In the Raman spectrum of augite (see above spectrum), the stretching modes of Si-O_nbr are observed to have the highest intensity at 1006 cm^-1, and weaker peaks at 863 cm^-1, 928 cm^-1, 1043 cm^-1 and 1102 cm^-1. The bands assigned to the stretching of the Si-O_br bonds are located at 667 cm^-1, 707 cm^-1 and 769 cm^-1. The 533 cm^-1 and 555 cm^-1 bands are attributed to the bending of O-Si-O bonds. The cation-oxygen vibrations appear at lower frequencies, below 400 cm^-1 (see below table).

Based on the chemical composition of the augite specimens from the Techereu area (Apuseni Mountains, Romania), the formula resulting for this mineral is the following: (Ca_0.844Na_0.034K_0.009Mg_0.113)(Mg_0.737Fe³⁺_0.226Fe²⁺_0,007Ti_0,014Al_0.014Mn_0.002)[(Si_1.832Al_0.168)O₆] (see below table - 2nd table) (Stoicovici, 1968). The high content of Ca and Mg makes the augite from Techereu very similar to the diopside structure. The Raman spectrum of augite is also similar to the diopside spectrum (see figure 2 from reference paper¹). The bands in the low region of the augite and diopside Raman spectra are located at a maximum difference of 5 cm^-1; the highest peak assigned to the stretching modes of Si-O_nbr is slightly lower in augite than in diopside (1006 cm^-1 and 1010 cm^-1, respectively), caused probably by the Al content in the tetrahedral sites. More bands appear in the augite spectrum, at 707 cm^-1, 769 cm^-1 and 1102 cm^-1. From the Raman spectra of the other augite sample studied (3974), it can be concluded that the specimen is also similar to diopside; a difference in the Raman spectra appears in the 700-1000 cm^-1 region (see below table), were more bands are observed in augite than in diopside, probably due to the presence of a high Al content.

The infrared spectrum of augite (see figure 3 from reference paper¹) revealed bands at frequencies of 87 cm^-1, 969 cm^-1 and 1070 cm^-1, attributed to the stretching of the Si-O bonds; 1070 cm^-1 is considered to be caused by the Si-O_nbr stretch, while the bands at smaller frequencies are assigned to the Si-O_br stretch. The bending modes are observed at 634 cm^-1, 665 cm^-1 and 673 cm^-1. Below 520 cm^-1, the vibrations between cation and oxygen are presented as four bands: 390 cm^-1, 399 cm^-1, 478 cm^-1 and 515 cm^-1. This assignment for IR bands of augite was also made by Makreski et al. (2006).

The group theory predicts for c2/c clinopyroxenes the following normal modes of vibration: 14A_g (R) + 16B_g (R) + 13A_u (IR) + 14B_u (IR) (Rustein and White, 1971). There are no modes which are Raman and IR active at the same time. An ideal SiO₄ tetrahedron has four fundamental modes: ν₁ symmetric stretching, νM₂ symmetric bending, ν₃ asymmetric stretching and ν₄ asymmetric bending. A large cation in the structure can distort the SiO₄ tetrahedra. This will make ν₁ and ν₂ Raman active, and ν₃ and ν₄ – IR active (Mills et al., 2005). In the case of our augite, we can assume that the Ca cation is producing the distortion and then the symmetric modes will be Raman active and the asymmetric ones will be IR active. Under these circumstances, the assignment made by Makreski et al. (2006) for the IR spectrum and ours for the Raman spectrum (see above table) are correct, based also on the rule that in SiO₄ tetrahedra ν₂< ν₄ and ν₁< ν₃ (Nakamoto, 2009).

• The Mineralogy Database [link]

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

• Stoicovici, E., (1968) - The augite in the diabase from Techerău (Hunedoara). Bul. Soc. de Șt. Geol., XI, 203–211. (In Romanian).

• 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.

• Makreski, P., Jovanovski, G., Gajović, A., Biljan, T., Angelovski, D., Jaćimović, R., (2006) - Minerals from Macedonia. XVI. Vibrational spectra of some common appearing pyroxenes and pyroxenoids. Journal of Molecular Structure, 788, 102–114.

• Rutstein, M.S., White, W.B., (1971) - Vibrational spectra of high-calcium pyroxenes and pyroxenoids. American Mineralogist, 56, 877–887.

• Mills, S.J., Frost, R.L., Kloprogge, J.T., Weier, M.L., (2005) - Raman spectroscopy of the mineral rhodonite. Spectrochimica Acta, Part A, 62, 171–175.

• Nakamoto, K., (2009) - Infrared and Raman Spectra of Inorganic and Coordination Compounds, Part A: Theory and Applications in Inorganic Chemistry (Sixth edition). John Wiley and Sons, New Jersey