STUDIES ON THE JOIN Cu₅FeS₄-CuFeS_2-x AS GEOTHERMOMETER

Abstract

To use unmixing chalcopyrite and bornite intergrowth as geothermometer, it is necessary to take into account the relation between composition of solid solution and its unmixing temperature. The relation is to be acquired from the solvus in the phase diagram of the join Cu₅FeS₄-CuFeS_2-x From this veiwpoint, thermal study on the solvus was carried out by the author by using bornite-chalcopyrite solid solutions synthesized by reaction between copper and iron sulfides, which were made by heating metallic copper, iron and crystal sulfur in glass tube sealed under vacuum. The results obtained are as follows:
1) The solvus on the bornite side in this a join is given in Fig. 4. From this solvus, it is found that unmixing temper a ture of the solid solution with 93.4%, 81.8%, 67.5%, 63.3% and 59.4% bornite in molecule are 375°C, 425°C, 500°C, 525°C and 540°C respectively. Thus the solvus between 375°C and 525°C is shown by a straight line having gentle slope, while it shows a steep inclination above 525°C. It is also presumed that the curve shows a steep gradient below 375°C.
2) The relation between lattice constant and composition of solid solution from 100% to 59.4% in bornite molecule is represented by a straight line between 10.946 ?? and 10.847 ?? as shown in Fig. 3.
3) As seen in Figs. 4 and 5, the area of solid solution on the bornite side is distinctly wider than that on the chalcopyrite side. It agrees with the fact that, in nature, chalcopyrite lamellae unmixed from bornite solid solution are very common, when examined under microscope, whereas exsolution bornite lamellae in chalcopyrite are very rare.
Using the relation between lattice constant and composition of the solid solution together with solvus in this join, unmixing or forming temperature of the bornite-chalcopyrite solid solution can be inferred.