A study was made of the formation and migration of point defects within the

magnesium sub-lattice in forsterite using a combination of empirical and quantum

mechanical modelling methods. Empirical models based on a parameterised force

field coupled to a high throughput grid computing infrastructure permitted rapid

evaluation of a very large number of possible defect configurations. An embedded

cluster approach revealed more accurate estimates of defect energetics for the most

important defect configurations. Considering all of the defects in their minimumenergy

equilibrium positions, it was found that the lowest-energy intrinsic defect

was the magnesium Frenkel type, where a magnesium atom moved from the M1

site to form a split interstitial defect. This defect had 2 four-coordinated

magnesium atoms located outside opposite triangular faces of an otherwise vacant

M1 octahedron. The split interstitial defect was more stable than normal

interstitials, where magnesium was located in either of the two structurally vacant

octahedral sites in the hexagonally close-packed oxygen lattice. The M1 vacancies

were also found to form when iron(II) oxidised to iron(III). The energy of the

defects away from the equilibrium positions permitted the energy barrier to

diffusion to be calculated. The migration of both magnesium vacancies and

interstitials was considered and it was found that vacancies were more mobile. When the energy contribution arising from defect formation was included, the

activation energies for vacancy diffusion then agreed with experiment.

A Computational Study of Magnesium Point Defects and Diffusion in Forsterite.

A.M.Walker, S.M.Woodley, B.Slater, K.Wright: Physics of the Earth and Planetary

Interiors, 2009, 172[1-2], 20-7