title: Carbon substitution for oxygen in silicates in planetary interiors.
abstract: Amorphous silicon oxycarbide polymer-derived ceramics (PDCs), synthesized from organometallic precursors, contain carbon- and silica-rich nanodomains, the latter with extensive substitution of carbon for oxygen, linking Si-centered SiO(x)C(4-x) tetrahedra. Calorimetric studies demonstrated these PDCs to be thermodynamically more stable than a mixture of SiO2, C, and silicon carbide. Here, we show by multinuclear NMR spectroscopy that substitution of C for O is also attained in PDCs with depolymerized silica-rich domains containing lithium, associated with SiO(x)C(4-x) tetrahedra with nonbridging oxygen. We suggest that significant (several percent) substitution of C for O could occur in more complex geological silicate melts/glasses in contact with graphite at moderate pressure and high temperature and may be thermodynamically far more accessible than C for Si substitution. Carbon incorporation will change the local structure and may affect physical properties, such as viscosity. Analogous carbon substitution at grain boundaries, at defect sites, or as equilibrium states in nominally acarbonaceous crystalline silicates, even if present at levels at 10-100 ppm, might form an extensive and hitherto hidden reservoir of carbon in the lower crust and mantle.
authors.affiliation: Department of Chemical Engineering and Materials Science and Peter A. Rock Thermochemistry Laboratory and Nanomaterials in the Environment, Agriculture, and Technology Organized Research Unit, University of California, Davis, CA 95616.
authors.forename: Sabyasachi
authors.initials: S
authors.lastname: Sen
authors.forename: Scarlett J
authors.initials: SJ
authors.lastname: Widgeon
authors.forename: Alexandra
authors.initials: A
authors.lastname: Navrotsky
authors.forename: Gabriela
authors.initials: G
authors.lastname: Mera
authors.forename: Amir
authors.initials: A
authors.lastname: Tavakoli
authors.forename: Emanuel
authors.initials: E
authors.lastname: Ionescu
authors.forename: Ralf
authors.initials: R
authors.lastname: Riedel
mesh_terms: D002244:Carbon / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D008956:Models, Chemical; D010100:Oxygen / Q000737:chemistry*; D016083:Planets; D017640:Silicates / Q000032:analysis*
chemical_list: D017640:Silicates; D002244:Carbon; D010100:Oxygen
keywords: Li-Si-O-C ceramics; carbon in silicates; deep Earth
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