Notes on the mechanisms of coal metamorphism in the Pennsylvania Anthracite Fields

Charred/devolatilized megasporinite and featureless vitrinite with devolatilization pores filled with pyrolytic carbon suggest that eastern Pennsylvania anthracites may have been altered by to localized episodes of metamorphism from superheated steam/CO ₂ /CO/hydrocarbons, in addition to the pervasive regional metamorphism. Based on the extant vitrinite textures, the lack of mesophase anisotropic textures, and on comparisons to laboratory studies of the thermoplasticity of bituminous coals, the rank at the time of metamorphism would have been high volatile C or lowermost high volatile B bituminous. The temperature of alteration would have been in the 350–400 °C range, corresponding to the critical point of steam (374.15 °C and 22.1 MPa; about 1-km depth). It is speculated that while increased pressure would have prolonged the duration of fluid properties, fluidity is still a transient property and the vitrinite would have returned to a solid state after a few hours. The heat would have persisted, dissipating over the course of days to months to years depending upon the depth and ambient (pre-heating) geothermal gradient, but contributing to the coal metamorphism long after the vitroplast solidified.