Crystallography, microstructure, and implications for the formation of the biomineralized system, nacre in red abalone

Biomineralized hard tissues (organic/inorganic composites) show extraordinary specialization and control over their form and microstructure. Biocomposites have crystallites of organized shape, distribution, orientation, and size within an organic framework. The resulting optimized microstructures provide an excellent combination of physical properties. The research goal here was to investigate microstructure of nacre from red abalone, a nano-laminated CaCO{dollar}sb3{dollar} (aragonite)-protein composite. Our objective was to elucidate organismal effects on crystal formation as compared to geological minerals in terms of crystallography, defect structure, morphology, and growth behavior.; Aragonite crystallites in nacre grow over prismatic (calcitic) section, which acts as a substrate. Detailed investigation of cross-sectional and face-on TEM samples across the nacre/prismatic interface showed: (i) Morphological correlation between aragonite and calcite crystals; (ii) Preferred orientation in nacre even within transitional aragonitic microstructure well before mature nacre is formed; (iii) Crystallographic correlation for individual nacreous aragonite crystallites, in terms of both their c-axis orientation ( (001) are normal to the interface plane), and a and b axes (twin relation) on the same layers; (iv) An organic filament runs through the center of successive aragonite platelets within a crystallographically coincident stack; (v) The transition between calcite and aragonite across the interface takes place typically over a spatial range of 10-20 {dollar}mu{dollar}m; (vi) The transitional region is not abrupt, but gradual, with myriad morphological forms called "frustrated biomineralization".; A model has been developed for the formation of nacre based on these and other morphological and crystallographic features. Central to this model is highly coincident growth of aragonite platelets in stacks from a common nucleation center connecting them vertically via an organic tether. Shape control effects studied in-vitro indicate that aragonite crystals grown in the presence of organic extracts are morphologically very similar to those in nacre.; These findings are significant in realization of critical mechanisms in biomineralization. The control of shape and apparent nucleation of aragonite point toward a potential synthetic means by which these types of complex but highly ordered materials could be produced. Insights achieved here could lead to practical applications of biomimetic principles in molecularly engineered materials with controlled, laminated, and hierarchical microstructures.