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

¹ Geology Department, Faculty of Science, Zagazig University, Zagazig City, El-Sharkia, Egypt
² Institut für Mineralogie und Kristallographie, Universität Wien – Geozentrum, Althanstraße 14, A-1090 Wien, Austria

^* Corresponding author, e-mail: eugen.libowitzky{at}univie.ac.at

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 ~340–180 cm^–1, those of the BiS₃ groups in bismuthinite at ~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.

Key-words: Raman spectroscopy, stibnite, bismuthinite, kermesite, stephanite, bournonite, sulfosalts.