Minerals from Macedonia. XVI. Vibrational spectra of some common appearing pyroxenes and pyroxenoids

Vibrational spectra of three pyroxenes: augite, (Ca,Na)(Mg,Fe,Mn,Al)(Si,Al)2O6, ferrojohannsenite, Ca(Fe,Mn)Si2O6 and carpholite, MnAl2Si2O6(OH)4 as well as two pyroxenoids: rhodonite, (Mn,Ca)MnSi2O6 and bustamite, (Ca,Mn,Fe)3Si3O9 were described and interpreted. The spectral behaviour was discussed in terms of their common structural characteristics since the pyroxenoids differ from pyroxenes by having distorted or twisted tetrahedral SiO4 chains. It results with the subtle differences in the IR spectra mostly pronounced in the 800–650 cm−1 region. The bands in this region could serve as an indication about the number of the tetrahedra in repeating unit. The existence of the Raman counterparts of the IR bands (so called IR–Raman doublets) were observed although the centre of inversion is present in the structures of the minerals. It was explained by the existence of TOT I-strips in the structures whose stacking sequence symmetry dominates over the crystal symmetry and determines the symmetry and the activity of the normal modes.