Erythrite or red cobalt is a secondary hydrated cobalt arsenate mineral with the formula Co 3(AsO 4) 2•8H 2O. Erythrite and annabergite , chemical formula (Ni 3As 4) 2•8H 2O, or nickel arsenate form a complete series with the general formula (Co,Ni) 3(AsO 4) 2•8H 2O.
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.
Erythrite was first described in 1832 for an occurrence in Grube Daniel, Schneeberg and takes its name from the Greek έρυθρος (erythros), meaning red. Historically, erythrite itself has not been an economically important mineral, but the prospector may use it as a guide to associated cobalt and native silver.
Erythrite occurs as a secondary mineral in the oxide zone of Co–Ni–As bearing mineral deposits. It occurs in association with cobaltite , skutterudite , roselite, pharmacosiderite, retgersite
Notable localities are Cobalt, Ontario; La Cobaltera, Chile, Schneeberg, Saxony, Germany , Czech Republic; Bou Azzer, Morocco; the Blackbird mine, Lemhi County, Idaho, Mexico and the Dome Rock copper mine, Mingary