Erythrite or red cobalt is a secondary hydrated cobalt arsenate mineral with the formula Co3(AsO4)2•8H2O. Erythrite and annabergite, chemical formula Ni3(AsO4)2•8H2O, or nickel arsenate form a complete series with the general formula (Co,Ni)3(AsO4)2•8H2O. Erythrite crystallizes in the monoclinic system and forms prismatic crystals. The color is crimson to pink and occurs as a secondary coating known as cobalt bloom on cobalt arsenide minerals. Well-formed crystals are rare, with most of the mineral manifesting in crusts or small reniform aggregates.