Thermodynamics of plagioclase II: Temperature evolution of the spontaneous strain at the I\bar 1 \leftrightarrow P\bar 1 phase transition in anorthite |
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Authors: | Simon A T Redfern Ekhard Salje |
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Institution: | 1. Department of Earth Sciences, University of Cambridge, Downing Street, CB2 3EQ, Cambridge, England
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Abstract: | The temperature dependence of the lattice parameters of pure anorthite with high Al/Si order reveals the predicted tricritical behaviour of the \(I\bar 1 \leftrightarrow P\bar 1\) phase transition at T c * =510 K. The spontaneous strain couples to the order parameter Q° as x i∝S xQ i Q°2 with S xQ 1 =4.166×10?3, S xQ 2 =0.771×10?3, S xQ 3 =?7.223×10?3 for the diagonal elements. The temperature dependence of Q° is $$Q^{\text{o}} = \left( {1 - \frac{T}{{510}}} \right)^\beta ,{\text{ }}\beta = \tfrac{{\text{1}}}{{\text{4}}}$$ A strong dependence of T c * , S xQ i and β is predicted for Al/Si disordered anorthite. |
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