ABSTRACT

Evaporation experiments using pyrrhotite single crystals (Fe_0.886S) were carried out at temperatures between 500 and 1300°C at 1 atm in an H₂-CO₂ gas flow (0.62-0.64 atm H₂), and at 500 and 900°C under an evacuated condition. Under the H₂-rich condition, spongy metallic iron layer was formed on the sulfide crystal surface at temperatures below the Fe-FeS eutectic point as a result of incongruent evaporation, and developed inward almost conserving its original shape. The thickness of the iron layer increases linearly with time at constant temperatures (linear rate law) due to transportation of evaporated gas species through pores in the spongy iron layers. If incongruent evaporation is controlled by diffusion of element(s) in an evaporation residue layer, a parabolic rate law is expected. The linear rate law shows that FeS evaporates more efficiently than expected based on a parabolic rate law. The linear rate constant obtained at various temperatures obeys the Arrhenius relation: k_FeS = (1.61 ± 0.42) × 10^-3exp(-115 ± 2 [kJ/mol]/RT) [m/sec]. A minor part of metallic iron in the surface layer diffused into the inner sulfide to form stoichiometric FeS (troilite) in the early evaporation stage. Thus, the experiments can be almost regarded as evaporation of troilite. Evaporation coefficients of FeS were obtained by comparing the experimental results with calculated rates using the Hertz-Knudsen equation. They are small (1.4 × 10^-4 ∼ 9.4 × 10^-6) due to slow surface reaction and/or slow escape of S-bearing gas species into the gas flow. Mass-dependent isotopic fractionation of S by the evaporation was not detected within an error of ±3‰ probably due to slow diffusivity of S in the sulfide crystal. In the evacuated experiments, evaporation occurred very slowly due to the absence of H₂ gas, which acts as a reducing agent. Iron residue layer was very thin or sometimes not detected probably because the evaporation rate of S from FeS became comparable to the evaporation rate of metallic iron, which can be neglected under the H₂-rich condition.