Given the fact that these spectra are very complex and that their quality is not very good (low signal-to-noise ratio), we divided the spectra (figs. 10a and 10b; see reference paper¹) into two major regions: 210-480 cm^-1 and 480-1200 cm^-1. Apart from this, these spectra were difficult to obtain due to high fluorescence. Riebeckite has a variety called crocidolite that is asbestiform in habit, while riebeckite is a non-fibrous sample. According to Bard et al. (1997), a comparison between the spectra of fibrous (crocidolite) and non-fibrous (riebeckite) samples shows no evident change in the 200-1200 cm^-1 spectral region; only the OH^- stretching region presents differences in the number of peaks, but our spectral domain is limited to 3400 cm^-1.

Figure 10a shows in the upper part a compressed spectral region from the same frequency range (210-480 cm^-1). In this compressed spectra, we can understand the vibrations ascribed to lattice modes, M-O and some of the Si₄O₁₁ deformation modes more easily. These spectra that were compressed are important in understanding the similar bands between all spectra presented in figure 10a, and in clearing any doubt regarding what bands are caused by a low signal-to-noise ratio.

The bands between 213 and 298 cm^-1 (with some peaks shifted or some bands absent) may be assigned to lattice vibrations. In the reference spectrum (R060028), one band appears at 329 cm^-1, while in our spectra (4-2, 4-2 and 4-2) this spectral line does not appear. In the 350-480 cm^-1 spectral region, some peaks appear at different frequencies; however, these bands cannot be compared because the low signal-to-noise ratio influences the interpretation of these peaks.

The assignment of the bands in the 480-1200 cm^-1 region follows the previous assignment of the peaks. The assignment of the spectral lines is summarized in table 9 (see reference paper¹).

The bands between 503 and 615 cm^-1 may be ascribed to Si₄O₁₁ deformation modes. These peaks (~537 and ~576 cm^-1 doublet peaks), at these values, are a factor in the discrimination between a riebeckite/crocidolite sample and other samples discussed above.

The following bands: 1042 and 1082 cm^-1 (for 4-2); 1041 and 1080 cm^-1 (4-2); 1042, 1076 and 1087 cm^-1 (4-2); 1042 and 1083 cm^-1 (R060028) correspond to the asymmetric stretching (ν_as) vibration modes of the Si-O_b-Si groups.

The Raman spectrum of the riebeckite samples (4-2 and 4-2) shows one peak at 2330 cm^-1 and one band at 2433 cm^-1 (in the case of the 4-2 spectrum); the first band (2330 cm^-1) may be assigned to H₃O⁺ vibration, and the second (2433 cm^-1) – to NH₄⁺. This is due to a substitution of cations from the M sites by H₃O⁺ or NH₄⁺.

• The Mineralogy Database [link]

• Crystal data (.cif file) from the American Mineralogist Crystal Structure Database [link]

• ¹APOPEI A. I., BUZGAR N. (2010) - The Raman study of amphiboles. Anal. Şt. Univ. “Al. I. Cuza” Iaşi, Geologie, LVI/1 [link]

• Bard, T., Yarwood, J., Tylee, B. (1997) - Asbestos fiber identification by Raman microspectroscopy. J. Raman Spectrosc., 28, 803-809

• Riebeckite spectrum from the RRUFF project [link]

• Downs, R. T. 2006. The RRUFF Project: an integrated study of the chemistry, crystallography, Raman and infrared spectroscopy of minerals. Program and Abstracts of the 19th General Meeting of the International Mineralogical Association in Kobe, Japan. O03-13.