Thermodynamics of Al/Al avoidance in the ordering of Al/Si tetrahedral framework structures |
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| Authors: | Eva R. Myers Volker Heine Martin T. Dove |
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| Affiliation: | (1) Cavendish Laboratory, University of Cambridge, Madingley Road, Cambridge CB3 0HE, UK. Fax: +44 01223 337356, GB;(2) Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK. Fax: +44 01223 333450, GB |
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| Abstract: | The main driving force behind Al/Si ordering in tetrahedral framework aluminosilicates is nearest-neighbour Al/Al avoidance. Computer simulation is used to explore the direct consequences of such Al/Al avoidance. 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 nearest-neighbour Al/Al avoidance, T c becomes zero for x less than some critical 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 as functions of x and T, which may be used to interpret experimental data or for extrapolation into ranges of x and T inaccessible to experiment. Received: 14 May 1997 / Revised, accepted: 2 June 1997 |
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| Keywords: | Loewenstein’ s rule Al/Al avoidance Al/Si ordering Bragg-Williams |
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