Erythrite: Properties and Occurrences

Erythrite or red cobalt is a secondary hydrated cobalt arsenate mineral with the formula [Co3(AsO4)2·8H2O]. is a wonderful mineral for people who like striking, unusual colors. Its characteristic bright red-purple color is very noticeable and was used to spot veins of cobalt-bearing ore. Erythrite was first described in 1832 as an occurrence in Grube Daniel, Schneeberg, Saxony, 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. Most erythrite is found in the form of crusts, but specimens from Morocco have larger crystals

General Information

  • Category: Arsenate mineral (Vivianite group)
  • Formula: Co3(AsO4)28H2O
  • Crystal system: Monoclinic
  • Crystal class: Prismatic (2/m) (same H-M symbol)

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.

  • Color: Crimson to peach-red, pale rose, or pink, maybe zoned
  • Crystal habit: Radial or stellate aggregates, fibrous, drusy; usually powdery and massive – rarely as striated prismatic crystals
  • Tenacity: Sectile
  • Mohs scale hardness: 1.5 – 2.5
  • Luster: Subadamantine, pearly on cleavages
  • Streak: Pale red to pink
  • Diaphaneity: Transparent to translucent
  • Specific gravity: 3.06
  • Optical properties: Biaxial (+)

Occurrence

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.

Notable localities are Cobalt, Ontario; Schneeberg, Saxony, Germany; Joachimsthal, Czech Republic; Cornwall, England; Bou Azzer, Morocco; the Blackbird Mine, Lemhi County, Idaho; Sara Alicia mine, near Alamos, Sonora, Mexico; Mt. Cobalt, Queensland and the Dome Rock copper mine, Mingary, South Australia.

 

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