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.