  Red abalone nacre is an intensely studied biomineral, and yet its formation
mechanism remains poorly understood. Here we report quantitative measurements
probing the degree of order of the aragonite tablets in nacre, and show that
order develops over a distance of about 50 microns. These observations indicate
that the orientational order of aragonite tablets in nacre is established
gradually and dynamically, and we show that a model of controlled assembly
based on suppression of the crystal growth rate along a specific direction,
when growth is confined in a layered structure, yields a tablet pattern
consistent with those revealed by detailed experimental measurements. This work
provides strong evidence that the organism s control of crystal orientation in
nacre occurs via regulation of crystal nucleation and growth as opposed to
direct templation or heteroepitaxial growth on organic molecules on the organic
matrix sheets.
