Detailed analysis of the crystal structures of minerals and inorganic compounds containing oxocentred [OCu4] tetrahedra reveals the following principle for relationships between oxocentred tetrahedra and tetrahedral oxyanions (Tn+O4)−(8−n) with T = S6+ or V5+, or triangular pyramidal oxyanions (SeO3)2− (which may be considered as (SeO3E)2− tetrahedra assuming the lone pair E of selenium to be a fourth ligand): (Tn+O4)−(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.
