THERMODYNAMICS OF AL AL AVOIDANCE IN THE ORDERING OF AL/SI TETRAHEDRAL FRAMEWORK STRUCTURES/

The main driving force behind Al/Si ordering in tetrahedral framework aluminosilicates is nearest-neighbour Al/Al avoidance. Computer simula tion is used to explore the direct consequences of such Al/Al avoidanc e. The main result is that the order-disorder transition temperature T -c falls dramatically as the concentration x of Al in the structure is reduced, and if the only interactions are those associated with neare st-neighbour Al/Al avoidance, T-c becomes zero for x less than some cr itical value x(c), where x(c) = 0.31 for the feldspar framework and x( c) = 0.34 for cordierite. Also a large degree of short range order is found above T-c. Both results differ radically from the standard Bragg -Williams model. Plots of entropy and enthalpy of ordering are given a s functions of x and T, which may be used to interpret experimental da ta or for extrapolation into ranges of x and T inaccessible to experim ent.