Erythrite Crystals

Erythrite, also known as red cobalt, is a secondary hydrated cobalt arsenate mineral with the formula Co₃(AsO₄)₂·8H₂O. It's a member of the vivianite group and forms a complete solid-solution series with annabergite (nickel arsenate). This means that the two minerals can mix and match their cobalt and nickel atoms, creating a range of colors from crimson red to pale green.

Erythrite occurs as a secondary mineral in the oxide zone of Co–Ni–As bearing mineral deposits. It occurs in association with cobaltite, skutterudite, symplesite, roselite-beta, scorodite, pharmacosiderite, adamite, morenosite, retgersite, and malachite.

Erythrite was first described in 1832 by François Sulpice Beudant and named after the Greek word "erythros" meaning "red."

Incredible Erythrite Crystals From Morocco

Incredible Erythrite Crystals . Credit: ShelterRockMinerals

Erythrite Properties

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.

Chemical Formula: Co₃(AsO₄)₂·8H₂O

Color: Crimson red to pink, also found in purple, peach, and gray. Lighter colors indicate higher nickel content.

Crystal System: Monoclinic, typically forming prismatic crystals (rare) or crusts, aggregates, and needle-like formations.

Hardness: 1.5 - 2.5 on the Mohs scale (soft, easily scratched)

Luster: Subadamantine, pearly on cleavages

Streak: Pale red to pink

Diaphaneity: Transparent to translucent

Solubility: Soluble in acids

Occurrence: Secondary mineral found in oxide zones of cobalt-arsenic deposits, often associated with cobaltite, skutterudite, and native silver.

Historical Use: Indicator mineral for cobalt and silver deposits, but not economically important itself.

Erythrite. Photo: Henk Smeets/Tomeik Minerals.

Where is Erythrite found

Erythrite is a secondary mineral, meaning that it forms from the weathering of other cobalt-bearing minerals. It's typically found in the oxidized zone of cobalt-nickel-arsenic deposits. Some notable localities for erythrite include: