A Computer Simulation Study of Point Defects in Diopside and the Self-Diffusion of Mg and Ca by a Vacancy Mechanism

Feridoon Azough1, Robert Freer1, Kate Wright2 and Robert Jackson3
1 Materials Science Centre, University of Manchester/UMIST, Grosvenor Street, Manchester M1 7HS, UK
2 Department of Earth Sciences, University of Manchester, Oxford Road, Manchester M13 9PL, UK
3 Department of Chemistry, University of Keele, Keele, Staffordshire, ST5 5BG, UK

Abstract: Computer simulation techniques have been used to investigate defect formation and the diffusion of Ca and Mg in diopside. It was found that isolated, non-interacting CaO and MgO Schottky defects had the lowest formation energies (3.66 and 3.97 eV respectively); oxygen Frenkel defects are the most favourable oxygen defects (formation energies 3.93 eV). Magnesium and calcium self-diffusion in the c-direction of diopside is easiest by a vacancy mechanism involving either direct jumps along the c-direction, or double jumps in the b-c plane. In the extrinsic regime, diffusion activation energies for Mg are predicted to be 9.82 eV (direct route) and 1.97 eV (double jump route); for Ca diffusion, activation energies are predicted to be 6.62 eV (direct route) and 5.63 eV (double jump route). If additional vacancies (oxygen or magnesium) are present in the vicinity of the diffusion path, Ca migration energies fall to 1.97–2.59 eV. At elevated temperatures in the intrinsic regime, diffusion activation energies of ⩾ 5.95 eV are predicted for Mg self-diffusion and 9.29–10.28 eV for Ca self-diffusion. The values for Ca diffusion are comparable with published experimental data. It is inferred that a divacancy mechanism may operate in diopside crystals.

Keywords: computer-simulation • point defects • diopside • vacancy mechanism • self-diffusion

Mineralogical Magazine; October 1998 v. 62; no. 5; p. 599-606; DOI: 10.1180/002646198547990
© 1998, The Mineralogical Society
Mineralogical Society (www.minersoc.org)