Thermal behaviour of magnesium-containing fluorapatite

Pure fluorapatite (Fap) and magnesium-substituted fluorapatite with various Mg contents were synthesised by precipitation reactions. Scanning electron microscopy micrographs showed that the use of Mg led to a change of the grain morphology. Chemical analysis and X-ray diffraction (XRD) indicated that Mg introduced in the solutions was incorporated into fluorapatite. However, samples heated at different temperatures showed that some Mg amount was adsorbed to the surface of the particles in an amorphous state, which crystallised in Mg2F(PO4) at 650°C. This phase was detected up to a temperature of 1120°C. Above this value, it disappeared following its dissolution in the liquid phase formed from a eutectic between fluorapatite and fluorine. The specific surface area (SSA) decreased strongly with increasing temperature. At low temperatures, the surface reduction was due to the agglomeration of the grains. With the improvement of the crystallisation, when the temperature increased, surface diffusion took place. At higher temperatures, before the densification occurred, surface diffusion was in competition with gaseous diffusion which became dominant.