The crystal chemistry of duftite, PbCuAsO 4 (OH) and the beta -duftite problem

Duftite, PbCu(AsO 4)(OH) is orthorhombic, space group P212121 with a = 7.768(1), b = 9. 211(1), c = 5.999(1) AE , Z = 4; the structure has been refined to R = 4.6 % and Rw = 6.5% using 640 observed reflections [F> 2 s(F)]. The structure consists of chains of edge-sharing CuO 6 ‘octahedra’, parallel to c; which are linked via AsO 4 tetrahedra and Pb atoms in distorted square antiprismatic co-ordination to form a three dimensional network. The CuO 6 ‘octahedra’ show Jahn-Teller distortion with the elongation running approximately along . The hydrogen bonding network in the structure was characterized using bond valence calculations. ‘b-duftite’ is an intermediate in the duftite‐ conichalcite series, which has a modulated structure based on the intergrowth of the two structures in domains of approximately 50 A E . The origin of the modulation is thought to be associated with displacements in the oxygen lattice and is related to the orientation of the Jahn-Teller distortion of CuO 6 ‘octahedra’. Approximately half of the strips show an elongation parallel to while the other strips are elongated parallel to [010]. This ordering results in an increase in the b cell repeat compared to duftite and conichalcite.