Joshua Martin Richard Muir , Richard Skelton, Andrew Walker

Water has a very strong effect on both the strength and fabric development of forsterite but the mechanism of this effect is unclear. In the paper we use Density Functional Theory Peierls-Nabarro modelling to examine the effect of water on the Peierls stress of different forsterite slip systems. We find that water in Mg vacancies will weaken [100](010) slip and thus produce A fabrics while water in Si vacancies weakens [001](100) slip and thus produces C fabrics. With a combination of DFT and forcefield based methods we find that while in the bulk hydrated vacancies typically occur as M1>M2>Si, near the [100] and [001] screw dislocation cores they occur as Si>M2>M1. Thus by simple modification of the Peierls stress and by segregation of Si vacancies to dislocations, water enhances C fabrics in forsterite. This production is enhanced by pressure and should be independent of water concentration above ~90 ppm H/Si in perfect forsterite. The shape of dislocations is not significantly modified by water and thus the stress exponent of dislocation glide should be unaffected. Water additionally reduces the activation volume of the Peierls mechanism by ~5 times by changing the pressure dependence of slip. E fabrics cannot be produced by a dislocation creep mechanism due to the consistently high Peierls stress of the [100](001) slip system and thus require another deformation method to form.