Raman spectra of some tectosilicates and of glasses along the orthoclase-anorthite and nepheline-anorthite joins

Raman spectra are reported for powdered crystalline anorthite, sanidine, leucite, α-nepheline, and α-carnegieite and glasses of CaAl₂Si₂O₈ (An), KAlSi₃O₈ (Or), and NaAlSiO₄ (Ne) compositions. Qualitative comparisons between features in the spectra of the isochemical crystals and glasses indicate that the glasses maintain short-range (coordination) relationships similar to those of the crystalline polymorphs. Intermediate-range order (TO₄ ring structures; T = Si, Al) of the glasses is inferred from the frequency of the dominant symmetric stretch v_s(T-O-T) band in their Raman spectra. The ring structure of An glass is similar to that of the crystalline feldspars, consisting predominantly of four-membered TO₄ rings. Its Raman spectrum indicates that the melt-quenched glass of Or composition contains domains of leucite-like structure, consisting of alternating four- and six-membered TO₄ rings, and regions of predominantly six-membered ring structures. The ring structure of Ne glass consists of six-membered TO₄ rings, similar to its isochemical crystalline polymorphs, although the rings are highly puckered. High-frequency antisymmetric-stretching features in the spectra of the tectosilicate glasses are explained in terms of site symmetries of TO₄ tetrahedra due to the distribution of SiO₄ and AlO₄ units in the glass. Variations in the ring sizes in glasses along the Or-An and Ne-An joins are reflected by shifts of the v_s(T-0-T) bands in the glass spectra. Antisymmetric-stretching features in the spectra of glasses along the Or-An join vary systematically as Si:Al changes from 3:1 in Or to 1:1 in An but vary only slightly in the spectra of glasses along the Ne-An join because of the common Si:Al of all glasses along that join.