Abstract
The lattice parameters of anorthites An98Ab2 and An100 have been measured from 22 to 1100 K. The spontaneous strain arising from the \(I\overline 1 - P\overline 1\) displacive transition in An98 follows second order Landau behaviour. The spontaneous strain (ɛ s) couples quadratically to the order parameter (Q 0) with ɛ s∝Q 02∝(T *c −T) and T *c =530 K in An98. This is in contrast to the tricritical behaviour observed in pure anorthite. These observations are consistent with a Landau model for the free energy of Ca-rich plagioclases in which Al/Si order and Na content renormalize the fourth order coefficient.
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Redfern, S.A.T., Graeme-Barber, A. & Salje, E. Thermodynamics of plagioclase III: Spontaneous strain at the \(I\overline 1 - P\overline 1\) phase transition in Ca-rich plagioclase. Phys Chem Minerals 16, 157–163 (1988). https://doi.org/10.1007/BF00203199
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DOI: https://doi.org/10.1007/BF00203199