Conversion of carbonaceous material to graphite during metamorphism

Mineral-templated growth of natural graphite films

“…During progressive diagenesis and metamorphism organic macromolecular aggregates gradually lose functional groups causing aromatic parts to grow in size until small sp 2 -bound carbon crystallites of approximately 4-10 stacked layers are formed (Bustin et al, 1995). Sustained conditions of elevated temperature and pressure cause in-plane growth, assimilation, alignment, and stacking of such crystallites leading ultimately to the formation of crystalline graphite with 3D structure (Beyssac et al, 2002b;Buseck and Huang, 1985;Bustin et al, 1995). The efficiency of this process depends on the initial micro-texture and the presence of strong cross-linking bonds (Franklin, 1951) within the organic precursor, the liberty of the formed crystallites to rearrange and align themselves, and the degree to which defects are eliminated and aromatic layers are annealed (Bustin et al, 1995).…”

“…The efficiency of this process depends on the initial micro-texture and the presence of strong cross-linking bonds (Franklin, 1951) within the organic precursor, the liberty of the formed crystallites to rearrange and align themselves, and the degree to which defects are eliminated and aromatic layers are annealed (Bustin et al, 1995). Metamorphosed CMs are therefore complex heterogeneous mixtures on a nanoscale, and can include micro-porous domains and structures such as carbon onion-rings (Beyssac et al, 2002b;Buseck and Huang, 1985). Earlier studies on shungites have…”

“…This has led to the loss of functional groups and heteroatoms, to aromatization of the carbonaceous phase, and ultimately to the formation of crystalline graphite. The steps involved in this maturation process have been studied in great detail (Bernard et al, 2010;Beyssac et al, 2003a;Beyssac et al, 2002b;Buseck and Huang, 1985;Bustin et al, 1995;Wopenka and Pasteris, 1993). It is generally accepted that graphitization is a continuous process in which the overall degree of structural order of bulk carbonaceous material (CM) -estimated by the average size of sp 2 -bound carbon crystallites -can be directly linked to progressive grades of peak metamorphic temperature.…”

“…On a nano-scale, however, CM of a certain metamorphic grade can display strong structural variation and can contain a wide variety of sp 2 -bound carbon nano-structures including graphitic globules (Kovalevski et al, 2001), carbon onion rings (Beyssac et al, 2002b;Buseck and Huang, 1985), graphite cones (Jaszczak et al, 2003), rolled graphitic structures (Jaszczak et al, 2007), and curled graphite structures (Papineau et al, 2010). In general this can be related to the fact that organic precursor materials are heterogeneous in both molecular structure and chemical composition.…”

“…highly aromatic compounds) and are easily converted to graphite, while others contain non-planar parts or 5-membered ring systems which make them particularly resistant to 4 graphitization. In addition, the presence of certain hetero-atoms could either inhibit or catalyze graphitization (Buseck and Huang, 1985, and references therein). Since natural biologic precursor materials are complex organic mixtures containing both graphitizing and non-graphitizing compounds, the resulting metamorphic CM thus commonly shows strong nano-scale structural variability.…”