Ferric iron in synthetic carbonate apatites: A Mössbauer effect study

Hydroxyapatites were synthesized by a precipitation method at pH 7.0 and 9.0 with carbonate and Fe³⁺ added during precipitation. The carbonate content of the samples was in the range of 0-4.0% and the Fe³⁺ uptake was up to 1200 ppm. The apatite phase of the compounds was identified by IR and x-ray diffraction methods. The Mössbauer spectrum of the samples at 85°K shows a quadrupole split doublet and at 4.1°K a magnetically split six-line spectrum was obtained. These characteristics of the spectra were obtained for all the samples studied, regardless of the amount of carbonate or iron in them. The isomer shift, quadrupole splitting and the hyperfine magnetic field are very similar to those of FeOOH and also to what was found for iron in bones and teeth in in vivo experiments. The results thus indicate that Fe³⁺ both in synthetic and biological apatites is present as FeOOH.