Ring distributions in alkali- and alkaline-earth aluminosilicate framework glasses- A raman spectroscopic study

Raman spectra of crystalline polymorphs of a number of tectosilicate minerals having various sizes of smallest rings of TO₄ tetrahedra (T = Si, Al) have been investigated to identify the bands that are sensitive indicators of the smallest rings in the network. The information obtained from the Raman spectra of tectosilicate minerals (e.g., SiO₂ polymorphs, NaAlSi₃O₈ (Ab), NaAlSiO₄ (Ne), KAlSi₃O₈ (Or), and KAlSi₂O₆ (Lc)) is used to interpret the Raman spectra of the isochemical glasses. It is shown that the frequency of the dominant ν_s (TOT) band in the spectra of both crystals and glasses is related to the dominant size of TO₄ rings in the structure. In agreement with previous X-ray RDF work, it is found that in the glasses of Ab and Jd (NaAlSi₂O₆) compositions, six-membered rings of TO₄ tetrahedra predominate. The Raman spectrum of Or glass, however, indicates that clusters of intermixed four- and six-membered rings of TO₄ tetrahedra, similar to those existing in crystalline leucite, are also present in the glass. Raman evidence indicates that four-membered rings of TO₄ tetrahedra predominate in the glass of An composition. Similarly, the higher frequency of the ν_s (TOT) band in the spectrum of Ne glass as compared with the frequency the ν_s (TOT) band in the spectra of crystalline cargenieite and nephelite indicates either an admixture of the four- and six-membered rings or the puckering of six-membered rings in the glass structure.