A hierarchical structure for apatite crystals.

Based on the experimental results taken from the references, a reasonable hierarchical structure of apatite (both fluorapatite and hydroxyapatite) crystals has been proposed for the first time. The structure consists of four levels of the hierarchy: the smallest level is made of single unit-cells and/or Posner's clusters with the linear dimensions slightly below 1 nm, the second level comprises X-ray coherent scattering blocks of 50-80 nm in size, the third level is represented by dislocation blocks of 0.3-2.0 microm in size and, finally, there are macroblocks of 35-50 microm in size.