Abstract Neodymium-bearing silicate-fluorapatites Ca 10− x Nd x (PO 4 ) 6− x (SiO 4 ) x F 2 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 Ca 9 Nd(PO 4 ) 5 (SiO 4 )F 2 . 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.