Carbon nanotubes were synthesized by chemical reduction of magnesium carbonate with metallic lithium at 600 degrees C. The nanotubes with an average length of 13 microm and diameter of 60 nm are made of short coaxial conical cylinder tubular graphite sheets with their cone axis parallel to the tube axis, different from the ordinary carbon nanotubes, composed of concentric cylindrical graphite layers with their normal perpendicular to the tube axis. It is suggested that nanoscale rough surface of lithium formed at the interface between supercritical carbon dioxide and liquid lithium takes the roles of both the reductant for reduction of carbon dioxide to carbon and the template for growth of carbon nanotubes.