Static and dynamic Jahn-Teller effects in Cu2+ oxysalt minerals

The details of Jahn-Teller distorted Cu(2+)phi(6) (phi: O2-, OH-, H2O) octahedral geometries in Cu2+ oxysalt minerals are examined. Usually the Cu(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(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(2+)phi(6), octahedra in mineral structures shows that there is no conclusive evidence of a holosymmetric Cu(2+)phi(6), octahedron. The presence of (2 + 4)-distorted Cu(2+)phi(6), octahedra in the structures of volborthite and KCu32+(OH)(2)[(AsO4)H(AsO4)], and a (2 + 2 + 2)-distorted Cu(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(2+)phi(6) oxysalt structures. The most persuasive example of a true (2 + 4)-distorted Cu(2+)phi(6) octahedron occurs in the structure of demesmaekerite.