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)(H 2 O)(AsO 4 )](H 2 O) 2 , is a new supergene mineral from the Tsumeb mine, Otjikoto (Oshikoto) region, Namibia. It occurs as thin platy crystals up to 80 μm long and a few μm 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 α = 1.601, β = 1.660, γ = 1.662, all ±0.002; 2V obs = 18(2)°, 2V calc = 20°, and the dispersion is r v , weak. Ianbruceite is monoclinic, space group P 2 1 / c , a = 11.793(2), b = 9.1138(14), c = 6.8265(10) A, β = 103.859(9)°, V = 712.3(3) A 3 , Z = 4, a : b : c = 1.2940:1:0.7490. The seven strongest lines in the X-ray powder diffraction pattern [ d (A), I , ( hkl )] are as follows: 11.29, 100, (100); 2.922, 17, (130); 3.143, 15, (2I02); 3.744, 11, (300); 2.655, 9, (230); 1.598, 8, (1I52); 2.252, 7, (222). Chemical analysis by electron microprobe gave As 2 O 5 36.27, As 2 O 3 1.26, Al 2 O 3 0.37, ZnO 49.72, MnO 0.32, FeO 0.71, K2O 0.25, H 2 O calc 19.89, sum 108.79 wt.%; the very high oxide sum is due to the fact that the calculated H 2 O content is determined from crystal-structure analysis, but H 2 O 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 A. Zigzag [ [5] Zn [6] Znφ 7 ] chains extend in the c direction and are linked in the b direction by sharing corners with (AsO 4 ) tetrahedra to form slabs with a composition [Zn 2 (OH)(H 2 O)(AsO 4 )]. The space between these slabs is filled with disordered (H 2 O) groups and minor lone-pair stereoactive As 3+ . The ideal formula derived from chemical analysis and crystal-structure solution and refinement is [Zn 2 (OH)(H 2 O)(AsO 4 )](H 2 O) 2 .