An ab initio study of hydrogen in forsterite and a possible mechanism for hydrolytic weakening

Even small amounts of water can profoundly effect the physical properties of minerals. In olivine < 1 H in every 1000 unit cells acts to increase creep rates of dunite by similar to 2 orders of magnitude, Although the mechanism for this is not known, it is not unreasonable to suggest that it is in some way related to an increase in the point defect population, In order to understand this better we have performed at initio pseudopotential calculations within the generalized gradient approximation on protonic defects in Mg2SiO4 forsterite. Three mechanisms for incorporating protons are considered: (1) interstitial, (2) binding at cation vacancies, and (3) binding at silicon vacancies. Assuming the existence of both Si and Mg vacancies, on energetic considerations, protons will initially populate Si vacancies until there are three protons in the vacancy. At this point, the addition of one more proton (to make a hydrogarnet substitution) is energetically unfavourable in comparison to populating the Mg vacancy, and the next proton will enter the Mg site. Interstitial protons will be very rare. Since it is > 2 eV more favourable to put the first; proton into the Si vacancy than the magnesium site, the presence of water will certainly act to increase the population of silicon vacancies, In fact, in the presence of water the energy required to form a Si vacancy is perhaps less than that to form an Mg vacancy, This is in stark contrast to dry olivine where Si vacancies are many eV less favourable. If creep is rate limited by the diffusion of the slowest species, silicon in olivine, then increasing the Si vacancy concentration could provide a mechanism for hydrolytic weakening.