Anisotropic lattice distortions in biogenic aragonite

Composite biogenic materials produced by organisms have a complicated design on a nanometre scale1-3. An outstanding example of organic-inorganic composites is provided by mollusc seashells, whose superior mechanical properties are due to their multi-level crystalline hierarchy 4,5 and the presence of a small amount (0.1-5wt%)6 of organic molecules2,7. The presence of organic molecules, among other characteristics, can influence the coherence length for X-ray scattering in biogenic crystals8-10. Here we show the results of synchrotron high-resolution X-ray powder diffraction measurements in biogenic and non-biogenic (geological) aragonite crystals. On applying the Rietveld refinement procedure to the high-resolution diffraction spectra, we were able to extract the aragonite lattice parameters with an accuracy of 10 p.p.m. As a result, we found anisotropic lattice distortions in biogenic aragonite relative to the geological sample, maximum distortion being 0.1% along the c axis of the orthorhombic unit cell. The organic molecules could be a source of these structural distortions in biogenic crystals. This finding may be important to the general understanding of the biomineralization process and the development of bio-inspired 'smart' materials.