Relationship between the colour change of hydroxyapatite and the trace element manganese

Hydroxyapatite containing the trace element manganese turns blue after sintering at high temperature in an oxidizing atmosphere. This phenomenon is related to the oxidation of manganese ions in the special crystal structure of hydroxyapatite. Combination of the crystal structure with electron spin resonance spectra gives an explanation for this oxidation of Mn2+ ions at Ca(l) sites into Mn5+ ions and the formation of MnO4(3-) ions at PO4(3-) sites, corresponding to the colour change of hydroxyapatite.