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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| Institution: | (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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