Displacive phase transition in anorthoclase; the “plateau effect” and the effect of T1-T2 ordering on the transition temperature

The effects of compositional changes and nonsymmetry-breaking Al-Si ordering on the displacive phase transition in anorthoclase have been studied using X-ray powder diffraction. The results were analyzed using a model based on Landau theory.

The observed transition temperature, $Tc*$⁠, was found to be independent of orthoclase content in the range 0 < X_Or < 0.02. For samples with X_Qr > 0.02, $Tc*$ decreases linearly with increasing X_Or. This plateau behavior is explained by the finite volume of the strain field around each K⁺ ion: Chemical mixing behavior is observed only for average K⁺-K⁺ distances of <20 Å.

Increasing Al-Si order between the T1 and T2 sites causes $Tc*$ to increase; this is unlike Tlo-Tlm ordering, which prevents the transition to monoclinic symmetry altogether. The range of $Tc*$ values reported in the literature is shown to be consistent with different degrees of T1-T2 order.