We reveal that the aragonite CaCO3 platelets in nacre of Haliotis laevigata are covered with a continuous layer of disordered amorphous CaCO3 and that there is no protein interaction with this layer. This finding contradicts classical paradigms of biomineralization, e.g., an epitaxial match between the structural organic matrix and the formed mineral. This finding also highlights the role of physicochemical effects in morphogenesis, complementing the previously assumed total control by biomolecules and bioprocesses, with many implications in nanotechnology and materials science.amorphous calcium carbonate ͉ biomineralization ͉ high-resolution transmission EM ͉ solid-state NMR T he delicate mineral structures produced by organisms in the process of biomineralization are widely recognized as inspiration for future materials science and nanotechnology because of their unique materials properties and their hierarchical order often over several length scales (1). Therefore, recent multidisciplinary research has focused on understanding biomineralization processes and exploring ways to mimic them (1). Particularly well investigated is nacre, possessing a 3,000-fold enhanced fracture resistance compared with pure aragonite, with implications on building material design. Nacre is composed of aragonite platelets, a usually metastable CaCO 3 polymorph, with [001] orientation toward protein-covered ␤-chitin layers (2).The present paradigm discusses an epitaxial match of acidic proteins adsorbed on the insoluble matrix with the atomic structure of the aragonite (001) plane (3). Indeed, two independent studies reported aragonite formation in the presence of soluble proteins extracted from a nacreous aragonite biomineral layer (4, 5). However, because the extracted macromolecules are disordered species and mixtures, too (5), an epitaxial match seems questionable. We therefore revisited nacre aragonite single crystalline platelets from the abalone Haliotis laevigata (6) with high-resolution transmission electron microscopy (HRTEM) supplemented by solid-state 13 C and 1 H NMR to obtain information on the organic-inorganic interface. Materials and MethodsNacre was obtained from the shell of the abalone H. laevigata, which belongs to the gastropoda. The structure of the nacreous layer is described in refs. 6-8. Thin cuts from the nacreous part were made with a diamond knife in a Leica ultracut UCT and transferred onto an amorphous carbon-coated copper grid. By using this technique, artifacts in the form of amorphous regions in the sample as can be observed by the ion milling technique (9) can be avoided. A Philips CM200 FEG transmission electron microscope, operating at 200 kV, equipped with a field emission gun was used. Alternatively, a JEOL 4010 operated at 400 kV equipped with a LaB 6 cathode was applied. The NMR experiments have been carried out by using an AVANCE 600 spectrometer (Bruker, Billerica, MA) using a double-resonant 7-mm probe at sample rotation magic angle spinning (MAS) frequencies of 6.5 kHz.1 H MAS NMR spectr...

Crystalline hexagonal-shaped superstructures of calcium carbonate, synthesized in the presence of ammonia, are shown to be assembled by a three-dimensional oriented attachment of vaterite nanoparticles. This unusual crystallographic lock-in mechanism enables the formation of complicated rounded structures with a crystallographic orientation from nanosized building blocks, which has so far only been found for transition metal systems.

Synthetic nacre morphologically indistinguishable from the natural archetype was synthesized with amorphous calcium carbonate precursors by confinement in the scaffold of the original insoluble nacre matrix. The precursors were generated using a synthetic polyelectrolyte highlighting the physicochemical aspects of biomineralization.