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/cI1 and C2/mC1, respectively. In both cases the phase transition occurs as a function of composition and is driven by the vanishing of the quantity C44C66C462, 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.

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