Vibrational spectra of (M3MS3)-M-I-S-III type synthetic minerals (M-I = Tl or Ag and M-III = As or Sb)

The vibrational (Raman and far infrared) spectra of four (M3MS3)-M-I-S-III type synthetic minerals (proustite, Ag3AsS3; ellisite, Tl3AsS3; pyrargyrite, Ag3SbS3; stibioellisite, Tl3SbS3) in the region from 600 to 100 cm(-1) (Raman) and 600 to 20cm(-1) (far infrared) were investigated. Their infrared and Raman spectra were compared with corresponding vibrational spectra of natural orpiment, As2S3, and stibnite, Sb2S3. In general, rather expressed similarity between the Raman spectra of the (M3AsS3)-As-I type synthetic minerals, and particularly between the (M3SbS3)-Sb-I type synthetic minerals (M-I = Ag or Tl) was observed. This is, most probably, due to the presence of the (MS3)-S-III pyramids (M-III = As or Sb) as main structural units in all minerals. The observed similarity between the Raman spectra of the (M3AsS3)-As-I type minerals and the corresponding spectrum of orpiment, As2S3 (also built up of (MS3)-S-III pyramids), justifies the treatment of those pyramids as a main vibrational units. The agreement between the Raman spectra of (M3SbS3)-Sb-I type minerals, on the one hand, and the Raman spectrum of stibnite, Sb2S3, on the other hand, is even much more pronounced. It was found, however, that the bands in the infrared spectra in the studied (M3MS3)-M-I-S-III type minerals, besides the sensitivity to the M-III atom (As or Sb), are significantly influenced by the nature of the M-I atom (Ag or Tl). This is most probably related to the different extent of the covalent character of the Ag-S bonds compared to the Tl-S bonds. (C) 2003 Published by Elsevier B.V