Natural arsenates

NATURAL ARSENATES (EN: arsenates; DE: Naturarsenate; FR: arseniates naturels; ES: arseniatos naturales; RU: арсенаты природные) are the class of minerals, namely, the salts of the orthoarsenic acid N3AsO4. It includes approximately 120 minerals. Within the complex anion AsO43-, arsenic may be isomorphically substituted for R and S. There are most characteristical for the natural arsenates the Ca, Cu, Co, Ni, Mg, Pb, Zn, Fe2+, Fe3+, UO22+, Mn2 cations, many of which isomorphically substitute each other within the wide limits, which influences the properties of the natural arsenates (coloration, optical constants, size of the unit cell, density, and so on).

There may be contained within the natural arsenates the additional anions: OH-, Cl-, PO43-, SO42-. They divide natural arsenates according to the peculiarities of their composition into the anhydrous ones, namely, mimetite Pb5[AsO4]3Cl, olivenite Cu2[AsO4]OH, duftite PbCu[AsO4]OH, and the hydrous ones, namely, erythrite, annabergite, scorodite, euchroite Cu2[AsO4]OH • 3H2O, metazeunerite Cu(UO2)2[AsO4]2 • 8N2O, and others. The crystal structures of the natural arsenates are insular (there is the AsO4- tetrahedron at their base), but there are distinguished, depending on the co-located situation of the coordination polyhedra of the cations and anions, the sub-chain, sub-layer, and sub-frame structures.

The crystal systems of the natural arsenates are the rhombic, monoclinic, triclinic ones. The crystals are small, acicular, or elongated-tabular. There are characteristical the earthy, colloform, and dense aggregates, incrustations, and crusts. The coloration is often bright, caused by the nature of the cation: Cu2+ is green of the various tints; UO22+ is yellow, greenish-yellow, emerald-green; Co2+ is pink, crimson; Ni2+ is apple-green; Mn2+ is pale-pink; Fe2+ is green, Fe3+ is yellowish, brown. The hardness and density vary, depending on the content of water, within the limits of 2.5-5.5 and 2900-7300 kilograms per cubic metre. The cleavage is determined by the type of the sub-structure. Most of the natural arsenates are the rare minerals, which are not forming the large clusters. Almost all natural arsenates are the supergene formations, which are associated with the processes of oxidation of the ores. Thanks to the bright colorations, they may be indicators of the ores of cobalt, nickel, uranium, lead. At the high concentrations, they may have the industrial significance, as the oxidized ores of the corresponding metals.