The Raman spectra of rhodonite and fowlerite are shown in figure 8 (see reference paper¹). Fowlerite is the zinc-rich variety of rhodonite, with the formula (Mn,Zn)SiO₃. In the 870-1045 cm^-1 region, the modes of Si-O_nbr bonds are observed, the highest band appearing as a doublet at ~973 cm^-1 and ~997 cm^-1, caused by the structure of rhodonite, where the five repeating tetrahedra of SiO₄ have different Si-O bond lengths. The region assigned in the pyroxenes to the stretching modes of the Si-O_br bonds is present in the rhodonite spectra at 667 cm^-1. No splitting of this peak is observed; weak bands appear at 714 cm^-1, and an additional one in fowlerite at 623 cm^-1. The bending modes of O-Si-O are observed at 510 cm^-1 and 557 cm^-1 in rhodonite, and 514 cm^-1 and 555 cm^-1 in fowlerite. The cation-oxygen vibration modes appear in the low region of the spectra below 420 cm^-1 (see below table). There are some differences in the Raman spectra of the studied samples. The stretching modes of Si-O_nbr are slightly shifted on higher frequencies in fowlerite, except for the ~877 cm^-1 and ~937 cm^-1 bands. This behaviour is also observed for the bending modes. In the region of the M-O modes, the bands at 250 cm^-1 and 265 cm^-1 in rhodonite appear in fowlerite as a single one at 258 cm^-1; the band at 327 cm^-1 in rhodonite is observed at 335 cm^-1 in fowlerite. The presence of Zn in fowlerite is causing these changes; a smaller size of Zn compared with Ca is creating stronger bonds, producing the shifting of the bands on higher wavenumbers in fowlerite.

Only a few studies were made on charoite (Rogova et al., 1978; Rozhdestvenskaya et al., 2009). Downs (2006) reported Raman spectra for charoite that are similar to the spectrum obtained in the present study; no discussions or band assignments for this mineral were found in the literature.

As in the case of wollastonite, the Raman spectrum of charoite is characterized by a strong fluorescence and background noise and the peaks are very weak and only a few can be distinguished. The structure of this mineral is still not clear, so an assigment of the bands is difficult to achieve. Like the other silicates wich contain chains of SiO₄ tetrahedra, we can assume that the 638 cm^-1, 675 cm^-1, 1054 cm^-1, 1116 cm^-1 and 1135 cm^-1 peaks are due to the Si-O bending/stretching modes. In the region of the M-O vibrations, the bands are very weak and only 242 cm^-1 and 434 cm^-1 were observed. In the high region of the spectrum, two new lines were observed at 2367 cm^-1 and 2403 cm^-1. Weinstein et al. (2001) reported these Raman bands for the vibrations of the N-H bonds in gallium nitride samples implanted with H⁺. It is possible that charoite contains NH₄⁺ ions in its structure, through the substitution of K⁺ atoms.

• 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]

• 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.