Molecular disordering in natural cokes approaching dyke and sill contacts

Abstract

Optical properties of vitrinites often display reversals in trend approaching contacts with igneous intrusions. Reflectance (maximum, average or random) is the most common property affected within thermal aureoles, but parameters of optical anisotropy, for example bireflectance or bireflectance ratio, may also show reversed trends close to intrusions. The suggestion that increasing randomness of the direction of polarization of discrete areas within affected coal material is responsible for a fall in bireflectance ratio, does not seem to explain all cases. The reversal phenomenon may possibly be related to more general molecular disordering, due to the thermal effects, and/or to difficulties in producing a polished surface of quality after the vitrinite has been transformed to carbonaceous material with a turbostratic or paracrystalline structure, or to fully ordered graphite. General molecular disordering as a cause is now a real possibility on the basis of correlated optical and X-ray diffraction data from samples analysed from within a thermal aureole of a Tertiary dyke emplaced in Permian coal-bearing strata. The influences of different factors (principally temperature level, time of exposure to elevated temperature, degree of consolidation of sediments and their porosities and permeabilities, initial maturation level of the vitrinite before intrusion, overburden pressure and stress) on the maturation level of the organic matter are considered. There is as yet no unequivocal single explanation for the reversals of property trends, but the reversals seem to be related to ‘optical graphites’ that have developed at much lower temperatures than those required to produce graphite in the laboratory. The reversals may be due to a combination of molecular disordering and variable surface quality, upon which differing orientations of highly optically anisotropic fragments in the samples may also have some bearing.