Synthesis, characterization and thermochemistry of calcium-magnesium hydroxyapatite and fluorapatite

Calcium-magnesium hydroxyapatite and fluorapatite solid solutions with the general formula Ca(10-x)Mgx(PO4)(6)X-2, where X = F or CH were synthesized by precipitation in an ammonia medium. They have been characterized by infr ared spectroscopy, X ray diffraction and chemical analysis. The nature of p hases precipitated and the limit of magnesium substitution and formula of s olid solutions were determined. Calcium- magnesium hydroxyapatite is obtain ed for a magnesium atomic fraction Mg/(Ca + Mg) up to 0.066. For more amoun t of magnesium, whitlockite (Ca18Mg2H2(PO4)(14)) and MgPO4NH4.H2O appear. F luorapatite solid solutions were obtained for fraction of magnesium up to 0 .101. Beyond this limit, MgPO4NH4.H2O phase appears. Using an isoperibol ca lorimeter the heats of solution of these products in a 9% weight nitric aci d solution were determined. Combining these results with crystallographic o nes allows to determine the substitution limits of magnesium in these produ cts. This limit is in the range 0.066-0.084 for hydroxyapatites and 0.073-0 .101 for fluorapatites. (C) 2001 Elsevier Science B.V. All rights reserved.