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 fe ldspar systems (Ca,Sr)Al2Si2O8 and disordered (Na,K)AlSi3O8 as functio ns of composition to simulate the ferroelastic phase transitions I2/c- I (1) over bar and C2/m-C (1) over bar, respectively. In both cases th e phase transition occurs as a function of composition and is driven b y the vanishing of the quantity C44C66 - C-46(2) without any of the in dividual elastic constants being strongly dependent on composition and without softening of an optic mode. In both cases, the strains epsilo n(4) and epsilon(6) are proportional to each other for small values of strain, but nonlinear coupling becomes dominant when \epsilon(4)\ bec omes larger than about 0.02. The results are consistent with experimen tal data and explain the nature of coupling of the displacive transiti on to Al-Si ordering in Al:Si 2:2 feldspars.