Acentric structure (P3) of bechererite, Zn7Cu(OH)13[SiO(OH)3SO4

Abstract

The crystal structure of bechererite from the Tonopah-Belmont mine, Arizona, was reinvestigated using a single-crystal X-ray diffractometer (MoKalpha radiation) equipped with a CCD area detector. The structure was refined in space group P3 (a = 8.319(2), c = 7.377(1) A) with 1247 unique reflections up to (sintheta )/lambda = 0.65 leading to R1 = 2.7%, wR2 = 6.4%. In agreement with the previously reported centric structure (P3), bechererite is composed of (001) brucite-like sheets formed by edge-sharing (Zn,Cu)O 6 octahedra and 1/7 ordered octahedral vacancies. The layers are connected parallel to c by ditetrahedral (Zn,Cu) 2 (OH) 7 units. Due to the attractive force of a weak hydrogen bond, the bridging O atom of the ditetrahedral unit is displaced from the threefold axis. A characteristic feature of the structure is isolated tetrahedra, which connect only with one apex to the octahedral sheet. The acentric structure (P3) reveals ordering between SiO(OH) 3 and SO 4 tetrahedra yielding characteristic distortions of the neighboring (Zn,Cu)O 6 octahedra caused by bond-valence requirements of the shared O atoms. The [SiO(OH) 3 ] (super 1-) tetrahedron is only 70(1)% occupied and is partly substituted by tetrahedral [O 4 H 7 ] (super 1-) clusters. The existence of H 2 O molecules in the [O 4 H 7 ] (super 1-) cluster is supported by FTIR spectroscopy.