ORTHOWALPURGITE, (UO2)BI4O4(ASO4)(2).2H(2)O, A NEW MINERAL FROM THE BLACK-FOREST, GERMANY

The new mineral orthowalpurgite was found associated with preisingerit e, quartz and anatase on a specimen from the dump of the Schmiedestoll en, Wittichen, Black Forest, Germany. It forms transparent, yellow, or thorhombic crystals with 2/m symmetry up to 0.3 mm, tabular parallel t o {010}, elongated parallel to [100]; crystallographic forms are {010} ,{010},{104}, and {100}; Mohs' hardness 41/2, density(calc.) = 6.51 g cm(-3). The crystals are brittle, show a conchoidal fracture, and an i ndistinct cleavage parallel to {001}. Orthowalpurgite is biaxial negat ive, n(x) = 1.98(2), n(y) = 2.00(2) and n(z) = 2.05 (calculated), 2V(x ) = 70(3)degrees (lambda = 589 nm); orientation: X parallel to c, Y pa rallel to a, and Z parallel to b. Neither pleochroism nor dispersion w ere observed. Electron-microprobe analyses gave UO3 17.86 wt.%, Bi2O3 64.21 wt.%, As2O5 16.11 wt.%, H2O (calculated from the idealized formu la) 2.43 wt.%, Sigma 100.61 wt.% this yielded the empirical formula (b ased on 16 oxygen atoms) (UO2)(0.92)Bi4.06O3.89(ASO(4))(2.07). 1.99H(2 )0, ideally (UO2)Bi4O4(AsO4)(2.2)H2O The cell parameters are a = 5.492 (1) Angstrom, b = 13.324(2) Angstrom, c = 20.685(3) A, V = 1513.6 Angs trom(3), space group Pbcm, Z = 4. A crystal structure investigation wa s performed using single-crystal X-ray data. The refinement converged at R = 0.080 and R(w) = 0.031 for 999 reflections with sin theta/lambd a less than or equal to 0.6 Angstrom(-1) [F-o > 2 sigma(F-o)] and 68 v ariable parameters. The coordination polyhedra around the Bi([4+2])and Bi-[5+2] atoms are edge- and corner-connected to Bi8O8 layers. The la yers are interconnected via (UO2)(AsO4)(2) chains to form a three-dime nsional network. The name orthowalpurgite was chosen to express the st ructural relationship to the triclinic polymorph walpurgite.