Solubility of sintered fluorapatite.

The apparent solubility of fluorapatite (Fap) sintered at 890degreesC for one hour under an argon atmosphere was determined at 25, 30, 37 and 50degreesC in dilute solutions of phosphoric acid of various concentrations. The Fap displayed retrograde solubility. When the concentration of (HPO4)-P-3 was below 20.10(-3) Mol/L infrared spectroscopy and X-ray diffraction patterns did not show any significant evolution of the solid after equilibration. For a concentration of 50. 10(-3) mol/L, the apatite phase was modified with formation of calcium fluoride and of an amorphous phosphated phase CaHP0(4). Scanning electron microscopy showed a grain growth when temperature increased. The evolution towards new phases was assigned to secondary reactions and to the surface effect of the solid particles.