Hydrogen diffusion in Ti-doped forsterite and the preservation of metastable point defects

Michael C. Jollands, Jose Alberto Padron-Navarta, Jorg Hermann, Hugh St C. O'Neill

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    The effect of trace concentrations of Ti on the rate and mechanism of hydrogen diffusion in pure forsterite was investigated experimentally. Forsterite doped with 350-400 ppm Ti (predominantly octahedral Ti3+, minor tetrahedral Ti4+) was prepared by diffusing Ti into pure synthetic forsterite at high temperature (1500 °C), very low oxygen fugacity (~QFM-5) at atmospheric pressure. The Ti-doped forsterite was then diffusively hydroxylated in a piston-cylinder apparatus at much lower temperatures (650-1000 °C) and higher oxygen fugacities, at 1.5-2.5 GPa, with chemical activities buffered by forsterite-enstatite or forsterite-periclase and partial pressure of H2O equal to total pressure. This produced hydrogen concentration-distance profiles of several hundred micrometers in length. Diffusion of hydrogen through the Ti-doped forsterite, even at very high fO2, does not lead to redox re-equilibration of the high Ti3+/STi ratio set during the synthesis of the starting material at extremely reducing conditions-the metastable point defects are partially preserved. Three main hydroxylated point defects are observed; hydroxyl is associated with Ti4+ (titano-clinohumite point defects), Ti3+ (and possibly other trivalent cations), and M-site vacancies. Concentration-distance profiles represent an interplay between diffusion and reaction (i.e., site rearrangement) to form the observed point defects. In all experiments, the concentration-distance profiles of the hydroxylated Ti defects coincide with the concentration-distance profiles of the M-site vacancy substitution, with the same crystallographic anisotropy. This suggests that the macroscopic movement of hydrogen through the crystal is due to one diffusion mechanism (the diffusion of hydroxylated M-site vacancies). The net H diffusion coefficient [logD(ΣH)], between 650-1000 °C, is logD(ΣH)=logD0(ΣH)+(-223(±8)kJ/mol/2.3RT) where the values of logD0(ΣH) parallel to [100] and [001] directions are-3.0 ± 0.4 and-2.2 ± 0.4, respectively; diffusion is therefore around one order of magnitude faster along the c axis than along the a axis. The diffusion of hydrogen is slightly faster in Ti-doped forsterite than in pure forsterite. There is no effect of chemical activity or oxygen fugacity on the rate of diffusion. Hydrogen diffusion profiles represent a complex interplay between the movement of H through the crystal lattice and point-defect reactions to maintain charge balance.

    Original languageEnglish
    Pages (from-to)1571-1583
    Number of pages13
    JournalAmerican Mineralogist
    Issue number7
    Publication statusPublished - 1 Jul 2016


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