Computational study of defects in ideal phengite

Abstract

Phengite is the name of a series designating the “potassic dioctahedral micas between, or close to, the joins muscovite-aluminoceladonite and muscovite-celadonite” (Rieder et al. 1998). These micas play a important role in most petrogenetic reactions occurring in high-pressure (HP) metamorphic environments; they are useful geothermobarometers and participate in reactions as H2O carriers in the subduction zone. In this work we have employed atomistic simulations techniques to model defects, evaluate the most stable defect species and determine the most likely diffusion mechanism in crystals. We have calculated the defect formation energies for vacancies, impurities and interstitials, Frenkel-type and Schottky-type for the whole set of atoms in a supercell 2 x 2 x 1 of the 2M1 politype of the ideal phengite [K(Mg0.5Al1.5)(Al0.5Si3.5)O10(OH)2]. In order to dilute defect concentrations we have used the embedded cluster method using the formulation of Mott and Littleton (Mott and Littleton, 1938) as implemented within General Utility Lattice Program (GULP), (Gale and Rohl, 2003) incorporating with semi-classical simulations methods based on empirical interatomic potentials.