REVISION OF THE CRYSTAL-STRUCTURE OF MRAZEKITE, BI2CU3(OH)2O2(PO4)2-CENTER-DOT-2H2O

Mrazekite, Bi2Cu3(OH)2O2(PO4)2.2H2O, is described from its second occu rrence in silicified barite veins from Gadernheim and Reichenbach, Ode nwald, Hesse, Germany. It forms blue crystals up to 0.5 mm across that are tabular on {101} and elongate parallel to [010]. The most promine nt forms are {012}, {101}, and {103BAR}. Mrazekite is biaxial negative , 2V = 66.0(5)degrees, r < nu strong; the mean index of refraction n i s 1.86-1.87, X LAMBDA c almost-equal-to 27-degrees, Y LAMBDA a = 15-de grees, and Z parallel-to b. Electron-microprobe analyses gave Bi2O3 50 .1 wt.%, CuO 25.4%, PbO 0.10%, P2O5 15.2%, As2O5 0.26%, V2O5 0.25%, H2 O (calc.) 6.0%, total 97.3%. This yielded the empirical formula 02.00[ (PO4)1.95(AsO4)0.02(VO4)0.02]SIGMA=1.99.1.88 H2O. A reinvestigation of the crystal structure of mrazekite was performed using single-crystal X-ray data. The space group is P2(1)/n, with a 9.065(1), b 6.340(l), c 21.239(3) angstrom, beta 101.57(1)degrees, V 1195.8(2) angstrom3, Z = 4. The structure refinement converged for 2995 observed unique refle ctions and 194 free parameters at R = 0.050. The [2+2]Bi3+O4 polyhedra are edge-connected to form Bi2O6 dimers, <Bi-O> being 2.12 angstrom a nd 2.41 angstrom for the short and longer bonds. The coordination is c ompleted by four ligands at a distance between 2.71 and 3.09 angstrom. The Cu atoms have four ligands with <Cu-O> = 1.95 angstrom; two addit ional ligands have Cu-O between 2.63 and 2.85 angstrom. The lengths of the hydrogen bonds in the hydroxyl groups are 2.67 (angstrom); the wa ter molecules have O(w)...O contacts greater than or equal to 2.98 ang strom. Two thirds of the CuO4 squares are comer-connected to rows in [ 010]; the PO4 tetrahedra are linked to these rows to form ribbons that are interconnected by the remaining third part of the Cu atoms. The r esulting corrugated sheets parallel to (103BAR) are linked by Bi atoms and hydrogen bonds.