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Experimental control of H-O-S gas mixtures with applications to Fe-Ni sulfideoxide-silicate reactions Lecheminant, Anthony Norman

Abstract

Sulfide-oxide and sulfide-oxide-silicate reactions commonly involve gas species in the H-O-S system. Experimental investigation of such reactions requires control of H-O-S gas mixture compositions. The reaction: Fe₃O₄ + 3/2 S₂ ⇄ 3 FeS + 2 O₂ has been studied using solid phase buffers in nested noble metal capsules to control the H-O-S vapour phase at temperatures between 550 and 700°C and total pressures to 3000 bars. Results for IW, QFM, and NNO buffers correspond closely to those predicted from thermochemical data. Reaction rates allow pyrrhotite compositions to be bracketed in experiments lasting less than 100 hours. Platinum and thin-walled Au capsules have been successfully used as hydrogen membranes at f[sub H₂] values specified by QFM or NNO external buffers. At sulfur and oxygen fugacities higher than those defined in NNO buffer experiments difficulties arise due to low f f[sub H₂] values, quench sulfide phases, and reaction between the H-O-S vapour phase and Pt capsules. Computer programs and data used to compute O-H and H-O-S gas mixture compositions controlled by buffer systems tested in this study are tabulated. Careful distinction must be made between configurations that control f[sub O₂]internally by solid phase oxygen buffers and those in which f[sub O₂] is specified indirectly through hydrogen diffusion. In the latter case f f[sub O₂]in the charge capsule may differ significantly from f[sub O₂] in the external buffer. Computations show H₂O, H₂, H₂S, and SO₂ are quantitatively the important species within geologically significant regions of f[sub S₂]- f[sub O₂] - P - T space. Experimental evidence supports calculations that predict hematite cannot coexist with either pyrite or pyrrhotite above 550°C at P[sub total] = 500 bars or 600°C at P[sub total] >2000 bars. The feasibility of using QFM or NNO buffers to investigate sulfide-oxide reactions in the Fe-Ni-H-O-S system has been established. Results demonstrate the importance of f[sub O₂] in defining the metal:sulfur ratio in (Fe,Ni)[sub 1-x]S monosulfide solid solution coexisting with an Fe-bearing spinel and vapour.

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