10:00 AM - 10:15 AM
[SIT15-05] Water in nominally anhydrous mantle minerals: Are you seeing them all?
★Invited Papers
Keywords:water, nominally anhydrous minerals, mantle, incorporation mechanisms, hydrogen bonding
Water is the most important fluid component that may be locked up in the crystal structures of all nominally anhydrous mantle minerals, and significantly alter various thermodynamic and physical properties (e.g., diffusivity, electrical conductivity, elastic properties and sound velocity), and contribute to the dynamics and evolution of the Earth and other planetary bodies. The effects of water on mineral properties strongly depend on how it is incorporated in the crystal structure, and thus it is indispensable to understand the water incorporation mechanisms and solubility. Extensive studies, mostly using infrared spectroscopy, have been performed thus far to investigate these issues for most mantle minerals, and a general picture of the distribution of the water in the deep Earth has been built (e.g., Peslier et al. 2017 Space Sci Rev; Ohtani 2021 Ann Rev Earth Planet Sci).
However, our comprehensive studies, using combined multi-nuclear NMR and vibrational spectroscopic measurements and first-principles calculations, on a variety of nominally mantle minerals (including enstatite and stishovite), which allowed us to gain unambiguous structural information, revealed that the general understandings about how water is incorporated in these minerals based on infrared spectroscopy were incomplete/wrong, and the true water solubilities (water contents) may have been significantly underestimated, requiring revision of the infrared absorption coefficients and reconsideration about how water may affect various properties.
In this presentation, we give an overview of our most recent findings on the incorporation mechanisms of water in MgSiO3 ortho- and clino- enstatite and aluminous orthoenstatite (Xue et al., submitted to Contrib Mineral Petrol; Xue et al., in preparation), and also discuss its implications for other important mantle minerals.
Our study revealed that for MgSiO3 orthoenstatite and low-pressure clinoenstatite, two types of OH defects, (2H)Mg (two protons in a Mg vacancy) and (4H)Si (four protons in a Si vacancy) are present, with the latter increasing in proportion with pressure. The (4H)Si defects were found to yield four OH stretching bands, two of high frequencies (A1/A2 bands) due to OH associated with nonbridging oxygens (O1/O2), and two of low-frequencies due to strongly hydrogen-bonded OH associated with bridging oxygens (O3). The latter are beyond the spectral range of infrared studied thus far, and thus have been overlooked. Therefore, the reported water solubilities for enstatite, especially for clinoenstatite synthesized at higher pressures, in which the (4H)Si defects dominate, may have been significantly underestimated. We have revised the infrared absorption coefficients by taking into consideration of the undetected low-frequency bands. As the important lower-mantle minerals, such as bridgmanite, have silica-rich compositions like enstatite, it is necessary to also investigate them to ascertain if there are any strongly hydrogen-bonded OH defects that may have been overlooked by infrared spectroscopy. Our first-principles calculations also revealed that the (4H)Si defects in the unquenchable high-pressure clinoenstatite takes a hitherto unknown configuration with 2H on the O2, and no H on one O3, exhibiting spectroscopic features of molecular H2O.
Our 2D 1H NMR measurements and first-principles calculations on aluminous orthoenstatite revealed that the generally believed paired substitutions of Al+H = Si, Al+H = 2Mg, although present, are not the dominant substitution mechanisms, and more than half of the OH defects are in the form of (2H)M2 defects adjacent to an Al. The interaction with Al significantly increases its stability and also alters the hydrogen bonding strength, explaining the enhancement of water solubility by Al and the infrared spectra reported thus far. The generally held views for the incorporation mechanisms of Al and H in other important mantle minerals need also to be reconsidered.
However, our comprehensive studies, using combined multi-nuclear NMR and vibrational spectroscopic measurements and first-principles calculations, on a variety of nominally mantle minerals (including enstatite and stishovite), which allowed us to gain unambiguous structural information, revealed that the general understandings about how water is incorporated in these minerals based on infrared spectroscopy were incomplete/wrong, and the true water solubilities (water contents) may have been significantly underestimated, requiring revision of the infrared absorption coefficients and reconsideration about how water may affect various properties.
In this presentation, we give an overview of our most recent findings on the incorporation mechanisms of water in MgSiO3 ortho- and clino- enstatite and aluminous orthoenstatite (Xue et al., submitted to Contrib Mineral Petrol; Xue et al., in preparation), and also discuss its implications for other important mantle minerals.
Our study revealed that for MgSiO3 orthoenstatite and low-pressure clinoenstatite, two types of OH defects, (2H)Mg (two protons in a Mg vacancy) and (4H)Si (four protons in a Si vacancy) are present, with the latter increasing in proportion with pressure. The (4H)Si defects were found to yield four OH stretching bands, two of high frequencies (A1/A2 bands) due to OH associated with nonbridging oxygens (O1/O2), and two of low-frequencies due to strongly hydrogen-bonded OH associated with bridging oxygens (O3). The latter are beyond the spectral range of infrared studied thus far, and thus have been overlooked. Therefore, the reported water solubilities for enstatite, especially for clinoenstatite synthesized at higher pressures, in which the (4H)Si defects dominate, may have been significantly underestimated. We have revised the infrared absorption coefficients by taking into consideration of the undetected low-frequency bands. As the important lower-mantle minerals, such as bridgmanite, have silica-rich compositions like enstatite, it is necessary to also investigate them to ascertain if there are any strongly hydrogen-bonded OH defects that may have been overlooked by infrared spectroscopy. Our first-principles calculations also revealed that the (4H)Si defects in the unquenchable high-pressure clinoenstatite takes a hitherto unknown configuration with 2H on the O2, and no H on one O3, exhibiting spectroscopic features of molecular H2O.
Our 2D 1H NMR measurements and first-principles calculations on aluminous orthoenstatite revealed that the generally believed paired substitutions of Al+H = Si, Al+H = 2Mg, although present, are not the dominant substitution mechanisms, and more than half of the OH defects are in the form of (2H)M2 defects adjacent to an Al. The interaction with Al significantly increases its stability and also alters the hydrogen bonding strength, explaining the enhancement of water solubility by Al and the infrared spectra reported thus far. The generally held views for the incorporation mechanisms of Al and H in other important mantle minerals need also to be reconsidered.