The article discusses the thermodynamic properties of hydrothermal systems at elevated temperatures and pressures, focusing on the calculation of equilibrium constants for various reactions. It outlines the methods used to determine thermodynamic data for minerals, aqueous species, gases, and concentrated sodium chloride solutions. The paper presents a detailed analysis of the thermodynamic properties of hydrothermal systems, including the calculation of solubilities of silicates, sulfides, carbonates, sulfates, and oxides in high-sodium chloride solutions at elevated temperatures. The results are presented in tables and diagrams to facilitate extrapolation to the supercritical region and to enable the use of numerical values in further investigations. The paper also discusses the thermodynamic properties of electrolyte solutions at high temperatures, including the calculation of equilibrium constants for reactions involving "acid" aqueous solutions with high sodium chloride concentrations. It highlights the importance of these solutions in processes such as high temperature metasomatism and hydrothermal rock alteration. The paper presents a variety of methods for predicting thermodynamic properties, including the use of average heat capacities, experimental data, and theoretical models. The paper provides a detailed analysis of the thermodynamic properties of various aqueous species, including HCO₃⁻, HSO₄⁻, HS⁻, H₂CO₃, CO₂(aq), H₂S(aq), KCl, NaCl, HCl, CaSO₄, MgSO₄, H₄SiO₄, and chloride and sulfate complexes of ore-forming metals. It also discusses the importance of these species in hydrothermal processes and their role in the transport and precipitation of metals and sulfides in ore-forming concentrations at elevated temperatures. The paper presents a detailed analysis of the thermodynamic properties of minerals, including the calculation of standard enthalpies of formation and entropies. It discusses the use of entropy correlation plots to estimate dissociation entropies and the importance of these estimates in calculating high temperature equilibrium constants. The paper also discusses the use of heat capacity power functions to approximate the thermodynamic properties of minerals and the importance of these approximations in calculating equilibrium constants for high temperature reactions. The paper provides a detailed analysis of the activity coefficients in sodium chloride solutions, including the calculation of stoichiometric mean activity coefficients and the use of these coefficients in the calculation of equilibrium constants for reactions involving sodium chloride. It discusses the importance of these calculations in understanding the behavior of sodium chloride solutions at elevated temperatures and pressures. The paper also discusses the use of Henry's law coefficients to calculate the activity coefficients of CO₂ in sodium chloride solutions and the importance of these calculations in understanding the behavior of CO₂ in hydrothermal systems.The article discusses the thermodynamic properties of hydrothermal systems at elevated temperatures and pressures, focusing on the calculation of equilibrium constants for various reactions. It outlines the methods used to determine thermodynamic data for minerals, aqueous species, gases, and concentrated sodium chloride solutions. The paper presents a detailed analysis of the thermodynamic properties of hydrothermal systems, including the calculation of solubilities of silicates, sulfides, carbonates, sulfates, and oxides in high-sodium chloride solutions at elevated temperatures. The results are presented in tables and diagrams to facilitate extrapolation to the supercritical region and to enable the use of numerical values in further investigations. The paper also discusses the thermodynamic properties of electrolyte solutions at high temperatures, including the calculation of equilibrium constants for reactions involving "acid" aqueous solutions with high sodium chloride concentrations. It highlights the importance of these solutions in processes such as high temperature metasomatism and hydrothermal rock alteration. The paper presents a variety of methods for predicting thermodynamic properties, including the use of average heat capacities, experimental data, and theoretical models. The paper provides a detailed analysis of the thermodynamic properties of various aqueous species, including HCO₃⁻, HSO₄⁻, HS⁻, H₂CO₃, CO₂(aq), H₂S(aq), KCl, NaCl, HCl, CaSO₄, MgSO₄, H₄SiO₄, and chloride and sulfate complexes of ore-forming metals. It also discusses the importance of these species in hydrothermal processes and their role in the transport and precipitation of metals and sulfides in ore-forming concentrations at elevated temperatures. The paper presents a detailed analysis of the thermodynamic properties of minerals, including the calculation of standard enthalpies of formation and entropies. It discusses the use of entropy correlation plots to estimate dissociation entropies and the importance of these estimates in calculating high temperature equilibrium constants. The paper also discusses the use of heat capacity power functions to approximate the thermodynamic properties of minerals and the importance of these approximations in calculating equilibrium constants for high temperature reactions. The paper provides a detailed analysis of the activity coefficients in sodium chloride solutions, including the calculation of stoichiometric mean activity coefficients and the use of these coefficients in the calculation of equilibrium constants for reactions involving sodium chloride. It discusses the importance of these calculations in understanding the behavior of sodium chloride solutions at elevated temperatures and pressures. The paper also discusses the use of Henry's law coefficients to calculate the activity coefficients of CO₂ in sodium chloride solutions and the importance of these calculations in understanding the behavior of CO₂ in hydrothermal systems.