Textures of calcite crystals from a variety of mineralized tissues belonging to organisms from four phyla were examined with high-resolution synchrotron x-ray radiation. Significant differences in coherence length and angular spread were observed between taxonomic groups. Crystals from polycrystalline skeletal ensembles were more perfect than those that function as single-crystal elements. Different anisotropic effects on crystal texture were observed for sea urchin and mollusk calcite crystals, whereas none was found for the foraminifer, Patellina , and the control calcite crystals. These results show that the manipulation of crystal texture in different organisms is under biological control and that crystal textures in some tissues are adapted to function. A better understanding of this apparently widespread biological phenomenon may provide new insights for improving synthetic crystal-containing materials.

Sea urchin skeletal elements are composed of single crystals of calcite. Unlike their synthetic counterparts, these crystals do not have well-developed cleavage and are consequently much more resistant to fracture. This phenomenon is due in part to the presence of acidic glycoproteins occluded within the crystals. By means of x-ray diffraction with synchrotron radiation, it is shown that the presence of the protein in synthetic calcite only slightly decreases the coherence length but significantly increases the angular spread of perfect domains of the crystals. In biogenic calcite, the coherence length is 1/3 to 1/4 as much as that in synthetic calcite and the angular spread is 20 to 50 times as wide. It is proposed that the presence of macromolecules concentrated at mosaic boundaries that are oblique to cleavage planes is responsible for the change in fracture properties. These results may be important in the material sciences, because of the unusual nature of this material, namely, a composite based on the controlled intercalation of macromolecules inside single-crystal lattices.

Biological matrices can direct the absolute alignment of inorganic crystals such as calcite. Cooperative effects at an organic-inorganic interface resulted in similar co-alignment of calcite at polymeric Langmuir-Schaefer films of 10,12-pentacosadiynoic acid (p-PDA). The films nucleated calcite at the (012) face, and the crystals were co-aligned with respect to the polymer's conjugated backbone. At the same time, the p-PDA alkyl side chains reorganized to optimize the stereochemical fit to the calcite structure, as visualized by changes in the optical spectrum of the polymer. These results indicate the kinds of interactions that may occur in biological systems where large arrays of crystals are co-aligned.

Carbonated hydroxyapatite is the mineral found in vertebrate bones and teeth, whereas invertebrates utilize calcium carbonate in their mineralized organs. In particular, stable amorphous calcium carbonate is found in many crustaceans. Here we report on an unusual, crystalline enamel-like apatite layer found in the mandibles of the arthropod Cherax quadricarinatus (freshwater crayfish). Despite their very different thermodynamic stabilities, amorphous calcium carbonate, amorphous calcium phosphate, calcite and fluorapatite coexist in well-defined functional layers in close proximity within the mandible. The softer amorphous minerals are found primarily in the bulk of the mandible whereas apatite, the harder and less soluble mineral, forms a wear-resistant, enamel-like coating of the molar tooth. Our findings suggest a unique case of convergent evolution, where similar functional challenges of mastication led to independent developments of structurally and mechanically similar, apatite-based layers in the teeth of genetically remote phyla: vertebrates and crustaceans.