HEAT-CAPACITY AND ENTROPY OF BORNITE (CU5FES4) BETWEEN 6-K AND 760-K AND THE THERMODYNAMIC PROPERTIES OF PHASES IN THE SYSTEM CU-FE-S

Heat capacities of synthetic bornite (Cu5FeS4) have been measured by q uasi-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 +/- 1 K associated with the antif erromagnetic ordering of the spins of the Fe3+ ions. Between 140 and 2 55 K, there is a broad hump in C-p(o) which, in our opinion, may be ca used by charge transfer involving Fe2+ - Fe3+ and Cu+ - Cu2+. At 470 /- 2 and 535 +/- 2 K there are sharp peaks in the heat capacity caused by ordering of the copper, iron and vacancies and the consequent crys tallographic changes. At 298.15 K, the heat capacity and entropy of bo rnite are 242.9 +/- 0.6 and 398.5 +/- 1.0 J.mol(-1) K-1 respectively. Enthalpies and Gibbs free energies of formation of chalcopyrite and bo rnite are evaluated based on heat capacities (5 to 760 K) and the entr opy for bornite, published calorimetric (Delta(f)H degrees and C-p(o)) data for chalcopyrite, thermodynamic data for pyrite, and sulfur fuga city data for the reaction: 5CuFeS(2) + S-2 = Cu5FeS4 + 4FeS(2). Previ ous studies show that all phases in this reaction are stoichiometric w ithin analytical uncertainty from 673 to 773 K. Therefore, no adjustme nts for solid-solution effects are needed. Revised expressions for the Gibbs free energy of formation for selected phases in the system Cu-F e-S are presented below relative to the elements and ideal S-2 gas. [G RAPHICS] ICS]