CRYSTALLOGRAPHY AND STRUCTURE OF CURVED BIOGENIC SINGLE CRYSTALS

Biomineralization is the study of mineral formation controlled by living organisms. With the aid of proteins, organisms are able to control the structure and shape of the mineral crystals, resulting in superior materials.

This research focuses on a species of brittle star (O. Wendtii, of the phylum Echinodermata) which possess small lens-shaped crystals on their arm plates. These lenses perform as a compound eye, focusing light onto nerve bundles, allowing the animal to notice moving forms and differences in light intensity. These lenses are superior to man-made lenses, corrected for spherical aberrations and birefringence.

It has been widely believed that nearly all skeletal elements of echinoderms are single crystals[1]. In order to achieve a curved complex structure, a special crystallization mechanism is required. In this example, it was found that the arm plates crystallize from an un-ordered amorphous phase in a "mold", taking its shape[2].

We focused on studying the crystallography of the calcite plates, using EBSD and sub-micron scanning synchrotron diffracatometry to show the plates are single crystals. Results show clearly that the lenses and the stereom beneath are one single crystal. Although it has been proven using x-ray diffraction, it has never before been demonstrated with such detail.

HRTEM measurements show nanometric intracrystalline inclusions and XRD measurements are used to understand the calcite structure, combined with other methods such as DSC, TGA etc.

[1] Towe K.M., Echinoderm calcite - single crystal or polycrystalline aggregate, Science, 157(3792), pp. 1048, 1967.

[2] Aizenberg J., Muller D.A., Grazul J.L., Hamann D.R., Direct fabrication of large micropatterned single crystals, Science, 299(5610), pp. 1205-1208, 2003.