Erythrite

Erythrite or red cobalt is a secondary hydrated cobalt arsenate mineral with the formula Co
₃(AsO
₄)
₂•8H
₂O. Erythrite and annabergite, chemical formula (Ni
₃As
₄)
₂•8H
₂O, or nickel arsenate form a complete series with the general formula (Co,Ni)
₃(AsO
₄)
₂•8H
₂O.

Erythrite from Morocco

Erythrite
Erythrite crystals
General
Category	Arsenate mineral Vivianite group
Formula (repeating unit)	Co3(AsO4)2·8H2O
IMA symbol	Ery[1]
Strunz classification	8.CE.40
Dana classification	40.03.06.03
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	C2/m
Identification
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	[2][3][4]

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,[4] and takes its name from the Greek έρυθρος (erythros), meaning red.[3] 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.[2]

Notable localities are Cobalt, Ontario; La Cobaltera, Chile, 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.[2]