Raman spectra are reported for crystalline anorthite (An), calcium Tschermak’s pyroxene (CaTs), gehlenite (Geh), and glasses of CaAl2Si2O8 (An), CaAlAlSiO6 (CaTs), and Ca2AlAlSiO7 (Geh) composition. A small degree of Si-Al disorder in anorthite is indicated by the presence of weak Si-O-Si antisymmetric stretching modes in its spectrum. The spectrum of the glass of An composition closely resembles that of crystalline An. This correspondence indicates that the predominant species in An glass is composed of a random network of four-membered rings of T04 tetrahedra (where T = Si or Al). The spectra of glasses of CaTs and Geh compositions do not resemble the spectra of their respective crystals.

In the spectrum of glass of CaTs composition the νs(T-0-T) band appears at lower frequency (∼564 cm-1) than the νs(T-0-T) band of crystalline CaTs (651 cm-1). The νas(T-O-T) bands in the 900-1200 cm-1 region are also at lower frequencies and are relatively stronger than the νas(T-0-T) bands in the glass of An composition. The lower frequency of the νas(T-0-T) is due to an increase in Al/Si from 1 in An glass to 2 in CaTs glass. The increase in the relative intensity of the νas(T-0-T) bands is due to nonbridging oxygens. The presence of free Ca2+ ions, and possibly a small amount of Al3+ in six-fold coordination, is responsible for these nonbridging oxygens in the glass.

In the spectrum of glass of Geh composition the νs(T-O-T) band appears at lower frequency (552 cm−1) than in the spectrum of crystalline Geh (626 cm−1). The shift of the band toward lower frequencies in the spectrum of the glass of Geh composition indicates that most of the Al3+ ions are tetrahedrally coordinated and act as network formers. A strong band at ∼896 cm−1 in the spectrum of Geh glass is attributed to the presence of an appreciable amount of SiO44- groups in the glass network. The structure of glass of Geh composition is thus highly polymerized with CaAl2O4 present in the network. The 0.33 nonbridging oxygen per network-forming cation (Si-Al), as required by the stoichiometry of the glass to charge-balance the Ca2+ ion, exists in the form of SiO44- groups in the glass network.

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