Davidlloydite, ideally Zn3(AsO4)2(H2O)4, is a new supergene mineral from the Tsumeb mine, Otjikoto (Oshikoto) region, Namibia. It occurs as elongated prisms (∼10:1 length-to-width ratio) that are flattened on {010}, and up to 100 × 20 × 10 μm in size. The crystals occur as aggregates (up to 500 μm across) of subparallel to slightly diverging prisms lying partly on and partly embedded in fine-grained calcioandyrobertsite. Crystals are prismatic along [001] and flattened on {010}, and show the forms {010} dominant and {100} subsidiary. Davidlloydite is colourless with a white streak and a vitreous lustre; it does not fluoresce under ultraviolet light. The cleavage is distinct on {010}, and no parting or twinning was observed. The Mohs hardness is 3 – 4. Davidlloydite is brittle with an irregular to hackly fracture. The calculated density is 3.661 g cm–3. Optical properties were measured with a Bloss spindle stage for the wavelength 590 nm using a gel filter. The indices of refraction are α = 1.671, β = 1.687, γ = 1.695, all ±0.002; the calculated birefringence is 0.024; 2Vobs = 65.4(6)°, 2Vcalc = 70°; the dispersion is r < v, weak; pleochroism was not observed. Davidlloydite is triclinic, space group P1, with a = 5.9756(4), b = 7.6002(5), c = 5.4471(4) Å, α = 84.2892(9), β = 90.4920(9), γ = 87.9958(9)°, V = 245.99(5) Å3, Z = 1 and a:b:c = 0.7861:1:0.7167. The seven strongest lines in the X-ray powder diffraction pattern [listed as d (Å), I, (hkl)] are as follows: 4.620, 100, (011, 10); 7.526, 71, (010); 2.974, 49, (200, 01); 3.253, 40, (021, 120); 2.701, 39, (10, 002, 1); 5.409, 37, (001); 2.810, 37, (210). Chemical analysis by electron microprobe gave As2O5 43.03, ZnO 37.95, CuO 5.65, H2O(calc) 13.27, sum 99.90 wt.%. The H2O content and the valence state of As were determined by crystal structure analysis. On the basis of 12 anions with H2O = 4 a.p.f.u., the empirical formula is (Zn2.53Cu0.39)Σ2.92As2.03O8(H2O)4.

The crystal structure of davidlloydite was solved by direct methods and refined to an R1 index of 1.51% based on 1422 unique observed reflections collected on a three-circle rotating-anode (MoKα radiation) diffractometer equipped with multilayer optics and an APEX-II detector. In the structure of davidlloydite, sheets of corner-sharing (As5+O4) and (ZnO4) tetrahedra are linked by ZnO2(H2O)4 octahedra. The structure is related to that of parahopeite.

Ianbruceite, ideally [Zn2(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 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 köttigite, 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; 2Vobs = 18(2)°, 2Vcalc = 20°, and the dispersion is r < v, weak. Ianbruceite is monoclinic, space group P21/c, a = 11.793(2), b = 9.1138(14), c = 6.8265(10) Å, β = 103.859(9)°, V = 712.3(3) Å3, Z = 4, a:b:c = 1.2940:1:0.7490. The seven strongest lines in the X-ray powder diffraction pattern [d(Å), I, (hkl)] are as follows: 11.29, 100, (100); 2.922, 17, (130); 3.143, 15, (202); 3.744, 11, (300); 2.655, 9, (230); 1.598, 8, (152); 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 R1 index of 8.6%. The As is tetrahedrally coordinated by four O anions with a mean As O distance of 1.687 Å. Zigzag [[5]Zn[6]Znϕ7] 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 [Zn2(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 [Zn2(OH)(H2O)(AsO4)](H2O)2.