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

摘要

Coupled substitution of Fe3+ 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% H2O and Fe/(Mg+Fe) of 0.099(2), respectively. Mossbauer spectra showed 13(4)% Fe3+/Sigma Fe and Fe3+ at the tetrahedral site with 5(3)% Fe-IV(3+)/Sigma Fe. Crystal structure refinement by single-crystal X-ray diffraction indicated that Fe (presumably Fe3+) 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 Fe3+ at the tetrahedral site confirms the coupled substitution of Fe3+ and H+ for Si in Fe-bearing hydrous wadsleyite.