On the real-structure of biomimetically grown hexagonal prismatic seeds of fluorapatite-gelatine-composites:: TEM investigations along [001]

The central parts of hierarchically ordered fractal aggregates of fluorapatite-gelatine-composites consist of an elongated hexagonal-prismatic seed (5-20 mm in length). The X-ray diffraction patterns of the composite-seeds (gelatine-content 2.3 wt.-%) correspond to fluorapatite single crystals, the fracture area perpendicular to the hexagonal axis, however, shows a radial structure completely different to the fracture surface of pure apatite. This observation had already led to the assumption of the formation of a nano-composite-superstructure. Here, we report on a TEM investigation into the real-structure of biomimetically grown seeds of the fractal composite aggregates. Because of the high complexity of the composite system the present work is first of all restricted to observations along [001] only. Compared to a pure inorganic apatite mineral the composite is characterised by a superstructure with average periodicity of about 10 nm. The superstructure seems to be caused by calcified (triple-helical) gelatine molecules (orientation of their long axes parallel to [ 001] of the composite seed) which act as nucleation centres for highly mosaic-controlled apatite-subareas at nano-scale.