Natural graphitization of anthracite: Experimental considerations

Abstract

An anthracite coal (Romax = 5.27%, fixed carbon = 95.5%) was deformed in the steady state at various pressures, temperatures, and experimental configurations to assess the effects of stress, strain, and strain energy on graphitization. In simple shear tests, graphite first appears at temperatures as low as 600 °C and samples tested at 900 °C are predominately graphitized, as evident from optical microscopy, XRD, and transmission electron microscopy. The graphite is lamellar, has punctual hkl reflections or DebyeScherrer (hkl) rings (triperiodic order), and long stiff and stacked lattice fringes typical of well-crystallized graphite. No graphite was formed in either hydrostatic or coaxial tests (they remain porous and turbostratic), although increased orientation of the basic structural units (BSUs) and increase in size of molecular orientation domains (MOD), attributed to the coalescence of neighboring pore walls, is evident in some coaxial deformed samples. Micro-Raman spectroscopy of deformed samples indicates the ongoing presence of defects (band at 1350 cm −), even in samples that by XRD and TEM prove to be mainly graphite. Results of our experiments indicate the independence of stress and the dependence on strain and strain energy in the formation of graphite. Samples deformed in simple shear at 900 °C are more highly graphitized than a sample heated to 2800 °C (HTT) at ambient pressure. Simple shear tests, in particular, haveimparted strain energy resulting in rupturing of pore walls, flattening of pores, and mechanical rotation of stacks of basic structural units (BSUs). These processes facilitate preferred parallel orientation of pore walls, coalescence of pores and, thus, growth of aromatic sheets 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. The occurrence of natural graphite in rocks that have never been subjected to temperatures in excess of about 300 °C may be accounted for by strain energy imparted during tectonic deformation.