The minerals boulangerite, falkmanite and Cu-free meneghinite : synthesis, new powder diffraction data and stability relations

Although falkmanite of the ideal composition proposed by Ramdohr & Ödman (1940), Pb₃Sb₂S₆, could not be synthesized even with the help of halide fluxes, a phase with composition similar to boulangerite, (Pb₅Sb₄S₁₁) but with dissimilar X-ray diffraction pattern was synthesized by sulfidation of bournonite at 400 and 500° C. This phase has been provisionally referred to as falkmanite in this study. Molar PbS:Sb₂S₃ ratio in boulangerite varied from 2.41 to 2.47 and that in falkmanite varied from 2.51 to 2.57. Powder X-ray-diffraction pattern for boulangerite furnished an orthorhombic cell with a = 23.443 Å, b = 21.183 Å and c = 4.505 Å, while that for falkmanite gave a monoclinic cell with a = 21.626 Å, b = 11.409 Å, c = 7.876 Å and β = 96.19°. This raises doubts as to the identity of falkmanite as a member of the boulangerite homologous series, as proposed by Mozgova et al., (1983). At 530° C, Cu-free meneghinite co-existed with falkmanite, signifying that the meneghinite solid solution field in the ternary Cu₂S-PbS-Sb₂S₃ touched the PbS-Sb₂S₃ binary between 500 and 530°C. From 530 to 560° C, the proportion of Cu-free meneghinite gradually increased relative to that of falkmanite implying increased overlap of the meneghinite solid solution field with the PbS-Sb₂S₃ binary. Falkmanite and boulangerite synthesized at 500° C and heated at 560° C for 2 hours provided similar X-ray-diffraction patterns, which corresponded with that of Cu-free meneghinite.