Effect of chemical environment on the hydrogen-related defect chemistry in wadsleyite

Abstract

The effect of chemical environment on the hydrogen-related defect chemistry in wadsleyite was investigated using Fourier-transform infrared (FTIR) spectroscopy. Samples were annealed at P = 14-16 GPa and T = 1230-1973 K using Kawai-type multi-anvil apparatus. The effect of oxygen fugacity (fO₂) was investigated using three metal-oxide buffers (Mo-MoO2, Ni-NiO, and Re-ReO2). The effect of water fugacity (fH₂₂O) was studied using two different capsule assemblies (“nominally dry” and “dry” assemblies). A range of total OH concentration (COH,Total) of studied wadslyeites varies between <50 H/106Si (<3 wt ppm H2O) and 23 000 H/106Si (1400 wt ppm H2O). The observed FTIR spectra were classified into four different classes, i.e., peaks at 3620 (“3620”), 3480 (“3480”), and 3205 cm-1 (“3205”) and the others (Group O), where the Group O includes peaks at 3270, 3330, and 3580 cm−1. The variation in OH concentration corresponding to each peak was analyzed separately. The OH concentrations correspond to “3620,” “3480,” and “3205” were found to be highly dependent on both fH₂₂O and fO₂₂. Assuming COH,Group O = 2[(2H)xM] (COH,Group O is OH concentration of Group O), present data were analyzed by using thermodynamic model for concentration of hydrogen-related defects. Based on analytical results, OH concentration of “3620” and “3480” was found to be reasonably explained by q = 1/2 and r = 1/12 (q and r are fH₂O and fO₂ exponents, respectively), whereas that of “3205” was consistent with q = 1/2 and r = -1/12. These results suggest that “3620” and “3480” correspond to HM′ whereas “3205” corresponds to H·, respectively, under the charge neutrality condition of [FeM′] = 2[VM″ ].