Minerals from Macedonia: XXIII. Spectroscopic and structural characterization of schorl and beryl cyclosilicates

IR and Raman spectroscopy study on two collected cyclosilicate species: schorl (from tourmaline group), Na(Fe,Mg)3Al6(BO3)3Si6O18(OH,F)4 and beryl (Be,Mg,Fe)3Al2Si6O18 were carried out. Although beryl is nominally anhydrous mineral, vibrational results strongly indicate that H2O molecules exist in the structural channels. The number of vibrational bands and their frequencies revealed the presence of H2O type II, in which C2 symmetry axis of the water molecule is parallel to the structural channel (and to the c-axis of beryl). On the other hand, it was found that observed bands in the IR and Raman OH stretching region of the other tourmaline varieties appear as a result of the cation combinations involving dominant presence of Mg and Fe cations in the Y structural sites. The strong indication derived from the vibrational spectroscopic results that the studied mineral represents a schorl variety, coincide very well with the results obtained by powder X-ray diffraction and X-ray microprobe analysis. Both minerals show IR spectral similarities in the region below 1500 cm−1, whereas the resemblance between the Raman spectra (1500–100 cm−1) is less expressed confirming that these spectra are more sensitive to compositional changes and to structural disorder. The identification of both minerals was additionally supported by studying the powder X-ray diffraction diagrams.