STATIC LATTICE SIMULATION OF FELDSPAR SOLID-SOLUTIONS - FERROELASTIC INSTABILITIES AND ORDER DISORDER/

Lattice energy minimisation calculations with empirical shell model po tentials 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 KxNa1-xAlSi3O8 and Sr (x)Ca(1-x)A1(2)Si(2)O(8) solid solutions. In both cases it is found th at the transitions are driven by an elastic softening without critical softening of an optic phonon. The results for the ferroelastic sponta neous strains are consistent with experimental observation, and explai n the nature of the coupling between the ferroelastic instability and Al/Si ordering.