INCONGRUENT EVAPORATION OF TROILITE (FES) IN THE PRIMORDIAL SOLAR NEBULA - AN EXPERIMENTAL-STUDY

Incongruent evaporation experiments on troilite (FeS) were carried out under H-2-rich conditions at total pressure 1.0-10(-6) atm (800-970 d egrees C) to elucidate the kinetics of incongruent evaporation of troi lite in the primordial solar nebula. Sulfur evaporates from troilite l inearly with time, and a porous residual layer of metallic iron is for med. It was concluded from the results and consideration on individual possible processes during the evaporation that the evaporation rate o f sulfur is controlled by the surface chemical reaction. The evaporati on rate at 1 atm total pressure depends largely on hydrogen pressure, p(H-2), while that under low p(H-2) conditions has a little dependence on p(H-2). These results indicate that sulfur evaporates from troilit e mainly as H2S under high p(H-2) conditions, while mainly as S-2 (and HS) under low p(H-2) conditions. The evaporation coefficients, a, whi ch represent the degree of kinetic constrains of evaporation, were obt ained from the experimental results and thermodynamic calculations: al pha(H2S) = 2.03 x 10(-3) p(H-2)(0.106) exp(-940/T), alpha(HS) = 1.94 x 10(-2) p(H-2)(-0.136) exp(-2040/T), and alpha(S2) = 0.922 exp(-2220/T ). Small values of alpha for evaporation as H2S (<0.001) may be due to a sluggish surface reaction between H-2 molecules and troilite. This causes the metastable reaction including S-2 under low p(H-2) conditio ns. Taking the present results and p-T conditions in the primordial so lar nebula into consideration, it was concluded that troilite would ev aporate incongruently as H2S in low temperature regions of the outer p rimordial solar nebula and as S-2 (and HS) in high temperature regions . Since residual metallic iron evaporates little, the incongruent evap oration of troilite can cause the Fe/S fractionation in the primordial solar nebula. Copyright (C) 1998 Elsevier Science Ltd.