Incorporation of H2 in vitreous silica, qualitative and quantitative determination from Raman and infrared spectroscopy

Abstract

Incorporation mechanisms of H 2 in silica glass were studied with Raman and infrared (IR) microspectroscopy. Hydrogenated samples were prepared at temperatures between 800°C and 955°C at 2 kbar total pressure. Hydrogen fugacities (f H2 ) were controlled using the double capsule technique with the iron-wüstite (IW) buffer assemblage generating f H2 of 1290-1370 bars corresponding to H 2 partial pressures (P H2 ) of 960-975 bars. We found that silica glass hydrogenated under such conditions contains molecular hydrogen (H 2 ) in addition to SiH and SiOH groups. H 2 molecules dissolved in the quenched glasses introduce a band at 4136 cmin the Raman spectra which in comparison to that of gaseous H 2 is wider and is shifted to lower frequency. IR spectra of hydrogenated samples contain a band at 4138 cm −1 which we assign to the stretching vibration of H 2 molecules located in non-centrosymmetric sites. The Raman and IR spectra indicate that the dissolved H 2 molecules interact with the silicate network. We suggest that the H 2 band is the envelope of at least three components due to the occupation of at least three different interstitial sites by H 2 molecules. Both, Raman and IR spectra of hydrogenated glasses contain bands at 2255 cm −1 which may be due to the vibration of SiH groups. Under the assumption that the reaction Si-O-Si + H 2 → Si-H + Si-O-H describes adequately the 'chemical dissolution' of H 2 molecules, the SiH concentrations in our samples were determined and the molar extinction coefficient for the SiH absorption band in the infrared ( 2255 (SiH)) could then be estimated to be 45 ± 3 l/mol cm. The solubility of molecular H 2 in our hydrogenated samples was determined using the IR absorption band at 4138 cm −1 and the extinction coefficient given by Shelby [J. Non-Cryst. Solids 179 (1994) 138]. Samples quenched with different cooling rates gave nearly identical Raman and IR spectra, suggesting that the chemical dissolution of hydrogen (SiH and SiOH) can be quenched to room temperature without changing relative concentrations and that no exsolution of hydrogen occurred during the quench.

Molecular hydrogen in mantle minerals

“…This confirms that H 2 is actually physically dissolved in the crystal lattice. It is known from previous studies on glasses that the interaction with the surrounding silicate matrix can produce an induced dipole in the H 2 molecule, which makes it slightly infrared active (Shelby, 1994;Schmidt and Holtz, 1998;Hirschmann et al, 2012). The H 2 frequency observed in this study is lower than in silicate glasses by 30-70 cm -1 (4105-4140 cm -1…”

“…), which in turn is ~50 cm -1 lower than in vapour (Shelby, 1994;Schmidt and Holtz, 1998;Hirschmann et al, 2012). The peak of H 2 in olivine is not obvious at 2.5 GPa and 1250 °C, unlike for opx, cpx and garnet, but is markedly enhanced at 7 GPa and 1300 °C (Fig.…”

Speciation and solubility of reduced C–O–H–N volatiles in mafic melt: Implications for volcanism, atmospheric evolution, and deep volatile cycles in the terrestrial planets

“…However, as conditions become more reduced, both the identities and the quantitative solubilities of volatile magmatic species are less well-established. Hydrogen-bearing species OH -and H 2 O persist to modestly reduced conditions, though concentrations diminish and molecular H 2 becomes increasingly important at highly reduced conditions (Schmidt, 1998;Hirschmann et al, 2012). For carbon-bearing species, the solubility of CO 3 2-diminishes proportionally to f O2 below the C-CO-CO 2 buffer Pawley et al, 1992;Hirschmann and Withers, 2008;Stanley et al, 2014), and CO 3 2-is replaced by one or more reduced entities.…”

Determination of diffusion coefficients of hydrogen in fused silica between 296 and 523K by Raman spectroscopy and application of fused silica capillaries in studying redox reactions

“…Previous experimental data for hydrogen permeation and/or diffusion in various types of silica were collected mostly by heating silica plates in hydrogen at temperatures above 1073 K, and the glasses containing SiOH or SiH were then heat-treated in air to derive the diffusivity (Williams and Ferguson, 1924;Stone et al, 1985;Shelby, 1994;Schmidt et al, 1998;Lou et al, 2003) assuming the hydroxyl or hydride pair formation mechanism (Barrer, 1941;Lee et al, 1962;Kats et al, 1962;Lee, 1963;Lou et al, 2003). Some studies determined molecular deuterium diffusivities in silica (e.g., Lee et al, 1962;Lee, 1963;Shelby, 1977), which were lower than molecular hydrogen diffusivities by about 20% (e.g., Lee, 1963).…”