic growth. During this transition the animal forms the structure of the shell through both mechanical and chemical actions. About 6 weeks after implantation a steady-state growth of aragonite tiles begins after shorter and more irregular tiles cover the outer surface of the spherulites. The growth rate of aragonitic spherulite during this transition period was calculated to be approximately 0.5 microm per day. An organic scaffolding is observed during the steady-state growth of tiled aragonite. Observations of mineral growth following the deposition of these membranes confirm the presence of mineral bridges originating from subsurface tiles and extending through the organic matrix, confirming the growth model proposed by Schäffer et al. [Schäffer TE, Ionescu-Zanetti C, Proksch R, Fritz M, Walters, DA, Almqvist N, et al. Does abalone nacre form by heteroepitaxial nucleation or by growth through mineral bridges? Chem Mater 1997;9:1731-40]. Field emission scanning electron microscopy of fractured deproteinated nacre shows the presence of mineral bridges existing between individual layers of tiles. Transmission electron microscopy provides further evidence of mineral bridges.