Minerals from Macedonia. XXIV. Spectra-structure characterization of tectosilicates

Spectra-structure studies were undertaken on the collected representatives of the alkali feldspar species: albite, NaAlSi3O8; microcline, KAlSi3O8 and sanidine, (K,Na)(Al,Si)4O8 by vibrational spectroscopy. Despite their common structural characteristics, it was shown that Raman (1200–100 cm−1) and particularly IR spectra (1600–370 cm−1) could clearly discriminate between albite, microcline and sanidine. The IR and Raman bands are greatly enhanced by ordering in the Si/Al site being much intense in microcline (highly ordered feldspar) and significantly weaker in sanidine (disordered feldspar). Frequency band shifting and increase of the bandwidths in the vibrational spectra of alkali feldspars is also related to the increase of the Al/Si disorder degree. The highly ordered feldspar specimen albite also exhibit more bands in the SiO stretching spectral region. In addition, an attempt for detailed interpretation of the powder vibrational spectra of rare-occurring zeolite mineral, stilbite (NaCa4Al8Si28O72·30H2O), was undertaken. The authenticity of the minerals was confirmed by X-ray powder diffraction and the chemical formulas were derived according to the results from the X-ray microprobe analysis.