Phase transitions in minerals are of interest to a wide spectrum of scientists - geolo gists, mineralogists, solid state chemists, and physicists. We have now reached the point where mean field theory or Landau Theory of phase transitions as a function of temperature, pressure, or chemical composition can be usefully applied to natural materials, resulting in an improved understanding of the thermodynamics of signifi cant constituents of the earth. Given the chemical complexity of so many silicate solid solutions, there are two distinct approaches to the problems posed by common minerals: one...