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An amount of disorder among tetrahedral silicon and aluminum in sillimanite too small to measure readily by means of x-rays may have significant effects on phase relations of the Al2SiO5 minerals. The effect is nearly as great as that due to solid solution of minor constituents and cannot be ignored.

A statistical thermodynamic treatment permits evaluation of the Gibbs' free energy of sillimanite as a function of disorder at any temperature on the assumption of a most stable degree of order at a temperature. Reasonable permutations of the assumed order and its associated temperature indicate that the enthalpy change associated with complete disorder is approximately 4 kilocalories and that sillimanite may become completely disordered at temperatures of 1,700°C or greater. The processes of ordering and disordering are likely to occur by homogeneous diffusion rather than by nucleation of domains.

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