Ianbruceite, ideally [Zn2(OH)(H2O)(AsO4)](H2O)2, a new arsenate mineral from the Tsumeb mine, Otjikoto (Oshikoto) region, Namibia: description and crystal structure

Ianbruceite, ideally [Zn-2(OH)(H2O)(AsO4)](H2O)(2), is a new supergene mineral from the Tsumeb mine, Otjikoto (Oshikoto) region, Namibia. It occurs as thin platy crystals up to 80 mu m long and a few mu m thick, which form flattened aggregates up to 0.10 mm across, and ellipsoidal aggregates up to 0.5 mm across. It is associated with coarse white leiteite, dark blue kottigite, minor legrandite and adamite. Ianbruceite is sky blue to very pale blue with a white streak and a vitreous lustre; it does not fluoresce under ultraviolet light. It has perfect cleavage parallel to (100), is flexible, and deforms plastically. The Mohs hardness is 1 and the calculated density is 3.197 g cm(-3). The refractive indices are alpha = 1.601, beta = 1.660, gamma = 1.662, all +/- 0.002; 2V(obs) = 18(2)degrees, 2V(calc) = 20 degrees, and the dispersion is r < v, weak. Ianbruceite is monoclinic, space group P2(I)/c, a = 11.793(2), b = 9.1138(14), c = 6.826500) angstrom, beta = 103.859(9)degrees, V = 712.3(3) angstrom(3), Z = 4, a:b:c = 1.2940:1:0.7490. The seven strongest lines in the X-ray powder diffraction pattern [d (angstrom), I, (hkI)] are as follows: 11.29, 100, (100); 2.922, 17, (130); 3.143, 15, (<(2)over bar>02); 3.744, 11, (300); 2.655, 9, (230); 1.598, 8, ((1) over bar 52); 2.252, 7, (222). Chemical analysis by electron microprobe gave As2O5 36.27, As2O3 1.26, Al2O3 0.37, ZnO 49.72, MnO 0.32, FeO 0.71, K2O 0.25, H2Ocalc 19.89, sum 108.79 wt.%; the very high oxide sum is due to the fact that the calculated H2O content is determined from crystal-structure analysis, but H2O is lost under vacuum in the electron microprobe. The crystal structure of ianbruceite was solved by direct methods and refined to an R-1 index of 8.6%. The As is tetrahedrally coordinated by four O anions with a mean As-O distance of 1.687 angstrom. Zigzag [(ZnZr)-Zn-[5]-Zr-[6]phi(7)] chains extend in the c direction and are linked in the b direction by sharing corners with (AsO4) tetrahedra to form slabs with a composition [Zn-2(OH)(H2O)(AsO4)]. The space between these slabs is filled with disordered (H2O) groups and minor lone-pair stereoactive As3+. The ideal formula derived from chemical analysis and crystal-structure solution and refinement is [Zn-2(OH)(H2O)(AsO4)](H2O)(2).