Aerugite has two ions As5+ in the form of As5+O4 anion, as well as one As5+ as the cation and eight O2- as the anion. I think that the reason for this is the different crystallographic position of As in the structure. According to [Fleet, M.E., Barbier, J. (1989) Structure of aerugite (Ni8.5As3O16) and interrelated arsenate and germanate structural series. Acta Crystallographica: B45: 201-205], aerugite has a close-packed layered structure with tetrahedral As5+ and octahedral cations As5 + and Ni5+. However, the authors give the chemical formula as Ni8.5As3O16, so the chemical formula on E-Rocks and Mindat reveals a more crystallographic value.
