ELECTRON-DENSITY AND BONDING IN RING SILICATES - BERYL, CORDIERITE, DIOPTASE

The electron density and bonding in the silicates beryl, cordierite and dioptase has been investigated. The replacement of Be, Si and Al atoms in the identical structural fragments leads to a deltarho-redistribution and changes the sigma-bond character from slightly polar to strongly polar. The condensation of [SiO4]-tetrahedra in the rings leads to the accumulation of electron density near bridge oxygens. This phenomenon is expressed especially in dioptase (pure ring silicate). Therefore, the electron density picture may be considered as a supplementary independent criterion for classification of crystal structures. The experimental deltarho-maps for H2O molecules in dioptase are in good agreement with theoretical ones. The deltarho-distribution of 3 d-electrons and bonding in the Jahn-Teller distorted Cu-octahedron can be interpreted from crystal-field-theory point of view.