AbstractLattice-energy minimization calculations have been performed on the feldspar systems (Ca,Sr)Al2Si2O8and disordered (Na,K)AlSi3O8as functions of composition to simulate the ferroelastic phase transitions I2/c-I1̄ and C2/m-C1̄, 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 Ɛ4and Ɛ6are proportional to each other for small values of strain, but nonlinear coupling becomes dominant when |Ɛ4| 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.