Davidlloydite, ideally Zn_3(AsO_4)_2(H_2O)_4, a new arsenate mineral from the Tsumeb mine, Otjikoto (Oshikoto) region, Namibia: description and crystal structure

摘要

Davidlloydite, ideally Zn_3(AsO_4)_2(H_2O)_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 x 20 x 10 ìm in size. The crystals occur as aggregates (up to 500 um 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.