Thermodynamic Model of the H2O–LiCl–NaCl System for Study of Fluid Inclusions: Calculations Using the Pitzer Equations

A thermodynamic model of the ternary fluid system H2O–LiCl–NaCl is proposed for the temperature range from –77 to +300°C. This model incorporates low-temperature phase transitions of freezing products of water–salt inclusions. The model is based on the Pitzer equations using new interaction parameters of Na, Cl, and the corresponding equilibrium constants of reactions involving the solid and liquid phases. Based on microthermometry data of fluid inclusions (T of phase transitions during heating after freezing), the model allows determination of the salt concentrations. The characteristics (T, wt %, LiCl and NaCl) of triple points with solid phases including the eutectic $${\text{E}}{\kern 1pt} ''$$ (ice + LiCl · 5H2O + NaCl · 2H2O), peritectic $${\text{P}}_{1}^{{{\text{''}}}}$$ (LiCl · 5H2O + NaCl · 2H2O + NaCl) and $${\text{P}}_{2}^{{{\text{''}}}}$$ (LiCl · 5H2O + LiCl · 3H2O + NaCl), and the cotectic, peritectic curves separating the phase fields (ice + L, NaCl · 2H2O + L, NaCl + L), as well as solubility isotherms of ice, hydrohalite, and halite calculated by the model, showed good agreement with the experimental data. As an example of the application of a model to a natural object, we determined the salt contents in lithium-bearing brine inclusions in late quartz veins of the Bolshie Keivy area (Fennoscandian Shield).