Erythrite

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
Erythrite crystals
General
CategoryArsenate mineral
Vivianite group
Formula
(repeating unit)		Co3(AsO4)2·8H2O
Strunz classification8.CE.40
Dana classification40.03.06.03
Crystal systemMonoclinic
Crystal classPrismatic (2/m)
(same H-M symbol)
Space groupC2/m
Identification
ColorCrimson to peach-red, pale rose, or pink, may be zoned
Crystal habitRadial or stellate aggregates, fibrous, drusy; usually powdery and massive - rarely as striated prismatic crystals
CleavagePerfect on {010}; poor on {100} and {102}.
TenacitySectile
Mohs scale hardness1.5 - 2.5
LusterSubadamantine, pearly on cleavages
StreakPale red to pink
DiaphaneityTransparent to translucent
Specific gravity3.06
Optical propertiesBiaxial (+)
Refractive indexnα = 1.626 - 1.629 nβ = 1.662 - 1.663 nγ = 1.699 - 1.701
Birefringenceδ = 0.073
PleochroismVisible: X = pale pinkish to pale rose; Y = pale violet to pale violet-rose; Z = deep red
References[1][2][3]

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; 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.[1]

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), hörnesite (Mg) and köttigite (Zn).

References
  1. http://rruff.geo.arizona.edu/doclib/hom/erythrite.pdf Handbook of Mineralogy
  2. http://www.mindat.org/min-1407.html Mindat.org
  3. http://webmineral.com/data/Erythrite.shtml Webmineral data
  • Dana's Manual of Mineralogy ISBN 0-471-03288-3
  • Manual of Mineral Science, 22nd Ed. C. Klein.ISBN 0-471-25177-1
  • Faye, G H; Nickel, E H (1968). "The origin of pleochroism in erythrite" (PDF). The Canadian Mineralogist. 9: 492–504.
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