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  • Print publication year: 1991
  • Online publication date: November 2009

2 - Thermodynamic systems and factors of petrogenesis

Summary

The Gibbs method of thermodynamic potentials (1931) has been used and extended by Korzhinskii (1959, 1969, 1976) to endogenic mineral formation. Petrogenic systems may be distinguished by different types of thermodynamic potentials – the characteristic functions of state whose minimum values are the condition for minerals to attain equilibrium. The following thermodynamic potentials of isochemical equilibrium are recognized: the Gibbs G(T, P) and Helmholtz F(T, V) free energies, the enthalpy or thermal function H(S, P) and the internal energy U(S, V). The respective functions of allochemical equilibrium – the Korzhinskii thermodynamic potentials Gz(T, P, μm, …, μf), Fz(T, V, μm, …, μf), Hz(S, P, μm, …, μf) and Uz(S, V, μm, …, μf) – characterize the reversible gain–loss of certain perfectly mobile components having constant chemical potentials.

Systems with perfectly mobile components thermodynamically can be called isopotential systems (μm, …, μf are constant). They can gain or lose perfectly mobile components (m … f) in their equilibrium state (dGz = O, dFz = O), just the same as isothermal systems can gain or lose heat at constant temperature without disturbance of equilibrium (dG = O). Chemical potentials of perfectly mobile components (μm,…,μf) are factors of mineral equilibria in corresponding systems in just the same way as temperature (‘potential of heat’) is a factor of mineral equilibrium in isothermal systems. Potentials Gz and Fz have been derived in order to describe a thermodynamic system of allochemical equilibria, in which the potentials of certain components are controlled by external conditions. conditions. This control is achieved by the infiltration through the systems of flows of volatile and perfectly mobile components (H2O, CO2, K2O, Na2O, HCl etc.).