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 o f minerals. In olivine < 1 H in every 1000 unit cells acts to increase cree p rates of dunite by similar to 2 orders of magnitude, Although the mechani sm for this is not known, it is not unreasonable to suggest that it is in s ome way related to an increase in the point defect population, In order to understand this better we have performed at initio pseudopotential calculat ions within the generalized gradient approximation on protonic defects in M g2SiO4 forsterite. Three mechanisms for incorporating protons are considere d: (1) interstitial, (2) binding at cation vacancies, and (3) binding at si licon vacancies. Assuming the existence of both Si and Mg vacancies, on ene rgetic considerations, protons will initially populate Si vacancies until t here are three protons in the vacancy. At this point, the addition of one m ore proton (to make a hydrogarnet substitution) is energetically unfavourab le in comparison to populating the Mg vacancy, and the next proton will ent er 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 magne sium site, the presence of water will certainly act to increase the populat ion of silicon vacancies, In fact, in the presence of water the energy requ ired 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 les s favourable. If creep is rate limited by the diffusion of the slowest spec ies, silicon in olivine, then increasing the Si vacancy concentration could provide a mechanism for hydrolytic weakening.