THE PHENOMENON OF LOW AL-SI ORDERING TEMPERATURES IN ALUMINOSILICATE FRAMEWORK STRUCTURES

The enthalpy of ordering of Al and Si between tetrahedral sites in fra mework structures is now known from several studies to be in the range 0.3-1.4 eV per two Al-O-Si linkages, depending on the structure and t ype of linkage between tetrahedra. On the basis of this value a simple Bragg-Williams model predicts an ordering phase transition at a tempe rature, T-c, on the order of a few thousand degrees after adjusting th e estimate to account for fluctuations in short-range order. However, there are several materials with T-c below 1800 degrees C (a typical m elting point), with some having T-c so low that it cannot be observed because of the slowness of the ordering kinetics, e.g., gehlenite and leucite. We discuss two mechanisms that can reduce T-c substantially: low effective dimensionality and low Al concentration. The cases of si llimanite, gehlenite, cordierite, and leucite, and some simple demonst ration systems, are treated quantitatively with the aid of Monte Carlo computer simulations. Other materials (kaisilite, anorthite, albite, and nepheline) are discussed qualitatively in terms of the same princi ples.