Heat capacities of synthetic bornite (Cu 5 FeS 4 ) have been measured by quasi-adiabatic calorimetry between 5 and 351 K and between 338 and 761 K by differential scanning calorimetry. The heat capacity of bornite exhibits a lambda -type anomaly at 65+ or -1 K associated with the antiferromagnetic ordering of the spins of the Fe (super 3+) ions. Between 140 and 255 K, there is a broad hump in C degrees p which, in our opinion, may be caused by charge transfer involving Fe (super 2+) -Fe (super 3+) and Cu (super +) -Cu (super 2+) . At 470+ or -2 and 535+ or -2 K there are sharp peaks in the heat capacity caused by ordering of the copper, iron and vacancies and the consequent crystallographic changes. At 298.15 K, the heat capacity and entropy of bornite are 242.9+ or -0.6 and 398.5+ or -1.0 J.mol (super -1) .K (super -1) , respectively. Enthalpies and Gibbs free energies of formation of chalcopyrite and bornite are evaluated based on heat capacities (5 to 760 K) and the entropy for bornite, published calorimetric (Delta f H degrees and C degrees p ) data for chalcopyrite, thermodynamic data for pyrite, and sulfur fugacity data for the reaction: 5CuFeS 2 + S 2 = Cu 5 FeS 4 + 4FeS 2 . Previous studies show that all phases in this reaction are stoichiometric within analytical uncertainty from 673 to 773 K. Therefore, no adjustments for solid-solution effects are needed. Revised expressions for the Gibbs free energy of formation for selected phases in the system Cu-Fe-S are presented below, relative to the elements and ideal S 2 gas.