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3.2 d. Hydrous ringwoodite (J.R. Smyth, D.J. Frost and C.A. McCammon)

Ringwoodite is the high-pressure spinel phase of olivine and is thought to be a major phase in the lower portion of the transition zone at depths of 525 to 670 km. Unlike olivine, wadsleyite (which is the stable form between 410 and 525 km depth) and ringwoodite can incorporate relatively large amounts of hydrogen. Whereas the incorporation of hydrogen into wadsleyite is fairly well understood and can be explained by an underbonded oxygen in this silicate structure, the high solubility of hydrogen in ringwoodite had been enigmatic. Therefore, crystals of hydrous ringwoodite (up to about 500 µm in size) have been synthesized at 1400°C and 20 GPa in the 5000 tonne multianvil press. The crystals contain about 2.2 weight percent H2O and have a composition of Mg1.63Fe0.22H0.4Si0.95O4, based on electron microprobe analysis. Mössbauer spectra indicate that approximately 10% of the iron present is in the ferric state. The crystals are optically isotropic and are dark blue in color. The crystal structure has been refined in space group Fd3m from 419 measured X-ray diffraction intensities of which 88 are unique and have intensities greater than 2. Crystal structure refinement indicates significant vacancies in both octahedral and tetrahedral sites. In addition, Fourier difference analysis indicates about ten percent occupancy of a normally unoccupied tetrahedral void at fractional coordinates (1/8, 1/8, 5/8). The oxygen position parameter is 0.2432(2); however the atom appears to be split with significant electron density displaced from the main position towards the partly occupied tetrahedral void.

Partial occupancy of normally vacant tetrahedral voids has been observed in both hydrous wadsleyite and hydrous wadsleyite II. Occupancy of adjacent tetrahedral voids in the spinel structure would create an Si2O7 group with a bridging oxygen, which would require a non-silicate oxygen elsewhere. A possible explanation is that ringwoodite may hydrate by creation of a wadsleyite-like defect with Si2O7 groups and non-silicate oxygens that would act as sites of protonation.

Identifying the maximum water solubility and the hydration mechanism of ringwoodite has important implications for H2O storage in the Earth's mantle. Ringwoodite and wadsleyite could act as reservoir minerals for H2O in the transition zone and in subducting slabs they may aid the transport of H2O into the deep mantle. In addition, the presence of H2O in the transition zone may affect physical properties such as electrical conductivity, elasticity and rheology.

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