Raman spectra of isolated and interconnected pyramidal XS3 groups (X = Sb,Bi) in stibnite, bismuthinite, kermesite, stephanite and bournonite

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 SbS3 groups with ideal trigonal symmetry, the former show distorted XS3 (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 SbS3 groups occur at similar to 340-180 cm(-1) those of the BiS3 groups in bismuthinite at similar to 280-150 cm(-1). The higher mass and the longer bond distances of the BiS3 groups readily explain the lower wavenumbers of the latter. However, even for the SbS3 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 SbS3 groups is not observed.