Static lattice simulation of feldspar solid solutions: Ferroelastic instabilities and order/disorder

Lattice energy minimisation calculations with empirical shell model potentials have been performed on feldspar solid solutions as a function of composition and tetrahedral Al/Si order. The monoclinic-triclinic phase transitions have been simulated across the K_xNa_1-xAlSi₃O₈ and Sr_xCa_1-xAl₂Si₂O₈ solid solutions. In both cases it is found that the transitions are driven by an elastic softening without critical softening of an optic phonon. The results for the ferroelastic spontaneous strains are consistent with experimental observation, and explain the nature of the coupling between the ferroelastic instability and Al/Si ordering.