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Complementary use of powder XRD and vibrational spectroscopy for identification of pyroxene and pyroxenoid minerals from Macedonia

The powder XRD and vibrational (infrared and Raman) spectroscopy were used for identification of some pyroxene [augite, (Ca, Na)(Mg, Fe, Mn, Al)(Si, Al)_2O_6 ; ferrojo-hannsenite, Ca(Fe, Mn)Si_2O_6] and pyroxenoid minerals [rhodonite, (Mn, Ca)MnSi_2O_6 ; bustamite, (Ca, Mn, Fe)_3Si_3O_9] collected from Macedonia. Since the pyroxenoids differ from pyroxenes by having distorted or twisted tetrahedral SiO_4 chains, the spectral behaviour was discussed in terms of their common structural characteristics. It resulted with the subtle differences in the IR spectra mostly pronounced in the 800-650 cm^-1 region. Namely, it was found that the bands that appear in this region served as an indication about the number of the tetrahedra in repeating unit. The existence of the Raman counterparts of the IR bands (although the centre of inversion is present in the structures of the minerals) was explained by the presence of tetrahedral-octahedral-tetrahedral (TOT) strips in the structures whose stacking sequence symmetry dominates over the crystal symmetry and determines the activity of the normal modes. The difficulties concerning the presence of impurities in rhodonite and ferrojohannsenite sample, despite by the vibrational techniques, were also outrun by the use of the powder XRD. Summarized discussion in that context will be given here.

Macedonian minerals ; X-ray diffraction ; Vibrational spectroscopy

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