NATURAL GRAPHITIZATION OF ANTHRACITE - EXPERIMENTAL CONSIDERATIONS

An anthracite coal (Romax = 5.27%, fixed carbon = 95.5%) was deformed in the steady state at various pressures, temperatures, and experiment al configurations to assess the effects of stress, strain, and strain energy on graphitization. In simple shear tests, graphite first appear s at temperatures as low as 600 degrees C and samples tested at 900 de grees C are predominately graphitized, as evident from optical microsc opy, XRD, and transmission electron microscopy. The graphite is lamell ar, has punctual hkl reflections or Debye-Scherrer (hkl) rings (triper iodic order), and long stiff and stacked lattice fringes typical of we ll-crystallized graphite. No graphite was formed in either hydrostatic or coaxial tests (they remain porous and turbostratic), although incr eased orientation of the basic structural units (BSUs) and increase in size of molecular orientation domains (MOD), attributed to the coales cence of neighboring pore walls, is evident in some coaxial deformed s amples. Micro-Raman spectroscopy of deformed samples indicates the ong oing presence of defects (band at 1350 cm(-1)), even in samples that b y XRD and TEM prove to be mainly graphite. Results of our experiments indicate the independence of stress and the dependence on strain and s train energy in the formation of graphite. Samples deformed in simple shear at 900 degrees C are more highly graphitized than a sample heate d to 2800 degrees C (HTT) at ambient pressure. Simple shear tests, in particular, have imparted strain energy resulting in rupturing of pore walls, flattening of pores, and mechanical rotation of stacks of basi c structural units (BSUs). These processes facilitate preferred parall el orientation of pore walls, coalescence of pores and, thus, growth o f aromatic sheets leading to the formation of graphite. We propose tha t a major component of the activation energy required for graphitizati on in our experiments and, by analogy in nature, is provided by strain energy. The occurrence of natural graphite in rocks that have never b een subjected to temperatures in excess of about 300 degrees C may be accounted for by strain energy imparted during tectonic deformation.