Actinolite is a solid solution composition between the endmembers of the following series: tremolite and ferro-actinolite. According to Jovanovski et al. (2009), solutions with more than 90 Mg are called tremolite, between 90-50% - actinolite, and less than 50% - ferro-actinolite. We will discuss the Raman spectra of these minerals separately.

The Raman spectra of these samples are shown in figure 5 (see reference paper¹). All of these spectra are very similar, with small differences in the case of very low intensity peaks. In the 650-1200 cm^-1 spectral region (the region of internal vibrations), five intense bands appear significantly at 670, 745, 928, 1027 and 1056 cm^-1 for the 2-1 sample; 670, 744, 929, 1027 and 1059 cm^-1 for 2-2, and 670, 744, 928, 1027, 1059 cm^-1 for the 2-3 actinolite sample. In the case of the 928 cm^-1 peak (for 2-2 samples this peak exhibits at 929 cm^-1), one or two overlapped bands with a very low intensity appear. These peaks (891, 956 cm^-1; 892, 949 cm^-1 and 946 cm^-1, respectively – see fig. 5; reference paper¹) are part of O-Si-O symmetric stretching vibrations (ν_s). As discussed in the case of grunerite, the bands at 1027, 1056 cm^-1 (2-1); 1027, 1059 cm^-1 (2-2) and 1027, 1059 cm^-1 (2-3), respectively, may be ascribed to the antisymmetric stretching vibrations (ν_as) of the Si-O_b-Si bridges; the bands at 744-745 cm^-1 are debatable, given the fact that 750 cm^-1 is the limit of νs O-Si-O and νs Si-O_b-Si vibrations. The most intense bands, which appear at 670 cm^-1 (in all samples), are ascribed to the ν₁ (Ag) symmetric stretching modes (ν_s) of the Si-O_b-Si bridges.

In the 210-650 cm^-1 region, the assignments of the M-O vibrations is problematic; three bands appear in the 300-450 cm^-1 spectral region (for 2-1: 369, 389 and 413 cm^-1; 2-2: 369, 392 and 415 cm^-1, and 369, 392 and 415 cm^-1 for 2-3), where we would expect to see vibrations produced by Ca, Mg and/or Fe²⁺ cations [Ca₂(Mg,Fe²⁺)₅Si₈O₂₂(OH)₂ – ideal chemistry for actinolite]. Nevertheless, the bands between 210-300 cm^-1 are assigned to lattice modes (for 2-1: 221, 247 and 292 cm^-1, for 2-2: 226, 247, 294 cm^-1, and 222 and 292 cm^-1 for 2-3).

The following bands: 479, 523 and 573 cm^-1 (for 2-1); 482, 522 and 577 cm^-1 (2-2), and 484, 522 and 581 cm^-1 (2-3) correspond to the deformation modes of Si₄O₁₁, with the observation that in this region there should be a librational and translational vibration of the OH^- group (probably 573, 577 and 581 cm^-1 bands; see fig. 5 - reference paper¹).

The Raman spectrum of the sample of actinolite (2-1) shows one peak at 2327 cm^-1; this band may be assigned to the H₃O⁺ vibration. This is due to a substitution between H₃O⁺ and a cation from the M sites.

• 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]

• Huang, E. (2003) - Raman Spectroscopic Study of Amphiboles. PhD thesis in Chinese.

• Jovanovski, G., Makreski, P., Kaitner, B., Boev, B. (2009) - Silicate Minerals from Macedonia. Complementary Use of Vibrational Spectroscopy and X-ray Powder Diffraction for Identification and Detection Purposes. Croatica chemica acta, 82 (2), 363-386.

• Shurvell, H. F., Rintoul, L., Fredericks, P. M. (2001) - Infrared and Raman spectra of jade and jade minerals. Internet Journal of Vibrational Spectroscopy, (www.ijvs.com) 5, 5, 4.