Journal
AMERICAN MINERALOGIST
Volume 101, Issue 7-8, Pages 1571-1583Publisher
MINERALOGICAL SOC AMER
DOI: 10.2138/am-2016-55681571
Keywords
Diffusion; point defects; nominally anhydrous minerals; FTIR spectroscopy
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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 degrees C), very low oxygen fugacity (similar to QFM-5) at atmospheric pressure. The Ti-doped forsterite was then diffusively hydroxylated in a piston-cylinder apparatus at much lower temperatures (650-1000 degrees 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 f(o2), does not lead to redox re-equilibration of the high Ti3+/Sigma Ti 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(Sigma H)], between 650-1000 degrees C, is log D(Sigma H) = log D-0(Sigma H) + [-223(+/- 8)kJ/mol/2.3RT] where the values of logD(0)(Sigma H) 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.
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