Abstract

Raman spectra are reported for crystalline anorthite (An), calcium Tschermak's pyroxene (CaTs), gehlenite (Geh), and glasses of CaAl 2 Si 2 O 8 (An), CaAlAlSiO 6 (CaTs), and Ca 2 AlAlSiO 7 (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 TO 4 tetrahedra (where T = Si or Al). In the spectrum of glass CaTs composition the v s (T-O-T) band appears at lower frequency ( approximately 564 cm (super -1) ) than the v s (T-O-T) band of crystalline CaTs (651 cm (super -1) ). The v as (T-O-T) bands in the 900-1200 cm (super -1) region are also at lower frequencies and are relatively stronger than the v as (T-O-T) bands in the glass of An composition. The lower frequency of the v as (T-O-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 v as (T-O-T) bands is due to nonbridging oxygens. In the spectrum of glass of Geh composition the v s (T-0-T) band appears at lower frequency (552 cm (super -1) ) than in the spectrum of crystalline Geh (626 cm (super -1) ). The shift of the band toward lower frequencies in the spectrum of the glass of Geh composition indicates that most of the Al (super 3+) ions are tetrahedrally coordinated and act as network formers. A strong band at approximately 896 cm (super -1) in the spectrum of Geh glass is attributed to the presence of an appreciable amount of SiO (super 4-) 4 groups in the glass network.--Modified journal abstract.

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