Raman study of anorthite, calcium Tschermak's pyroxene, and gehlenite in crystalline and glassy states

Raman spectra are reported for crystalline anorthite (An), calcium Tschermak's pyrox-ene (CaTs), gehlenite (Geh), and glasses of CaAl2Si2Os (An), CaAlAlSiO6 (CaTs), and CazAlAlSiOz (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 TOa 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 y,(T-O-T) band appears at lower frequency (-564 cm-') than the z"(T-O-T) band of crystalline CaTs (651 cm-'). The 2,,(T-O-T) bands in the 900-1200 cm-r region are also at lower frequencies and are relatively stronger than the zu,(T-O-T) bands in the glass of An composition. The lower frequency of the zu,(T-O-T) is due to an increase in Al/Si from I in An glass to 2 in CaT