erythrite, arsenate mineral in the vivianite group, hydrated cobalt arsenate [Co3(AsO4)2ยท8H2O]. Erythrite, which is used as a guide to the presence of cobalt-nickel-silver ores because of its crimson or peach-red colour, occurs as radiating crystals, concretions, or earthy masses in the oxidized zone of cobalt and nickel deposits. It forms a complete solid-solution series with annabergite, in which nickel replaces cobalt in the erythrite structure. As the nickel content increases, the colour lightens to white, gray, or pale green. Erythrite occurs in Schneeberg, Ger.; Allemont, France; Cornwall and Cumberland, Eng.; Chile; Morocco; and the southwestern United States. For detailed physical properties, see arsenate mineral (table).