ERYTHRITE
Erythrite – Co3(AsO4)2.8H2O – is a rare arsenate, typical of the oxidation zone of Co ores.
It is called “cobalt bloom” (“Kobaltblüte” in German) because it forms on primary minerals of Co and As, denouncing the existence of these. It is not a major ore mineral, but it is extremely useful for recognizing these ores. Many ore mineral books simply ignore eryhthrite; do not provide any information about it.
Erythrite is classified in the Vivianite Group and forms three solid solution series with annabergite, hörnesite and köttigite. It can contain Fe, Zn and Ni and there is a variety rich in Mg.
Idiomorphic erythrite crystals are rare, up to 10 cm, prismatic to long prismatic, typically flattened in [010], striated. There are many combined forms, It can be botryoidal or reniform.
1. Characteristics
Crystal system: Monoclinic prismatic.
Color: Crimson red to peach red, pale pink or pink, can be zoned.
Habit: Radial or stellate aggregates, fibrous, drusiform, usually powdery or massive.
Cleavage: {010} perfect, {100} poor, {-102} poor. Striations paralell to [001].
Tenacity: Sectile, flexible in thin sheets.
Twinning: No.
Fracture: Splintery.
Mohs Hardness: 1.5 – 2.5
Parting: No.
Streak: Pale red to pink.
Lustre: Subadamantine, waxy, pearly on cleavages.
Diaphaneity: Transparent.
Density (g/cm³): 3.06
2. Geology and Deposits
Erythrite is a secondary mineral typical of oxidation zones of Co-Ni-As deposits.
It can also occur in cornubianites of contact metamorphic rocks.
3. Mineral Associations
It associates with some common gangue minerals (quartz and calcite) and evidently with Co minerals, both primary (cobaltite, skutterudite) and secondary (roselite, beta-roselite, smolyaninovite).
It also occurs with Zn minerals (nickelline, adamite), Cu minerals (malachite, azurite, conicalcite, tyrolite), Ni minerals (morenosite, retgersite), other arsenates (pharmacosiderite, scorodite, symplesite) and secondary Fe minerals (goethite ).
With other common sulfides (pyrite, arsenopyrite, chalcopyrite), native silver, native bismuth and pitchblende.
4. Transmitted Light Microscopy
Refraction indices: nα = 1.626 – 1.629 nβ = 1.662 – 1.663 nγ = 1.699 – 1.701.
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Pleochroic between
X = paleish pink to pale rose;
Y = pale violet to pale violet-rose;
Z = deep red.
Relief: Moderate.
Cleavage: {010} perfect, {100} poor, {-102} poor
Habits: Radial or stellate aggregates, fibrous, massive.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Maximum birefringence of 0.073, corresponding to colors of up to 4th order, dominating pink and green colors, masked by the strong own color of the mineral.
Extinction: Probably oblique.
Elongation sign: No information available.
Twins: No.
Zoning: No information available.
CONVERGENT LIGHT
Character: B(+) or B(-)
2V angle: up to 90º
Alterations: Erythrite may be replaced by annabergite if Ni is present in the environment. Forms pharmacolite if Ca-rich fluids are present. In Mg-rich environments, erythrite is replaced by hörnesite. Can breakdown into amorphous or poorly crystalline arsenates.
May be confused with: No information available.
5. Reflected Light Microscopy
Sample preparation: the polishing of erythrite offers no difficulties. It acquires a good polish even before the associated opaque minerals.
PLANE POLARIZED LIGHT – PPL
Reflection color: Dark gray.
Pleochroism: No.
Reflectivity: Low (~5%)
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Anisotropy was not observed.
Internal reflections: Widespread in various shades of red, from very dark to intense vivid, blood red, magenta and in various shades of pink; they can be almost white.
May be confused with: other transparent minerals of the same color that may occur associated.
Roselite, beta-roselite, and cobalt calcite are three other secondary Co minerals, all three having colors nearly the same as erythrite. However, all three are extremely rare,
Wendwilsonite, a hydrated arsenate of Ca and Mg, is another secondary mineral that can occur in association and has a color between red and pink.
