On the basis of the P-T partition function, Landau theory of phase transitions, and thermodynamics of solid solutions, an equation of state for minerals was derived. It is based on the standard principle of the minimization of the Gibbs thermodynamic potential (G) of a mineral in an equilibrium state. For a substance with a λ-transition, the Gibbs free energy is defined as a function of temperature (T), pressure (P), and ordering parameter (Xa). The equilibrium mole fraction of ordered particles (Xa) is determined from the condition G = f(P, T, Xa) = min. Application to the system coesite-stishovite-α-quartz-β-quartz demonstrated that the equation allows the representation of experimental data within errors and shows good extrapolation characteristics. The equation is efficient for designing internally consistent thermodynamic databases.
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|Published - Nov 1998