Erythrite is a secondary hydrated arsenate of cobalt minerals with the formula (Co3(AsO4)2·8H2O). Erythrite and annabergite (Ni3(AsO4)2·8H2O) (nickel arsenate) form a complete series with the general formula (Co,Ni)3(AsO4)2·8H2O. In addition iron, magnesium and zinc also substitute for the cobalt position creating three more mineral varieties: parasymplesite (Fe), hornesite (Mg), and kottigite (Zn).
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. The nickel variety, annabergite, occurs as a light green nickel bloom on nickel arsenides.
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