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 (Tn+O4) (super -(8-n)) with T = S (super 6+) or V (super 5+) , or triangular pyramidal oxyanions (SeO 3 ) (super 2-) (which may be considered as (SeO 3 E) (super 2-) tetrahedra assuming the lone pair E of selenium to be a fourth ligand): (T (super n+) O4) (super -(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.

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