The details of Jahn-Teller distorted Cu (super 2+) phi 6 (phi : O (super 2-) , OH (super -) , H 2 O) octahedral geometries in Cu (super 2+) oxysalt minerals are examined. Usually the Cu (super 2+) phi 6 octahedron is (4+2)-distorted, although both (2+4)-distorted and holosymmetric octahedra have been reported in mineral structures. To a first order, the Jahn-Teller theorem indicates that either a (4+2) or (2+4) distortion of the Cu (super 2+) phi 6 octahedron is equally likely to occur, in apparent conflict with the dominance of (4+2)-distorted octahedra; this requires an extension of the Jahn-Teller theorem. Examination of reported holosymmetric Cu (super 2+) phi 6 octahedra in mineral structures shows that there is no conclusive evidence of a holosymmetric Cu (super 2+) phi 6 octahedron. The presence of (2+4)-distorted Cu (super 2+) phi 6 octahedra in the structures of volborthite and KCu (super 2+) 3 (OH) 2 [(ASO 4 )H(ASO 4 )], and a (2+2+2)-distorted Cu (super 2+) phi 6 octahedron in the structures of bayldonite and cyanochroite is attributed to a dynamic Jahn-Teller effect, rather than the static distortion that is usual in Cu (super 2+) phi 6 oxysalt structures. The most persuasive example of a true (2+4)-distorted Cu (super 2+) phi 6 octahedron occurs in the structure of demesmaekerite.

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