Thermomagnetic study of coal and associated roof shale

Thermomagnetic studies of Pennsylvanian coals and associated roof shales indicate that hematite and minor goethite are present as ferrimagnetic phases, in addition to the paramagnetic mineral pyrite. Laboratory studies show that the carrier of the stable natural magnetic remanence (NRM) is mainly single domain hematite. X-ray diffraction studies of the samples has directly identified illite, kaolinite and minor amounts of chlorite as the natural suite of clay minerals typical in coal and overclay. Mössbauer spectroscopy identifies hematite and geothite as ferrimagnetic phases within the clay structures. Thermomagnetic studies at higher temperatures show that paramagnetic pyrite is converted to ferrimagnetic hematite through a strongly ferromagnetic phase maghemite. This transition occurs within the 400-500°C temperature interval and results in a significant increase in the ferrimagnetic polarization and paramagnetic susceptibility of the samples.