Sabyasachi Sen^a, Scarlett J. Widgeon^a,b, Alexandra Navrotsky^a,b,*, Gabriela Mera^c, Amir Tavakoli^b, Emanuel Ionescu^c, and Ralf Riedel^c

^aDepartment of Chemical Engineering and Materials Science and
^bPeter A. Rock Thermochemistry Laboratory and Nanomaterials in the Environment, Agriculture, and Technology Organized Research Unit, University of California, Davis, CA 95616
^cInstitut für Materialwissenschaft, Technische Universität Darmstadt, D-64287 Darmstadt, Germany

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_xC_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_xC_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.

Reference
Sen S Widgeon SJ, Navrotsky A, Mera G, Tavakoli A, Ionescu E and Riedel R (in press) Carbon substitution for oxygen in silicates in planetary interiors. PNAS
[doi:10.1016/j.icarus.2013.09.020]

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