Synthesis and thermochemistry of calcium-lead fluorapatites.

Lead-substituted calcium fluorapatites Ca(10-x)Pb-x(PO4)(6)F-2 with 0 less than or equal to x less than or equal to 10 have been precipitated in an ammonia medium (pH=11) containing fluorine ions. They have been characterized by X ray diffraction, infrared spectroscopy and chemical analysis. Measurements of solution enthalpies for Ca(10-x)Pb-x(PO4)(6)F-2 have been carried out in 25% w/w nitric acid solution using an isoperibol calorimeter. Standard enthalpies of formation of these products have been deduced from these results together with thermochemical results picked from literature or obtained by dissolving (or diluting): other products and entities. The heat of formation varies linearly with the amount of incorporated lead, ranging from -13550 kJ mol(-1) to -8466 kJ.mol(-1) correspondig to a lead atomic fraction X-Pb=O and X-Pb=10 respectively. The variation of the standard mixing enthalpy versus lead concentration seems to be symmetrical with a maximum at about X-Pb=0.5. However, this symmetry is fictitious since the parameter of binary interactions Delta(mix)H degrees/(X-Ca*X-Pb) can be represented by two Lines crossing at X-Pb=0.45. These results are in agreement with the non-statistical distribution of Pb2+ in the lattice of the apatite, which has been demonstrated previously elsewhere.