Enthalpies of formation of polyhalite: A mineral relevant to salt repository

•Enthalpies of formation of polyhalite have been determined using drop-solution calorimetry.•Standard Gibbs free energy of formation of polyhalite has been derived.•This work provides fundamental parameters for modeling stability behavior of polyhalite.

Polyhalite is an important coexisting mineral with halite in salt repositories for nuclear waste disposal, such as Waste Isolation Pilot Plant (WIPP) in Carlsbad, New Mexico. The thermal stability of this mineral is a key knowledge in evaluating the integrity of a salt repository in the long term, as water may release due to thermal decomposition of polyhalite. Previous studies on structural evolution of polyhalite at elevated temperatures laid the basis for detailed calorimetric measurements. Using high-temperature oxide-melt drop-solution calorimetry at 975 K with sodium molybdate as the solvent, we have determined the standard enthalpies of formation from constituent sulfates (ΔH°f,sul), oxides (ΔH°f,ox) and elements (ΔH°f,ele) of a polyhalite sample with the composition of K2Ca2Mg(SO4)4·1.95H2O from the Salado formation at the WIPP site. The obtained results are: ΔH°f,sul = −152.5 ± 5.3 kJ/mol, ΔH°f,ox = −1926.1 ± 10.5 kJ/mol, and ΔH°f,ele = −6301.2 ± 9.9 kJ/mol. Furthermore, based on the estimated formation entropies of polyhalite, its standard Gibbs free energy of formation has been derived to be in the range of −5715.3 ± 9.9 kJ/mol to −5739.3 ± 9.9 kJ/mol. These determined thermodynamic properties provide fundamental parameters for modeling the stability behavior of polyhalite in salt repositories.