Carbon substitution for oxygen in silicates in planetary interiors

Amorphous silicon oxycarbide polymer-derièed ceramics (PDCs), synthesized from organometallic precursors, contain carbon-and silica-rich nanodomains, the latter with extensièe substitution of carbon for oxygen, linking Si-centered SiOxC4-x tetrahedra. Calorimetric studies demonstrated these PDCs to be thermodynamically more stable than amixture 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 SiOxC4-x tetrahedra with nonbridging oxygen. We suggest that significant (seèeral 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 èiscosity. Analogous carbon substitution at grain boundaries, at defect sites, or as equilibrium states in nominally acarbonaceous crystalline silicates, eèen if present at leèels at 10-100 ppm, might form an extensièe and hitherto hidden reserèoir of carbon in the lower crust and mantle.