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The Gibbs free energy of nukundamite (Cu3.38Fe0.62S4): A correction and implications for phase equilibria

Robert R. Seal,·E. E. Inan·Bruce S. Hemingway
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Abstract

The Gibbs free energy of formation of nukundamite (Cu 3.38 Fe 0.62 S 4 ) was calculated from published experimental studies of the reaction 3.25 Cu 3.38 Fe 0.62 S 4 + S 2 = 11 CuS + 2 FeS 2 in order to correct an erroneous expression in the published record. The correct expression describing the Gibbs free energy of formation (kJ·mol −1 ) of nukundamite relative to the elements and ideal S 2 gas is Δ f G° nukundamite, T(K) = −549.75 + 0.23242 T + 3.1284 T 0.5 , with an uncertainty of 0.6%. An evaluation of the phase equilibria of nukundamite with associated phases in the system Cu–Fe–S as a function of temperature and sulfur fugacity indicates that nukundamite is stable from 224 to 501°C at high sulfidation states. At its greatest extent, at 434°C, the stability field of nukundamite is only 0.4 log f (S 2 ) units wide, which explains its rarity. Equilibria between nukundamite and bornite, which limit the stability of both phases, involve bornite compositions that deviate significantly from stoichiometric Cu 5 FeS 4 . Under equilibrium conditions in the system Cu–Fe–S, nukundamite + chalcopyrite is not a stable assemblage at any temperature.

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Robert R. Seal,, E. E. Inan, Bruce S. Hemingway. 2001. The Gibbs free energy of nukundamite (Cu3.38Fe0.62S4): A correction and implications for phase equilibria. https://doi.org/10.2113/gscanmin.39.6.1635

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