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

The Gibbs free energy of formation of nukundamite (Cu_3.38Fe_0.62S₄) was calculated from published experimental studies of the reaction 3.25 Cu_3.38Fe_0.62S₄ + 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 Δ_fG°_{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(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.