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International audienceCrystals of ferrous and ferric iron-bearing hydrous wadsleyite have been synthesized at 1400 ºC and 12–13.5 GPa in a multi-anvil press. Crystal structures (atom positions, occupancies, and cell parameters) have been refined by single-crystal X-ray diffraction at ambient conditions. Assuming cation vacancies to be in the M3 site only, their concentration has been estimated from the unit-cell parameter b/a ratio. Total refined site Fe contents are consistent with microprobe chemical analysis. There appears to be up to 11% iron (presumably ferric) in the tetrahedral site, consistent with reduced silica content (<1 Si per 4 O atoms) in the chemical analysis. Also the volume of the tetrahedron increases with increasing ferric iron content. Strong ordering of Fe in the octahedral sites is apparent in the order FeM3 ≥ FeM1 >> FeM2. The presence of ferric iron in the mantle transition zone is expected to partition preferentially into wadsleyite and may expand the stability region of wadsleyite relative to olivine and ringwoodite. Also, the observation of tetrahedral ferric iron in these samples increases the likelihood that there is compositional continuity between wadsleyite and the spinelloid III phase field observed in the Mg-free system fayalite-magnetite

The humite-group minerals on the brucite-olivine join may be important dense hydrous magnesium silicate (DHMS) phases in the subducting slab. Fluorine and titanium can be incorporated into the crystal structures through the substitution mechanisms (OH)-= Fand Mg 2+ + 2(OH)-= Ti 4+ + 2O 2-. These substitutions have significant effects on the hydrogen bonding behavior in their crystal structures. Crystal structure refinements and in situ high-temperature Raman and Fourier transform infrared (FTIR) measurements were conducted on natural humite and norbergite crystals. Both minerals crystallize in space group Pbnm, and the isobaric Grüneisen parameters for the lattice and SiO 4 internal vibrations are compared with those of chondrodite, clinohumite, brucite and forsterite. For the humite-group minerals, the OH-stretching modes above 3450 cm-1 are affected by local H-H repulsion, whereas the behavior of those below 3450 cm-1 can be explained by Fand Ti 4+ substitutions, either of which may relieve the H-H repulsion effect. The Raman-active OH bands below 3450 cm-1 are affected by Ti 4+ substitution, while the IR-active bands can be affected by either For Ti 4+ substitutions. Based on an analysis of the high-T Raman and FTIR spectra, the OH vibrations above and below 3450 cm-1 behave differently as a function of temperature, and similar behavior has also been observed for other dense hydrous silicate phases in the hydrous peridotite system. Hence, the lengths of the oxygen-oxygen edges in MgO 6 octahedra where protonation can occur become similar to each other at elevated temperatures. This may provide an This is the peer-reviewed, final accepted version for American Mineralogist, published by the Mineralogical Society of America. The published version is subject to change. Cite as Authors (Year) Title. American Mineralogist, in press.