Synthése, caractérisation et thermochimie d’apatites calco-magnésiennes hydroxylées et fluorées

Calcium-magnesium hydroxyapatite and fluorapatite solid solutions with the general formula Ca(10−x)Mgx(PO4)6X2, where X=F or OH were synthesized by precipitation in an ammonia medium. They have been characterized by infrared spectroscopy, X ray diffraction and chemical analysis. The nature of phases precipitated and the limit of magnesium substitution and formula of solid solutions were determined. Calcium-magnesium hydroxyapatite is obtained for a magnesium atomic fraction Mg/(Ca+Mg) up to 0.066. For more amount of magnesium, whitlockite (Ca18Mg2H2(PO4)14) and MgPO4NH4·H2O appear. Fluorapatite solid solutions were obtained for fraction of magnesium up to 0.101. Beyond this limit, MgPO4NH4·H2O phase appears. Using an isoperibol calorimeter the heats of solution of these products in a 9% weight nitric acid solution were determined. Combining these results with crystallographic ones allows to determine the substitution limits of magnesium in these products. This limit is in the range 0.066–0.084 for hydroxyapatites and 0.073–0.101 for fluorapatites.