Properties of Heterogeneous Apatites Containing Magnesium,Fluoride,and Carbonate

Abstract

Biological apatites present in the mineral phases of normal and pathological calcifications contain magnesium, Mg, and carbonate, CO3. As a consequence of fluctuations in the composition of the micro-environment, these apatites may sometimes form by heterogeneous precipitation. The purpose of this study was to investigate the properties of (Mg, CO₃)-apatites formed heterogeneously in the presence of fluoride, F. Two types of fluoridated (Mg, CO₃)-apatites formed from solutions with low and high levels of Mg were prepared at 80°C, pH 7.4. We prepared FMgCO₃-MgCO ₃AP (Type 1) by adding the F-containing solution to those containing calcium, Mg, and phosphate ions during the first half of the precipitation period. We prepared MgCO₃-FMgCO₃Ap (Type 2) by adding the F-containing solution during the final half of the period. The apatites were analyzed by x-ray diffraction (XRD), infrared absorption spectroscopy, and scanning electron microscopy (SEM).

SEM and XRD analyses showed evidence of mixed crystals in the heterogeneous apatites. The presence of Mg inhibits, while F promotes, apatite crystal growth. In addition, Mg incorporation increased with increasing fluoride concentration. The extent of dissolution in acid buffer of both types of heterogeneous apatites increased with Mg: Type 1 > Type 2.

These results suggest that the crystal and dissolution properties of heterogeneous fluoridated (Mg, C03)-apatites are greatly affected by the mode of F incorporation and Mg concentrations in the environment.