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

Abstract

. Raman spectra are reported for powdered crystalline anorthite, sanidine, leucite, a-nepheIme, an~ ~-carnegie~te ~nd glasses .ofCaA12Si20s (An), KAlSi30s (Or), and NaAlSiO. (Ne) composItIOns. Quahtatlve compansons between features in the spectra of the isochemical c~stal~ a~d glasses indicate that the glasses maintain short-range (coordination) relationShIPSsImIlar to those of the crystalline polymorphs. Intermediate-range order (TO. ring structures; T = Si,Al) of the glasses is inferred from the frequency of the dominant symmetric stretch v,(T -0- T) band in their Raman spectra. The ring structure of An glass is similar to that of the crystalline feldspars, consisting predominantly off our-membered TO. rings. Its Raman spectrum indicates that the melt-quenched glass of Or composition contains domains ~fleucite-like structure, consisting of alternating four- and six-membered TO. rings, and ~egIons ~fpredominantly six-membered ring structures. The ring structure ofNe glass consIsts of sIx-membered TO. 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 TO. tetrahedra due to the distribution of SiO. and AIO. units in the glass. Variations in the ring sizes ~n glasses along the Or-An and Ne-An joins are reflected by shifts of the v,(T-0T) bands m 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.