Lattice simulation studies of the ferroelastic phase transitions in (Na,K)AlSi₃O₈ and (Sr,Ca)Al₂Si₂O₈ feldspar solid solutions

Lattice-energy minimization calculations have been performed on the feldspar systems (Ca,Sr)Al₂Si₂O₈ and disordered (Na,K)AlSi₃O₈ as functions of composition to simulate the ferroelastic phase transitions I2/c-I1 and C2/m-C1, respectively. In both cases the phase transition occurs as a function of composition and is driven by the vanishing of the quantity C₄₄C₆₆ - C²₄₆, without any of the individual elastic constants being strongly dependent on composition and without softening of an optic mode. In both cases, the strains ε₄ and ε₆ are proportional to each other for small values of strain, but nonlinear coupling becomes dominant when |ε₄| becomes larger than about 0.02. The results are consistent with experimental data and explain the nature of coupling of the displacive transition to Al-Si ordering in Al:Si 2:2 feldspars.