Thermodynamics of plagioclase III: Spontaneous strain at the I\overline 1 - P\overline 1 phase transition in Ca-rich plagioclase

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 (? _s) couples quadratically to the order parameter (Q ⁰) with ? _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.