This study presents the first description of liquid–liquid phase separation in a vapor-saturated CdSO4 solution at temperatures above 222.2 °C in fused silica capillary tubing, in which a sulfate-rich liquid (Srich) phase separates from an initially homogeneous aqueous solution and coexists with the remaining sulfate-poor liquid (Spoor) phase and a vapor phase. This phase behavior is characterized by a lower critical solution temperature (∼222.2 °C), which is a macroscale property of polymer mixtures. In situ Raman spectroscopy shows an increase in Cd2+–SO42− associations with increasing temperature, especially in the immiscible Srich phase. Phase behavior observations and in situ Raman spectroscopic analyses confirm our previous conclusion that strong ion associations are responsible for the liquid–liquid phase separation in inorganic solutions. The effects of pressure and the fluid composition on liquid–liquid phase separation are also investigated. The temperature of liquid–liquid phase separation increases almost linearly with pressure at a rate of ∼0.5 °C/MPa and decreases with the addition of methanol. Both decreases in pressure and increases in the methanol concentration decrease the dielectric constant of the solutions, favoring the Cd2+–SO42− interaction and thus liquid–liquid phase separation. The capillary size also exerts a strong influence on liquid–liquid phase separation because the immiscible Srich phase is more stable in fused silica capillary tubing with an inner diameter of ≤300 μm. Therefore, liquid–liquid phase separation can occur in hydrothermal fluids hosted in porous rocks/sediments, especially those that are rich in low-dielectric-constant components and/or at low pressures.
Liquid–liquid phase separation may play an important role in the formation of Mississippi-valley-type ore deposits because the ore-forming fluids are enriched in low-dielectric-constant components. The occurrence of liquid–liquid phase separation can promote the enrichment of Zn and S in fluids and accelerate thermochemical sulfate reduction as well as the mineralization of sphalerite. In addition, liquid–liquid phase separation may contribute to the enrichment and transport of metals in other hydrothermal systems because it significantly increases the capability of fluids to carry certain components. Finally, we observe crystallization within the Srich droplet, confirming that large clusters and separated dense liquids can act as precursors for crystallization. Therefore, crystallization through liquid–liquid phase separation can represent an important mechanism of nucleation and crystal growth in hydrothermal fluids.
