Abstract: Studying the crystallization field of polyhalite in the Na⁺, K⁺, Mg²⁺, Ca²⁺∥Cl^-, $SO_{4}^{2-}$-H₂O hexary seawater system is essential for understanding the origin of polyhalite evaporites. It is also helpful for developing potassium extraction techniques for polyhalite resources utilization. However, the phase equilibrium relations between polyhalite and the more generally CaSO₄-based salts (e.g., gypsum, anhydrite, hemihydrates, syngenite, gorgeyite, glauberite and hydroglauberite) are far from clear in terms of both experimental evidence and thermodynamic models. The lack of reliable equilibrium data between polyhalite and multicomponent brines has created huge obstacle to constraining the polyhalite origin conditions and utilizing polyhalite resources. In this study, we re-examined the crystallization field of polyhalite in the Na⁺, K⁺, Mg²⁺, Ca²⁺∥Cl^-, $SO_{4}^{2-}$-H₂O hexary system at 25 ℃, both experimentally and by thermodynamic simulation. Our new experimental results using long equilibration time indicate the thermodynamically stable crystallization field of polyhalite is very large even at 25 ℃ and several times larger than the widely adopted experimental results. Moreover, our experimental results well support the predictions by the temperature dependent thermodynamic models. These more reliable phase equilibrium data provided solid physiochemical constrains to the origin of polyhalite evaporites, indicating a diverse stable mineral assemblage of polyhalite in the evaporite basin. Moreover, the potassium/magnesium concentration needed for polyhalite formation is not as high as previously suggested based on phase diagram. The improved knowledge implies that polyhalite is more likely to be an indicator for potassium-rich brines unsaturated with sylvine, rather than for large-scale soluble potassium salts (e.g., sylvine, carnallite, kainite and picromerite).

Key words: polyhalite, seawater system, crystallization field, phase diagram, thermodynamic simulation