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

Detailed analysis of the crystal structures of minerals and inorganic compo unds containing oxocentred [OCu4] tetrahedra reveals the following principl e for relationships between oxocentred tetrahedra and tetrahedral oxyanions (Tn+O4)(-(8-n)) with T = S6+ or V5+, or triangular pyramidal oxyanions (Se O3)(2-) (which may be considered as (SeO3E)(2-) tetrahedra assuming the lon e 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 C u-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 relat ionships as 'face-to-face', meaning that the oxocentred and cation-centred tetrahedra 'meet' along their 'whole' triangular faces. Geometric character istics of 'face-to-face' relationships are discussed.