Thermomagnetic studies of conversion of pyrite and marcasite in different atmospheres (vacuum, H2, He and CO)

Abstract

The results of a study of the conversion of pyrite (obtained from coal) and marcasite, when heated to about 485 degrees C in several different atmospheres (vacuum, He, H2 and CO) are reported. Changes in the magnetic susceptibility chi and in the mass of the sample are noted as the sample is first heated to 485 degrees C and then cooled to room temperature at the rate of about 2 degrees C min-1. The composition parameter chi of the converted pyrohotites Fe1-xS is determined from the known location of the various magnetic and structural transitions in Fe1-xS and from the magnitudes of the observed chi . The mass changes indicated the conversion process FeS2 to Fe1-xS. Pyrite, when heated in vacuum and He, forms monoclinic pyrohotites with x approximately 0.11 to 0.13 whereas in H2 atmosphere it converts to hexagonal pyrohotites with x approximately 0.08 to 0.10. Prolonged heating in H2 eventually produces FeS (x=0). In CO atmosphere the end product seems to be an antiferromagnetic pyrrhotite with x approximately 0.05. For marcasite, the conversion products in vacuum and H2 atmospheres are essentially the same as that for pyrite except that the conversion efficiencies are somewhat higher. The magnetic properties of coal pyrites vis-a-vis mineralogical pyrites are also examined.