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Erythrite crystals
Category Arsenate mineral
Vivianite group
(repeating unit)
Strunz classification 8.CE.40
Dana classification
Crystal system Monoclinic
Crystal class Prismatic (2/m)
(same H-M symbol)
Space group C2/m
Color Crimson to peach-red, pale rose, or pink, may be zoned
Crystal habit Radial or stellate aggregates, fibrous, drusy; usually powdery and massive - rarely as striated prismatic crystals
Cleavage Perfect on {010}; poor on {100} and {102}.
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 (+)
Refractive index nα = 1.626 - 1.629 nβ = 1.662 - 1.663 nγ = 1.699 - 1.701
Birefringence δ = 0.073
Pleochroism Visible: X = pale pinkish to pale rose; Y = pale violet to pale violet-rose; Z = deep red
References [1][2][3]

Erythrite or red cobalt is a secondary hydrated cobalt arsenate mineral 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.

Erythrite from Morocco

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, Saxony,[3] and takes its name from the Greek έρυθρος (erythros), meaning red.[2] 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, symplesite, roselite-beta, scorodite, pharmacosiderite, adamite, morenosite, retgersite, and malachite.[1]

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.[1]

Other varieties[edit]

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), hörnesite (Mg) and köttigite (Zn).