Equation of state of minerals for thermodynamic databases used in petrology

On the basis of the P-T partition function, Landau theory of phase transiti ons, and thermodynamics of solid solutions, an equation of state for minera ls was derived. It is based on the standard principle of the minimization o f the Gibbs thermodynamic potential (G) of a mineral in an equilibrium stat e. For a substance with a lambda-transition, the Gibbs free energy is defin ed as a function of temperature (T), pressure (P), and ordering parameter ( X-a). The equilibrium mole fraction of ordered particles (X-a) is determine d from the condition G = f(P, T, X-a)= min. Application to the system coesi te-stishovite-alpha-quartz-beta-quartz demonstrated that the equation allow s the representation of experimental data within errors and shows good extr apolation characteristics. The equation is efficient for designing internal ly consistent thermodynamic databases.