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 shea r configurations at temperatures up to 900 degrees C and confining pre ssures up to 1 GPa. Additional tests were carried out at ambient press ures at temperatures up to 2800 degrees C. In simple shear, graphite a ppears, with an anthracite starting material at temperatures as low as 600 degrees C; samples tested at 900 degrees C are predominately grap hitized, as is evident from optical microscopy, X-ray diffraction (XRD ) and transmission electron microscopy (TEM). In tests on high volatil e bituminous coal, graphite first appears in simple shear tests at tem peratures of 800 degrees C and is common at 900 degrees C. In TEM obse rvations graphite particles are lamellar, have punctual hkl reflection s or Debye-Scherrer hkl rings (triperiodic order) and long, stiff and stacked lattice fringes typical of well crystallized graphite. No grap hite was formed in either the hydrostatic or coaxial tests (they remai n porous and turbostratic). Micro-Raman spectroscopy of deformed sampl es indicates the presence of defects (band at 1350 cm(-1)) even in sam ples that prove to be mainly graphite by XRD and TEM. With increasing experimental temperatures there is an overall increase in maximum refl ectance and bireflectance, Samples deformed in simple shear locally ha ve reflectance values typical of graphite. In anthracite the highest r eflectance and bireflectance values occur in zones of kink banding or cataclasis, indicating the importance of localized areas of high strai n on graphitization. In high volatile bituminous coal localization of graphite appears to reflect compositional heterogeneity as well as str ain partitioning during the experiments, The occurrence of low reflect ance zones and mesoporousturbostratic particles in samples otherwise c omposed of graphite is interpreted as reflecting localized areas of lo w strain (strain shadows) during deformation. Comparison of anthracite and high volatile bituminous coal samples tested under the same gener al conditions indicate that anthracite is more graphitizable under all conditions. The importance of simple shear experiments is that, becau se of their geometry, a significant component of strain is imparted to the samples. Strain energy has facilitated additional flattening of e xisting pores, with likely mechanical rotation of stacks of basic stru ctural units (BSUs) and rupturing of pore walls. Thus, strain facilita tes coalescence of pores, parallelism of BSUs and, therefore, the grow th 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.