Planar defects and dissociation of dislocations in a K feldspar

Abstract

We investigate some slip systems active in sanidine, the high-temperature form of K feldspar (KAlSi₃O₈) by atomistic computer modelling of possible dislocation and planar defect structures. As a first approximation, we assume that the disordered Al-SiO₂ network behaves as a purely covalent SiO₂ network, allowing the use of a Keating-like potential previously fitted to quartz. Our conclusions are that the dissociated screw dislocation in the (010)[001] slip system, with a stacking fault located within the double crankshaft of the Al-SiO₄ tetrahedra, is the most energetically favoured dislocation. Also the high energy of the stacking fault on (010) with the displacement vector a/2 makes dissociation difficult and hence makes the appearance of the slip system (010)[100] rare. Both of these findings are in good agreement with observations by electron microscopy.