Thermochemical characterization of Ca₄La₆(SiO₄)₆(OH)₂ a synthetic La- and OH-analogous of britholite: implication for monazite and LREE apatites stability

In this contribution, monazite (LREEPO₄) solubility is addressed in a chemical system involving REE-bearing hydroxylapatite, (Ca, LREE)₁₀(PO₄,SiO₄)₆(OH)₂. For this purpose, a synthetic (La)- and (OH)-analogous of britholite, Ca₄La₆(SiO₄)₆(OH)₂, was synthesised and its thermodynamic properties were measured. Formation enthalpy of -14,618.4±31.0 kJ·mol^-1 was obtained by high-temperature drop-solution calorimetry using a Tian-calvet twin calorimeter (Bochum, Germany) at 975 K using lead borate as solvent. Heat capacities (Cp) were measured in the 143-323 K and 341-623 K ranges with an automated Perkin-Elmer DSC 7. For calculations of solubility diagrams at 298 K, the GEMS program was used because it takes into account solid solutions. In conditions representative of those expected in nuclear waste disposal, calculations show that La-monazite is stable from pH = 4 to 9 with a minimum of solubility at pH = 7. La-bearing hydroxylapatite precipitates at pH > 7 with a nearly constant composition of 99% hydroxylapatite and 1% La-britholite. Each mineral buffers solution at extremely low lanthanum concentrations (log{La} = 10^-10-10^-15 mol·kg^-1 for pH = 4 to 13). In terms of chemical durability, both La-monazite and La-rich apatite present low solubility, a requisite property for nuclear-waste forms.