Standard enthalpy of formation of neodymium fluorbritholites

Neodymium-bearing silicate-fluorapatites Ca10−xNdx(PO4)6−x(SiO4)xF2 with 1 ≤ x ≤ 6 have been synthesized by solid state reaction at high temperature. The heat of solution in 46 wt% nitric acid has been measured at 298 K with a differential calorimeter. From the measurements on several compositions of the solid solution, the mixing enthalpy has been determined. In order to determine the enthalpies of formation, a thermochemical cycle was proposed and complementary experiences were performed to obtain the formation enthalpy of the monosubstituted britholite Ca9Nd(PO4)5(SiO4)F2. This quantity was combined with the mixing enthalpies to obtain the formation enthalpies of the remaining britholites. The stability of the solid solution decreases as the rate of substitution increases.