A comparative study of the properties of marcasite and pyrite

Marcasite (FeS₂) and pyrite (FeS₂) are major sources of Fe and S‐bearing mineral matter in hot coals. Recent studies have indicated ¹ that three minerals may act beneficially as catalysts in coal conversion (of course, their negative role in pollution is well‐known). In order to better understand the roles of these minerals in various technological processes, we have, in recent years, studied the various properties (magnetic, dielectric, optical) of pyrite² and similar studies on marcasite are currently underway. Magnetic susceptibility (χ) of a natural sample of marcasite, studied from 4.2 to 380 K³, suggests a low‐spin configuration for Fe²⁺ and Van Vleck paramagnetism, somewhat similar to the case of pyrite. However, near 300 K, χ(marcasite?3χ(pyrite), suggesting large differences in the crystal‐field splittings in the two cases. This is further confirmed by the recent electrical resistivity (77–400 K) measurements which yield the band gap Eg?0.33 eV (for pyrite Eg?0.84 eV²), the first such estimate for marcasite. This large difference in the values of Eg and χ for the two minerals, though consistent with each other, suggests different transformation properties of coal processes in the two cases.