The effect of a magmatic intrusion on the optical properties (reflectance, anisotropy and optical character) of vitrinite in low-rank bituminous coals was examined. The study focused on coal samples collected at different distances from an intrusion emplaced into seam no. 416 in Sośnica mine near Gliwice in the Upper Silesian Coal Basin, Poland. In the immediate contact zone, coal transformed into natural coke may be seen.

For comparison, laboratory carbonization of coals weakly changed or apparently unchanged by the intrusion was carried out. The results obtained confirmed that during carbonization of the low-rank coal, heating alone (with no external pressure) makes possible the reorganization of carbon planes that is responsible for the increase of vitrinite reflectance.

The heating leads to the destruction of coal-structure cross-linkages (aliphatic groups or heteroatoms) and various further transformations of the structural net, in addition to the removal of volatile carbonization products. External pressures affect the transformation process, especially by promoting the rearrangement of small structural units to form further elements of anisotropic structure. Thus, in the case of coal from the immediate contact of the intrusion, pressures resulted in values of anisotropy parameters higher than those characterizing coal heated in the laboratory up to 1200 °C, but in the absence of stress. The source of pressure that strongly affected the coal in the mine was probably intrusion related. Weak tectonism in the region of the Sośnica mine could explain the fine biaxial optical character of the low rank parent coals more generally. Pressures associated with thermally evolving gasses conceivably led to the growth of structural anisotropy in the walls of degassing pores. Finally, some structural changes seen in coal adjacent to the intrusion may reflect hydrothermal activity.