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The Canadian Mineralogist; December 2001; v. 39; no. 6; p. 1635-1640; DOI: 10.2113/gscanmin.39.6.1635
© 2001 Mineralogical Association of Canada
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Articles

THE GIBBS FREE ENERGY OF NUKUNDAMITE (Cu3.38Fe0.62S4): A CORRECTION AND IMPLICATIONS FOR PHASE EQUILIBRIA

Robert R. Seal, II1,§, E. Esra Inan2 and Bruce S. Hemingway3

1 U.S. Geological Survey, 954 National Center, Reston, Virginia 20192, U.S.A.
2 Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, U.S.A.
3 U.S. Geological Survey, 953 National Center, Reston, Virginia 20192, U.S.A.

§ E-mail address: rseal{at}usgs.gov

The Gibbs free energy of formation of nukundamite (Cu3.38Fe0.62S4) was calculated from published experimental studies of the reaction 3.25 Cu3.38Fe0.62S4 + S2 = 11 CuS + 2 FeS2 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 S2 gas is {Delta}fG°nukundamite, T(K) = –549.75 + 0.23242 T + 3.1284 T0.5, 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(S2) 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 Cu5FeS4. Under equilibrium conditions in the system Cu–Fe–S, nukundamite + chalcopyrite is not a stable assemblage at any temperature.

Keywords: nukundamite, bornite, thermodynamic properties, Gibbs free energy, phase equilibria.






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