Heat capacity of lazulite, MgAl2(PO4)2(OH)2, from 35 to 298 K and a (S–V) value for P2O5 to estimate phosphate entropy

The heat capacity of synthetic lazulite, MgAl₂(PO₄)₂(OH)₂, has been measured between 35 and 298 K by means of low-temperature adiabatic calorimetry. A lazulite third-law entropy (S⁰₂₉₈) value of 204(3) J K⁻¹ mol⁻¹ was retrieved after extrapolation of the lazulite heat capacity by a cubic temperature function, down to absolute temperature. Identical measurements on α-Al₂O₃ yielded S⁰₂₉₈ = 51.2 J mol⁻¹ K⁻¹ instead of the recommended 50.94 J mol⁻¹ K⁻¹ value. This new lazulite third-law entropy along with other phosphate entropy data either derived from low-temperature adiabatic calorimetry or experimental phase equilibria under pressure and temperature, were computed to retrieve a (S–V)P₂O₅ value of 57.4(2.9) to predict phosphate entropies from oxide summation. The predicted third-law entropy for trolleite, Al₄(PO₄)₃(OH)₃, amounts to 285.4(8.5) J mol⁻¹ K⁻¹ ~13% higher than the previously published value. We propose a new formation enthalpy from the elements of −6504.2 kJ mol⁻¹ and a new third-law entropy of 281.7 J mol⁻¹ K⁻¹ for trolleite, compatible with the predicted entropy from oxide summation as well as with the published brackets on the trolleite = berlinite + corundum + water reaction.