Abstract
The thermodynamic behaviour of Na-rich feld-spars is derived from a Landau-type free energy expression which contains two order parameters. One order parameter describes the structural distortion during the displacive phase transition between monalbite and analbite, the other the Al, Si order. The structural phase transition\(C2/m - C\bar 1\) in sodium feldspar under thermal equilibrium is due to both order parameters and is accompanied by a crossover between a thermal regime with low Al, Si order (high-albite) and a thermal regime with high Al, Si order (low-albite). The crossover is a smooth function of temperature. Simple formulae are given for the calculation of thermodynamic potentials for thermal equilibrium and for metastable phases with temperature independent Al, Si order.
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Salje, E. Thermodynamics of sodium feldspar I: Order parameter treatment and strain induced coupling effects. Phys Chem Minerals 12, 93–98 (1985). https://doi.org/10.1007/BF01046833
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DOI: https://doi.org/10.1007/BF01046833