MECHANISMS OF GRAPHITE FORMATION FROM KEROGEN - EXPERIMENTAL-EVIDENCE

To resolve the role of strain in the formation of natural graphite, a 'hard' carbon-based anthracite and a 'soft' carbon-based high volatile bituminous coal were deformed in hydrostatic, coaxial and simple shear configurations at temperatures up to 900 degrees C and confining pressures up to 1 GPa. Additional tests were carried out at ambient pressures at temperatures up to 2800 degrees C. In simple shear, graphite appears, with an anthracite starting material at temperatures as low as 600 degrees C; samples tested at 900 degrees C are predominately graphitized, as is evident from optical microscopy, X-ray diffraction (XRD) and transmission electron microscopy (TEM). In tests on high volatile bituminous coal, graphite first appears in simple shear tests at temperatures of 800 degrees C and is common at 900 degrees C. In TEM observations graphite particles are lamellar, have punctual hkl reflections or Debye-Scherrer hkl rings (triperiodic order) and long, stiff and stacked lattice fringes typical of well crystallized graphite. No graphite was formed in either the hydrostatic or coaxial tests (they remain porous and turbostratic). Micro-Raman spectroscopy of deformed samples indicates the presence of defects (band at 1350 cm(-1)) even in samples that prove to be mainly graphite by XRD and TEM. With increasing experimental temperatures there is an overall increase in maximum reflectance and bireflectance, Samples deformed in simple shear locally have reflectance values typical of graphite. In anthracite the highest reflectance and bireflectance values occur in zones of kink banding or cataclasis, indicating the importance of localized areas of high strain on graphitization. In high volatile bituminous coal localization of graphite appears to reflect compositional heterogeneity as well as strain partitioning during the experiments, The occurrence of low reflectance zones and mesoporousturbostratic particles in samples otherwise composed of graphite is interpreted as reflecting localized areas of low strain (strain shadows) during deformation. Comparison of anthracite and high volatile bituminous coal samples tested under the same general conditions indicate that anthracite is more graphitizable under all conditions. The importance of simple shear experiments is that, because of their geometry, a significant component of strain is imparted to the samples. Strain energy has facilitated additional flattening of existing pores, with likely mechanical rotation of stacks of basic structural units (BSUs) and rupturing of pore walls. Thus, strain facilitates coalescence of pores, parallelism of BSUs and, therefore, the growth of aromatic sheets (by coalescence of neighbouring pores), leading to the formation of graphite. We propose that a major component of the activation energy required for graphitization in our experiments and, by analogy, in nature, is provided by strain energy.