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

Abstract

IR and Raman spectroscopy study on two collected cyclosilicate species: schorl (from tourmaline group), Na(Fe,Mg) 3Al 6(BO 3) 3Si 6O 18(OH,F) 4 and beryl (Be,Mg,Fe) 3Al 2Si 6O 18 were carried out. Although beryl is nominally anhydrous mineral, vibrational results strongly indicate that H 2O molecules exist in the structural channels. The number of vibrational bands and their frequencies revealed the presence of H 2O type II, in which C 2 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.