Order disorder in sillimanite

AP³⁺-Si⁴⁺ disordering in sillimanite cannot be considered as ideal mixing on tetrahedral sites. Ideal free energy of mixing in sillimanite corresponding to slight disordering results in changing the kyanite-sillimanite phase equilibrium boundary by impossible magnitude. A regular solution model for the sites yields the following expression for the free energy of mixing: 1 $$G_{sill}^M = W { (X_{Al - T1} )^2 + (X_{Al - T2} )^2 } + RT ln X_{Al - T1} X_{Al - T2}$$ . Calculations using the phase equilibrium data show that even small disequilibrium in intra-crystalline distribution results in significant change in pressure of the kyanite-sillimanite “equilibrium”. If information on interaction energies such as W is not available for disordering silicates, it is not advisable to use the ideal configurational entropy in calculations of phase equilibria.