‘Face-to-Face’ relationships between oxocentred tetrahedra and cation-centred tetrahedral oxyanions in crystal structures of minerals and inorganic compounds

Abstract Detailed analysis of the crystal structures of minerals and inorganic compounds containing oxocentred [OCu 4 ] tetrahedra reveals the following principle for relationships between oxocentred tetrahedra and tetrahedral oxyanions (T n + O 4 ) −(8− n ) with T = S 6+ or V 5+ , or triangular pyramidal oxyanions (SeO 3 ) 2− (which may be considered as (SeO 3 E) 2− tetrahedra assuming the lone pair E of selenium to be a fourth ligand): (T n + O 4 ) −(8− n ) tetrahedra are ‘attached’ to the oxocentred tetrahedra so that their grounds (O-O-O and Cu-Cu-Cu) are parallel to each other and the mutual orientation is regular (O corners are exactly under Cu corners). We propose to describe these relationships as ‘face-to-face’, meaning that the oxocentred and cation-centred tetrahedra ‘meet’ along their ‘whole’ triangular faces. Geometric characteristics of ‘face-to-face’ relationships are discussed.