Lithium can reach high concentrations in some geological environments, such as rare element pegmatites, continental brines, and hydrothermal systems. The H2O-NaCl-LiCl system was investigated as a model for fluid inclusion studies in lithium-rich fluids. We first compiled experimental data (freezing point depression measurements and halite solubility data). Synthetic fluid inclusions were then prepared in quartz crystals, in order to obtain new solubility data and to observe phase transitions in a system characterized by the presence of numerous hydrates. The combination of Raman microspectrometry and microthermometry leads to identify undoubtedly the phase transition succession. Raman revealed partial crystallization and metastable phase transitions.

A new spectrum for LiCl pentahydrate, the stable phase melting at the eutectic, is presented. The Pitzer model was used to calculate phase solubility using published expressions of activity coefficients for the H2O-NaCl and H2O-LiCl systems. We finally propose a representation of the H2O-NaCl-LiCl phase diagram for the interpretation of phase transitions in Li-rich chloride-dominated fluid inclusions in the range −50° to +100°C.

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