Computer simulations of the Al-Si disorder in synthetic cordierites: Configuraltional entropy constrained by 29Si NMR data |
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| Authors: | Victor L Vinograd |
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| Institution: | 1. Theoretical Geochemistry Program, Institute of Earth Sciences, Uppsala University, S-75236, Uppsala, Sweden
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| Abstract: | The published 29Si NMR data on synthetic Mg-cordierites have been used to estimate the changes in configurational Al-Si entropy of the samples due to metastable disorder. The results show that with the increase of the time of annealing in the range of 2 min-2000 h at 1185 °C the entropy of disorder in cordierite decreases from 17.1 to 6.4 J/mol K, while at 1400 °C in the range of 2 min–88 h the entropy changes from 15.4 to 8.8 J/mol K. The decrease in entropy is followed by the appearance and increase of long-range ordering which is reflected in changes of T1 and T2 site occupancies, decrease in the number of Si-O-Si and Al-O-Al groupings around O1 oxygens and in a decrease in the number of Al-O-Si-O-Al contacts among hexagonal 6T2-rings. The derived values of entropy effects together with published calorimetric data suggest that the enthalpy of metastable disordering strongly depends on the temperature of annealing. |
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