Synthesis of minerals in the Cu — Fe — Bi — S system under hydrothermal condition and their phase relations

Abstract

Chalcopyrite, bornite, nukundamite, intermediate solid solution, wittichenite, emplectite, cuprobismutite, and phases CuBi₃S₅ (E), Cu₈Bi₈S₁₉ (Y) and Cu₈.₄Fe₁.₂Bi₁₀.₈S₂₂ (Z) in the system Cu — Fe — Bi — S were synthesized by a temperature gradient transport method using gold tubing under hydrothermal conditions at 300 °C and 420 °C. 5m NH₄Cl aqueous solution was used as solvent and various sulfides produced by evacuated glass tube method were employed as nutrient materials. The minerals synthesized were either a single crystal or aggregates. They were identified by ore microscope and Gandolfi or Guinier camera, and electron probe methods. According to these data, bornite and wittichenite contain 13.9 weight percent bismuth at 420 °C and 1.0 weight percent iron at 300 °C, respectively, while chalcopyrite has a very small amount, less than 0.1 weight percent, of bismuth. Emplectite and cuprobismutite have the compositions of CuBiS₂ and Cu₁₀B₁₂S₂₃, respectively, and occur as a stable phase at 300 °C and at 420 °C, respectively. A four component phase, Z, was found at 300 °C and 420 °C. It is prismatic form (monoclinic) and has a = 17.483 Å, b = 3.902 Å, c = 12.869 Å and β = 108.11° cell parameters. The stable assemblages formed at 300 °C were as follows : chalcopyrite + bornite + wittichenite + nukundamite, chalcopyrite + wittichenite + nukundamite + phase Z, wittichenite + emplectite + phase Z + phase Y, wittichenite + nukundamite + phase Z + phase Y, chalcopyrite + nukundamite + phase Z + phase Y, chalcopyrite + phase E + phase Z + phase Y, emplectite + phase E + phase Z + phase Y, intermediate solid solution + pyrrhotite + bismuthinite + Bi etc. The chalcopyrite˗wittichenite assemblage which is often found in natural ore occurs at 300 °C, but it disappears at 420 °C because the assemblages of bornite˗nukundamite˗phase Z and bornite˗nukundamite˗cuprobismutite become stable.