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Protonation in germanium equivalents of ringwoodite, anhydrous phase B, and superhydrous phase B

¹ GeoForschungsZentrum Potsdam (GFZ), Section 4.1, Telegrafenberg, 14473 Potsdam, Germany
² Institute for Mineralogy/Petrography, University Innsbruck, Innrain 52, A-6020 Innsbruck, Austria

To gain insight into hydroxyl solubilities and possible hydration mechanisms of mantle silicates, as well as to test the utility of germanium analog models in studies of water-related defects, our present work is focused on the protonation of germanium analogs of silicates.

For this purpose Ge-analogs of ringwoodite, anhydrous phase B (anhB), and for the first time, superhydrous phase B (shyB), were synthesized in a piston cylinder device at 2 GPa and 950–1000 °C under water-excess conditions. Electron probe microanalysis (EPMA), transmission electron microscopy (TEM), and X-ray diffraction as well as Raman and infrared (IR) spectroscopy were used to characterize the experimental products.

Ge-ringwoodite incorporates from 900 to 2200 ppm H₂O by weight, which is much less than Smyth et al. (2003) observed for the Si-equivalent synthesized at 22 GPa and 1500 °C, but 200x more than published for -Mg₂GeO₄ by Hertweck and Ingrin (2005). In addition to this discrepancy, the incorporation mechanism of H in Ge-ringwoodite also differs from that of Si-ringwoodite.

Ge-anhB, which is currently believed to be anhydrous in the Si-system, contains from 2400 to 5300 ppm water by weight. A hydration model for germanate anhB was constructed based on single-crystal X-ray diffraction analysis and IR spectroscopy, in which OH is incorporated via the hydrogarnet substitution [V_Ge·4(OH)_O]x and via vacant Mg sites [V_Mg·2(OH)_O]^x.

For Ge-shyB the water concentration and incorporation mechanism obtained in this study are identical to results reported for the silicate phase synthesized at 22 GPa and 1200 °C (Koch-Müller et al. 2005). Thus, germanates are good low-pressure analogs for hydrous mantle silicates in which protonation is controlled by stoichiometry. However, for nominally anhydrous minerals we cannot recommend the use of germanates as high-pressure models in water-related studies. In these Ge-analogs, which are usually synthesized at much lower pressures, i.e., lower water fugacities, OH incorporation seems to differ from the high-pressure silicate equivalents qualitatively and quantitatively, as hydroxyl solubility is governed by other factors such as water fugacity and intrinsic defects.

Key Words: Germanates • ringwoodite • nominally anhydrous minerals • FTIR • anhydrous phase B • superhydrous phase B