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Oriented single-crystals of stibnite, bismuthinite, kermesite, stephanite and bournonite were investigated by polarized Raman spectroscopy. The obtained spectra were compared to those of the sulfosalt minerals tetrahedrite and pyrargyrite. Whereas the latter show isolated SbS 3 groups with ideal trigonal symmetry, the former show distorted XS 3 (X = Sb,Bi) groups with lower symmetry. Moreover, in stibnite and isostructural bismuthinite the pyramidal groups are interconnected to infinite ribbons, and even to sheets in kermesite. The internal vibrations, i.e. the stretching and bending modes, of the SbS 3 groups occur at ~340–180 cm −1 , those of the BiS 3 groups in bismuthinite at ~280–150 cm −1 . The higher mass and the longer bond distances of the BiS 3 groups readily explain the lower wavenumbers of the latter. However, even for the SbS 3 groups a negative correlation between bond distances and vibrational wavenumbers is observed. Moreover, in those minerals with a wide range of pyramidal bond distances, i.e. stibnite, kermesite, bismuthinite, a more extended range of vibrational modes is observed. A clear distinction between Raman spectra of separated and interconnected SbS 3 groups is not observed.

Although falkmanite of the ideal composition proposed by Ramdohr & Odman (1940), Pb 3 Sb 2 S 6 , could not be synthesized even with the help of halide fluxes, a phase with composition similar to boulangerite, (Pb 5 Sb 4 S 11 ) 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:Sb 2 S 3 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 A, b = 21.183 A and c = 4.505 A, while that for falkmanite gave a monoclinic cell with a = 21.626 A, b = 11.409 A, c = 7.876 A 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 Cu 2 S-PbS-Sb 2 S 3 touched the PbS-Sb 2 S 3 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-Sb 2 S 3 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.