ABSTRACT

A remarkable isotopic effect of sulfur has been confirmed in the thermal decomposition of pyrite at 600°C in vacuo. The reaction concerned may be written as pyrite(FeS₂) → pyrrhotite(FeS) + elemental sulfur(S). ³⁴S/³²S ratio of an early fraction of the released elemental sulfur is some 12‰ lower than that of the source pyrite sulfur. Toward the end of the reaction the pyrrhotite dominant solid residue has the isotopic ratio up to about 4.5 ‰ higher than the initial material. A “Rayleigh model” which involves two isotopic fractionation factors, α₁(FeS/FeS₂) and α₂(S/FeS₂), seems to hold in the case. The estimated α₁ and α₂ are 0.9951 and 0.9883, respectively. The result may have significant bearing on sulfur isotope geochemistry, particularly because of the very large isotopic effect at a high temperature.