Spectres vibrationnels des silicates et germanates renfermant des anneaux, M 3O 9 (M  Si, Ge)—I. Attribution des fréquences caractéristiques de l'anneau M 3O 9, dans les composés de type bénitoïte, wadéite et tétragermanate

Abstract

A number of cyclo-silicates and germanates with the benitoïte, wadeite or tetra-germanate structure have been investigated by i.r. and Raman spectroscopy, so as to evidence the essential spectroscopic features of the Si 3O 9 and Ge 3O 9 rings and to show how these features are modified under the influence of a change of structure, with or without modification of the local symmetry of the ring. One band in the i.r. spectrum and two bands in the Raman spectrum have been found to be highly specific of the ring structure. The two Raman bands are observed in the 600-500 cm −1 region for the silicates, and 550-450 cm −1 region for the germanates; they correspond to bending motions of the ring, involving a predominant displacement of the ring oxygens, and no large displacement of the cations, as evidenced by the small 28Si- 30Si isotopic shift. The characteristic i.r. band is observed in the 760-700 cm −1 region for the silicates, and the 550-500 cm −1 region for the germanates, with a large (14 cm −1) 28Si- 30Si isotopic shift. This important mass effect of the cation is also reflected in the large frequency ratio ν(silicate) ν(germanate). This band is unique as long as the ring has the local C 3 h symmetry, irrespective of the crystal structure (benitoïte or wadeite); but it is split into a multiplet for distorted rings. It is thus a sensitive means of detecting the local symmetry of the ring.