In most mineral reactions studied experimentally, at least one fluid component is directly involved. Usually these reactions involve either dehydration or decarbonation or both.
For the thermodynamic treatment of fluid components a function of state is needed.
For pure H2O and pure CO2, a number of functions of state exist (e.g. Halbach & Chatterjee, 1982; Shmulovich & Shmonov, 1975). For mixtures of H2O and CO2, different functions of state have been proposed (Holloway, 1976; Flowers, 1979; Bowers & Helgeson, 1983; Kerrick & Jacobs 1982). Functions of state for either H2O or CO2 are characterized in most cases by high precision. Functions valid for mixed H2O/CO2 fluids sacrifice some of their precision for the sake of flexibility.
To preserve consistency in all calculations, the fugacities of pure H2O and CO2 and activities of H2O-CO2 mixtures were calculated using the equation of state by Kerrick & Jacobs (1981). This function of state reproduces the available experimental volume data reasonably well in the temperature and pressure range of 350-1000°C and 50-1000 MPa (Ferry & Baumgartner, 1987). Above 1000 MPa, the function of state by Belonoshko & Saxena (1992) was used. For pure H2O and CO2 fluids at temperatures between 200-350°C, the function of state for CO2 by Shmulovich & Shmonov (1975) and the function of state for H2O by Halbach & Chatterjee (1982) were used.