The role of strain energy in creep graphitization of anthracite

Abstract

RECENT research on ceramics and natural minerals has demonstrated that non-hydrostatic stress can affect some polymorphic transitions and can increase reaction rates^1,2. One such example is the graphitization of anthracite. Under natural conditions graphite forms at temperatures of 300–500° C and confining pressures of ∼500 MPa (refs 3–9). But in simple heating experiments at ambient pressure and high confining pressure (up to 1 GPa), temperatures of ∼ 2,000 °C are required for graphite formation10–13. Here we report creep experiments on natural anthracite at temperatures of 300–600 °C, using variable strains and strain rates and a constant confining pressure of 500 MPa. The experiments yield an apparent activation energy of 68.6 kJ mol^-1 for the steady-state process(es) leading to graphite formation. This value is in marked contrast with simple heating experiments, which require an activation energy of 1,000 kJ mol^-1 (ref. 10). We suggest that in our experiments, and also under natural conditions, graphitization is facilitated by available strain energy associated with non-hydrostatic stress; such stresses typify conditions of natural graphitization.