Coupled substitution of Fe3+and H+for Si in wadsleyite: A study by polarized infrared and Mössbauer spectroscopies and single-crystal X-ray diffraction

Abstract

Coupled substitution of Fe 3+ and H + for Si in wadsleyite was studied by polarized infrared and Mossbauer spectroscopies and single-crystal X-ray diffraction. Single crystals of Fe-bearing hydrous wadsleyite were synthesized at 16 GPa and 1870 K using a Kawai-type multi-anvil apparatus. Water and Fe contents of the sample were 0.19–0.26 wt% H 2 O and Fe/(Mg+Fe) of 0.099(2), respectively. Mossbauer spectra showed 13(4)% Fe 3+ /∑Fe and Fe 3+ at the tetrahedral site with 5(3)% IV Fe 3+ /∑Fe. Crystal structure refinement by single-crystal X-ray diffraction indicated that Fe (presumably Fe 3+ ) occupied 4.9(5)% of the tetrahedral site. Infrared light polarized with the electric vector E // a and c was absorbed at 3477(2) cm −1 while no absorption was observed at the region in spectra with E // b . The pleochroic behavior of the 3477 cm −1 band can be interpreted as protonation of silicate oxygen O3 in Fe-bearing hydrous wadsleyite. The protonation of O3 together with the presence of Fe 3+ at the tetrahedral site confirms the coupled substitution of Fe 3+ and H + for Si in Fe-bearing hydrous wadsleyite.