Erythrite or red cobalt is a secondary hydrated arsenate of cobalt minerals with the formula (Co3(AsO4)28H2O). Erythrite and annabergite (Ni3(AsO4)28H2O) (nickel arsenate) form a complete series with the general formula (Co,Ni)3(AsO4)28H2O.

Hardness: 1.5-2.5 on the Mohs scale of hardness.

Luster: (In crystals) Adamantine to vitreous, but pearly on cleavage. (As a crust) Earthy.

Specific Gravity: 3.06.

Cleavage: {010} Perfect.

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 takes its name from the Greek 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 has also been used to color glass.

Notable localities are Cobalt, Ontario , Schneeberg, Saxony, Germany, and Bou Azzer, Morocco, discovered in 2000.

  • Other varieties
  • The nickel variety, annabergite, occurs as a light green nickel bloom on nickel arsenides. In addition iron, magnesium and zinc can also substitute for the cobalt position, creating three other minerals: parasymplesite (Fe), hornesite (Mg), and kottigite (Zn). Also vivianite, a rare mineral which is a weathering product of primary iron-manganese phosphates in pegmatites.