any of a group of naturally occurring compounds of arsenic, oxygen, and various metals, most of which are rare, having crystallized under very restricted conditions. At the mineralogically famous Långban iron and manganese mines in central Sweden, more than 50 species of arsenate minerals have been described, many peculiar to the locality. Such compounds occur in open cavities and resulted from the reaction of arsenic acid (H₃AsO₄) upon pyrochroite 【manganese hydroxide; Mn(OH)₂】 at moderate to low temperature. Arsenates at other localities are often oxidation products of arsenide ores and are deposited at low temperatures in late-stage veins and open cavities.

Only a few arsenate minerals have economic importance. Because the transition metals (e.g., cobalt, copper, nickel) give brilliant colour to some of the arsenates, these can be used to advantage in prospecting; such arsenate oxidation products, or blooms, as erythrite (bright pink) and annabergite (green) are indicators of nickel and cobalt arsenide ores. Many of the nickel and cobalt deposits at Sudbury and Cobalt, Ont., were located in this manner.

The arsenate minerals, which are salts of arsenic acid, contain arsenate groups (AsO₄) in which four oxygen (O) atoms are arranged at the corners of a tetrahedron about a central arsenic (As) atom. Each arsenate tetrahedron has a net electric charge of -3, which is neutralized by large, positively charged metal ions (e.g., calcium, manganese, or ferrous iron) outside the tetrahedron. Unlike the similar silicate tetrahedra, which link to form chains, sheets, rings, or frameworks, arsenate tetrahedra are insular.

The crystal structure of the arsenate minerals is very similar to that of the phosphate and vanadate minerals; indeed, many arsenate minerals form solid solutions with both the phosphates and the vanadates.