Properties of lithium under hydrothermal conditions revealed by in situ Raman spectroscopic characterization of Li2O-SO3-H2O (D2O) systems at temperatures up to 420 °C

In submarine hydrothermal systems, various contact ion pairs should be important Li species during the high-temperature leaching of Li from basalts (e.g., > 200 °C). The formation of strong and various Li+-SO42 − pairs can enhance the leaching of Li because the mobility and diffusion of the Li+-SO42 − complex has been reported to be stronger than that of “free” ions. During the low-temperature (e.g., < 150 °C) uptake of Li+ into secondary minerals (e.g., clays), free Li+ and outer-sphere ion pairs are the dominant Li+ species. Contact ion pairs are also important Li species during the hydrothermal alteration/precipitation of Li-rich minerals in pegmatite systems. Moreover, liquid-liquid phase separation was found to play important roles in the formation of low-temperature minerals (e.g., calcite). However, similar crystallization pathways have rarely been documented for inorganic systems under high temperatures. Anhydrous Li2SO4 was observed to precipitate within the dense liquid phase at ≥ 360 °C in 1.5 m Li2SO4, confirming that the immiscible dense liquid phase can serve as a precursor for the crystallization of sulfates and other minerals under hydrothermal conditions. The possible processes for the dense-liquid-based crystallization by particle attachment can be summarized as (1) strong and reversible ion association induces liquid-liquid phase separation, (2) nucleation and crystallization within the dense liquid phase, and (3) continued growth on the surface of early formed solid phase. Submarine hydrothermal fluids often contain low-dielectric components such as methane and other organic components, which lower the dielectric constant of hydrothermal fluids and promote ion association. Therefore, liquid-liquid phase separation may occur in submarine hydrothermal fluids and can play important roles in the precipitation of sulfates and other minerals.