Biological and Synthetic Apatites

The name ‘apatite’ describes a family of compounds having similar structure (hexagonal system, space group, P63/m) in spite of a wide range of composition. ^1–3 The similarity of the x-ray diffraction (XRD) patterns of enamel, dentin and bone to those of mineral apatites (hydroxyapatite, HA, fluorapatite, FA) ^3,4 together with chemical analyses showing calcium and phosphate as principal constituents led to the conclusion, as early as 1926, ^3,4 that the inorganic phases of bone and teeth are basically calcium hydroxyapatite, HA, idealized as Ca₁₀(PO₄)₆(OH)₂. To date, the exact structure of biological apatites remain undefined. The submicroscopic dimensions of biological apatites precluded the use of single crystal analysis methods to determine their structure. Several observations which include the non-stoichiometry of these apatites and association of principal minor constituents, magnesium and carbonate, provided evidence that biological apatites are not pure HA and should be instead classified as carbonate apatites. ^5–8