The Raman spectra of the orthopyroxene samples (hypersthene and bronzite) studied are shown in figure 1 (see reference paper or above spectrum - in the case of bronzite) and the bands are listed in the below table, compared with the values obtained by Huang et al. (2000).

In theory, orthopyroxenes have 240 vibration modes: 30A_1g (R) + 30B_1g (R) + 30B_2g (R) + 30B_3g (R) + 30A_1u + 30B_1u (IR) + 30B_2u (IR) + 30B_3u (IR) (R – Raman; IR – infrared) (Chopelas, 1999). Fewer modes were observed in our Raman spectra.

Both spectra are dominated by intense bands, 1003 cm^-1 in hypersthene and 1010 cm^-1 and 1027 cm^-1 in bronzite, assigned to the symmetric stretching of Si-O_nbr bonds. The same assignment is made for the 870 cm^-1 and 939 cm^-1 bands in hypersthene, and 858 cm^-1, 928 cm^-1 and 1097 cm^-1 in bronzite. In bronzite, the 1027 cm^-1 band is more intense than the 1010 cm^-1 one. The intensity of the peaks may vary with the orientation of the crystal (Huang et al., 2000). The 650-750 cm^-1 region is assigned to the symmetric stretching of Si-O_br. The stronger mode from this region appears as two peaks (662 cm^-1 and 681 cm^-1 in bronzite; 656 cm^-1 and 674 cm^-1 in hypersthene) due to the Pbca structure which has two symmetrically distinct tetrahedral chains. The peaks between 500-600 cm^-1 are attributed to the O-Si-O bending modes. The bands below 500 cm^-1 are due to the vibration modes of M-O bonds (Huang et al., 2000).

Both hypersthene and bronzite spectra are similar, with small variations of the peak positions. Huang et al. (2000) showed that these variations in enstatite-ferrosilite series are due to the Fe²⁺ content. The wavenumbers of the Raman bands decrease with the increasing of Fe concentration (from enstatite to ferrosilite), with the exception of the ~860 cm^-1 and ~930 cm^-1 bands, where a positive correlation is observed (see below table). This characteristic appears in our spectra, as well. A few more weak bands were observed in the present study: in bronzite – 480 cm^-1, 585 cm^-1, 731 cm^-1 and 1097 cm^-1, and in hypersthenes – 290 cm^-1, 427 cm^-1, 454 cm^-1 and 581 cm^-1. The hypersthene bands of 406 cm^-1 and 1013 cm^-1, reported by Huang et al. (2000), were not observed in our spectra.

In orthopyroxene spectra, the wavenumbers of the Raman bands decrease with the increasing of the Fe concentration (from enstatite to ferrosilite), caused by the slightly greater size of the Fe²⁺ cation, compared to that of the Mg²⁺. The frequency of a Raman band is determined by the bond strength and the atomic mass, the frequency increasing with the increasing of the bond strength. Atoms with small ionic radius form shorter bonds, therefore the bonds will have greater strength. Mg²⁺-O bonds are stronger than Fe²⁺-O bonds, causing a restricted motion of the other bonds and lowering the position of the bands in samples with a higher Fe concentration (see below table).

• The Mineralogy Database [link]

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

• Chopelas, A., (1999) - Estimates of mantle relevant Clapeyron slopes in the MgSiO₃ system from high-pressure spectroscopic data. American Mineralogist, 84, 233–244.

• Huang, E., Chen, C.H., Huang, T., Lin, E.H., Xu, J., (2000) - Raman spectroscopic characteristics of Mg-Fe-Ca pyroxenes. American Mineralogist, 85, 473–479.