Diffusion and solubility of hydrogen and water in periclase
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Cylinders of synthetic periclase single crystals were annealed at 0.15–0.5 GPa and 900–1200 °C under water-saturated conditions for 45 min to 72 h. Infrared spectra measured on the quenched products show bands at 3,297 and 3,312 cm−1 indicating V OH − centers (OH-defect stretching vibrations in a half-compensated cation vacancy) in the MgO structure as a result of proton diffusion into the crystal. For completely equilibrated specimens, the OH-defect concentration, expressed as H2O equivalent, was calculated to 3.5 wt ppm H2O at 1,200 °C and 0.5 GPa based on the calibration method of Libowitzky and Rossmann (Am Min 82:1111–1115, 1997). This value was confirmed via Raman spectroscopy, which shows OH-defect-related bands at identical wavenumbers and yields an H2O equivalent concentration of about 9 wt ppm using the quantification scheme of Thomas et al. (Am Min 93:1550–1557, 2008), revised by Mrosko et al. (Am Mineral 96:1748–1759, 2011). Results of both independent methods give an overall OH-defect concentration range of 3.5–9 (+4.5/−2.6) ppm H2O. Proton diffusion follows an Arrhenius law with an activation energy E a = 280 ± 64 kJ mol−1 and the logarithm of the pre-exponential factor logDo (m2 s−1) = −2.4 ± 1.9. IR spectra taken close to the rims of MgO crystals that were exposed to water-saturated conditions at 1,200 °C and 0.5 GPa for 24 h show an additional band at 3,697 cm−1, which is related to brucite precipitates. This may be explained by diffusion of molecular water into the periclase, and its reaction with the host crystal during quenching. Diffusion of molecular water may be described by logDH2O (m2 s−1) = −14.1 ± 0.4 (2σ) at 1,200 °C and 0.5 GPa, which is ~ 2 orders of magnitude slower than proton diffusion at identical P-T conditions.
KeywordsPericlase Brucite OH defect IR spectroscopy Raman spectroscopy Proton diffusion Water diffusion
We thank Monika Koch-Müller for support during FTIR- and Raman-spectroscopic measurements. The support of Anja Schreiber, who produced FIB foils, Richard Wirth, who performed TEM measurements, Nico Küter, Max Wilke in the IHP and Hans-Peter Nabein in the hydrothermal lab is gratefully acknowledged. We thank Jim Van Orman and an anonymous reviewer for detailed reviews who helped to improve the paper. This work was funded by the Deutsche Forschungsgemeinschaft within the framework of FOR741, project HE 2015/9-1 (Wilhelm Heinrich), which is gratefully acknowledged.
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