Characterization of natural carbon particles formed at low temperature UHP conditions

Micro-to-nanosized carbon particles forming symmetrically oriented trails in ultrahigh pressure (UHP) garnet have been studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman spectroscopy. Micrometer-sized particles include two distinct morphological types: minor rod-shaped particles, and abundant equidimensional particles, interpreted on the basis of the Raman spectra, as graphite and diamond, respectively. The graphite spectra show structural disorder greater than that found in graphite of other UHP rocks. Most diamond spectra shows additional Raman features due to sp2 carbon phases and weaker and broader Raman bands than metamorphic diamond. The SEM images reveal clear topotactic relationships between garnet, micrometer-sized rods of graphite and equidimensional particles of diamond. The TEM study shows that garnet also contains multiwall nanotubes, nanocrystalline diamond particles and polyhedral carbon + silicate structures. The morphology and the orientation relationships between garnet and micro-to-nanometer graphite-like particles are compatible with formation through co-precipitation of garnet and metastable carbon allotropes. It is suggested that precipitation of disordered sp2 carbon can act as a precursor for diamond formation within the stability field of diamond. The presented data reveal the importance that the metastable carbon structures can have on the formation of diamond. (C) 2015 Elsevier B.V. All rights reserved.