Natural Arsenates

a group of minerals; salts of or-thoarsenic acid with different cations—Ca, Mg, Cu, Pb, Ni, Co, Fe, and others. They are crystallochemically related to the carcasslike, islandlike, chain, and laminated types of structure. All the minerals of the group natural arsenates are divided according to their chemical composition into anhy-drates (mimetesite Pb₅(AsO₄)₃Cl, olivenite Cu₂(AsO₄)OH) and hydrates (annabergite Ni₃(AsO₄)₂ 8H₂O, erythrite Co₃(AsO₄)₂ 8H₂O, and scorodite Fe(AsO₄) 2H₂O). The natural arsenates crystallize most frequently in the lower systems—rhombic and triclinic—more rarely in hexagonal and trigonal systems, and very rarely in a cubic system. The natural arsenates form fine crystals; the most characteristic forms of deposits are accumulative and earthy masses. The density of the anhydrate natural arsenates is 7,190–7,250 kg/m³; of the hydrates, 2,950–3,300 kg/m³. Their hardness on the mineralogical scale is 2.5–5.5. The natural arsenates are formed during exogenic processes, usually in the oxidation zone of sulfide deposits. Many natural arsenates have industrial significance as ores of different metals.