Wilhelmkleinite, ZnFe23+(AsO4)(2)(OH)(2), a new mineral from Tsumeb, Namibia

Wilhelmkleinite, a new zinc iron hydroxy-arsenate, ideally ZnFe23+ (ASO(4)) (2)(OH)(2), is monoclinic, space group P2(1)/n, with a = 6.631(1) Angstrom, b = 7.611(1) Angstrom, c = 7.377(1) Angstrom, beta = 91.80(1), Z = 2, from single crystal studies. The strongest reflec tions in the X-ray powder dif fraction data are (d(meas.),l,hkl) 3.385(100)(021), 3.315(78)(200/012), 2.9 39(47)(112). The mineral occurs at the Tsumeb mine, Namibia. It forms twinn ed spearhead-shaped crystals up to 5 mm in length. The blackish green trans lucent crystals which show strong pleochroism from olive-green to emerald-g reen and reddish-brown are found on AI-rich scorodite. They show no fluores cence, their streak is green and the hardness on Mohs' scale is 4.5. The re fractive index is about 1.94, the calculated density results in 4.364 g/cm( 3). Electron microprobe analyses yielded ZnO 13.81, Fe2O3 33.60 and As2O5 4 6.69.