In situ observations of liquid–liquid phase separation in aqueous MgSO4 solutions: Geological and geochemical implications

Abstract

A previously unknown liquid–liquid phase separation in vapor-saturated aqueous MgSO4 solutions containing 1.19–19.36mass% of MgSO4 was observed in fused silica capillary capsules during heating at temperatures above 259°C. Under these conditions, we observed that MgSO4-rich droplets were separated from the original aqueous MgSO4 solutions during heating, and these two coexisting liquid phases homogenized during cooling. The newly discovered liquid–liquid phase separation in MgSO4 solutions was characterized by a lower critical solution temperature phenomenon, which was considered to be a macro-scale chemical property of polymeric mixtures. In situ Raman spectroscopic investigations identified a distinctly new ν1(SO42-) mode at ∼1020cm−1 in the MgSO4-rich droplets; the new ν1(SO42-) mode was predicted to be present in MgSO4 polymer(s) in aqueous solutions. As mentioned above, both the phase behavior and relevant Raman spectra indicate the existence of polymer(s) in MgSO4 solutions. The recognition of the liquid–liquid phase separation and polymerization of MgSO4 in aqueous MgSO4 solutions is important for the experimental investigation of thermochemical sulfate reduction (TSR), because (1) the emergence of the MgSO4-rich droplets will substantially increase the local MgSO4 concentration, which is not representative of the geologic environments where TSR occurs; and (2) the formation of various ion pairs and MgSO4 polymers makes the mechanism of TSR far more complex than that occurring at relatively low temperatures (i.e., <200°C).