Solution, Mixing and Formation Enthalpy Determination of Neodymium-Bearing Silicate–Oxyapatite

Apatite is a common phosphate mineral in crustal rocks, recognized as being the main feedstock for fertilizer industry, but also due to its rich composition, which makes this mineral a source for rare earth elements. Moreover, apatite has a structure stability and durability under radiation conditions in geological situation and in synthetic products. Thus, britholite, which is a phospho-silicate-apatite-containing rare earth elements, is considered as one of the possible candidates for actinides and fission products immobilization. Hence, the management of apatite, in this environment protection view, suggests the knowledge of their stability, thermodynamic properties, dissolution, and potential reactions. The calorimetry of dissolution is a powerful technique to study the stability of materials in various solvents, like water and acid solutions, which leads to the determination of numerous thermodynamic values. In this work, the thermochemical stability of an apatite solid solution was studied, named Neodymium-bearing silicate-oxyapatite or britholite Ca10−xNdx(PO4)6−x(SiO4)xO having different substitution rate x, with 1 < x < 6. The calorimetric study consisted of measuring the enthalpies of a solution in nitric acidic aqueous solution (46% w) at 298.15 K using a swinging differential Tian-Calvet calorimeter. The formation enthalpies from the elements were reached by a thermochemical cycle and complementary experiments. Moreover, the mixing enthalpies in the studied solid solution were also deduced. The results revealed that the solution reactions are exothermic and display a notable increase of the absolute values of energies with the rate of substitution. Furthermore, the mixing enthalpies are positive and the formation enthalpies are decreasing with the amount of substitution x. Such experimental results are readily usable thermodynamic data for geochemical studies on the stability of apatite.