Erythrite
Cobalt bloom: crimson blades at hardness 1.5, which is the single fact that governs how you buy one and how you keep it.
Erythrite is hydrated cobalt arsenate, Co3(AsO4)2·8H2O, hardness 1.5 to 2.5 — softer than a fingernail. Beudant named it in 1832 from the Greek for red, and the type locality is the Daniel mine at Schneeberg in Saxony. Bou Azzer in Morocco is the source of nearly every collection specimen in circulation.
In short
- Erythrite is Co3(AsO4)2·8H2O, hardness 1.5–2.5. That softness governs everything: what to buy, what to pay attention to, and how to store it.
- Named by Beudant in 1832 from Greek erythros, red. Type locality the Daniel mine, Schneeberg, Saxony.
- It is the cobalt member of the vivianite group and forms a complete series with green annabergite, the nickel analogue. Intermediate material runs grey-pink to dull violet.
- As a surface stain it is cobalt bloom — the classic field indicator that cobalt arsenides lie underneath.
- Bou Azzer in Morocco supplies almost all the good crystallised material. Schneeberg and Sonoran specimens exist but are old and scarce.
| Species | Formula | Colour | Hardness | Note |
|---|---|---|---|---|
| Erythrite | Co3(AsO4)2·8H2O | Crimson to magenta | 1.5–2.5 | Blades and radiating sprays |
| Annabergite | Ni3(AsO4)2·8H2O | Apple green | 1.5–2.5 | The nickel end of the same series |
| Roselite | Ca2(Co,Mg)(AsO4)2·2H2O | Rose-red to deep pink | 3.5 | Blocky rhombs; a Bou Azzer classic |
| Wendwilsonite | Ca2(Mg,Co)(AsO4)2·2H2O | Rose-red to pink | 3.5 | Type locality Bou Azzer, 1987; not separable from roselite by eye |
| Spherocobaltite | CoCO3 | Magenta to raspberry | 4 | Cobalt carbonate; Bou Azzer and the Congo |
| Cobaltoan calcite / dolomite | (Ca,Co)CO3 and equivalents | Bright pink | 3–4 | A variety name, not a species; Congo botryoids |
Cobalt bloom, and what the colour is telling you
Erythrite forms where primary cobalt arsenides — skutterudite, safflorite, löllingite, cobaltite — meet oxygenated groundwater near surface. The cobalt and the arsenic recombine as a hydrated arsenate, and the result is a crimson that no other common secondary mineral produces.
Miners used that. A crimson smear on the wall of a working, "cobalt bloom", meant cobalt arsenides in the rock behind it, and the association is reliable enough that it was used as a prospecting indicator long before anyone could analyse for cobalt. For a collector the same logic runs backwards: a crimson bloom on a specimen tells you the piece came out of an oxidised arsenide vein, and that skutterudite or safflorite is likely to be somewhere in the matrix.
The saturated colour is a red flag in a different sense too. Dull, thin, powdery erythrite crusts are extremely common and worth very little; what is scarce is erythrite that grew as free-standing blades in a cavity.
The vivianite group and the annabergite series
Erythrite is the cobalt member of the vivianite group, an isostructural family of hydrated arsenates and phosphates that includes vivianite itself (iron phosphate), annabergite (nickel arsenate) and köttigite (zinc arsenate). Erythrite and annabergite form a complete solid-solution series, so cobalt-rich material is crimson, nickel-rich material is apple green, and everything between exists.
In practice this matters when you are looking at pale, greyish or lilac material from a cobalt-nickel deposit. A weak pink is not necessarily poor-quality erythrite; it may be an intermediate composition. Localities that produce both — Schneeberg among them — are where the ambiguity is real.
Bou Azzer, Schneeberg and the Sonoran material
Bou Azzer, in Morocco's Anti-Atlas, is one of very few places on earth mined with cobalt as the primary metal rather than a by-product, and its oxidised zone is why nearly every good erythrite in a modern collection is Moroccan. The best of it is deep crimson to magenta, in sprays of bladed crystals standing off quartz or altered serpentinite. Because the mines are still working, new material reaches the market regularly.
Schneeberg in Saxony is the type locality and the historical source. Nineteenth-century Schneeberg specimens exist in old collections, but they are typically crusts and small sprays rather than the showy plates Morocco produces, and they are bought for what the label says as much as for what is on the rock.
The Sara Alicia mine near Álamos in Sonora, Mexico produced a small quantity of exceptional erythrite — sharp crimson sprays on goethite — that collectors still chase. It is a limited find rather than a continuing source.
Buying erythrite: condition is the whole price
At hardness 1.5 to 2.5, erythrite is damaged by contact with practically anything, including another specimen in the same drawer and the tissue it was wrapped in. The consequence is that undamaged material is scarce far out of proportion to how much has been mined, and that a small perfect spray beats a large scuffed plate on every measure that matters.
Look, in order, at: whether the blade tips are intact under a lens; whether the sprays stand proud or have been flattened against the matrix; colour saturation, which varies enormously; and whether the piece has been consolidated with a lacquer or a dilute adhesive. Soft arsenates are among the most frequently stabilised species in the trade. Stabilisation is not dishonest in itself — undisclosed stabilisation is.
A raking light across the specimen shows both crushed tips and an artificial surface sheen better than direct light does.
Keeping erythrite once you own it
Its own box, on a mount, never travelling loose. That is most of the answer. A spray of blades that survived a hundred million years in a vein can be flattened by one careless slide of a drawer.
Do not clean it. Water will not dissolve erythrite, but a brush will destroy the blades and an ultrasonic cleaner will remove them from the matrix entirely. If it arrives dusty, it stays dusty.
On safety: it is a cobalt arsenate. A solid specimen in a case is not a hazard, but this is a mineral you never brush, saw or sand, and you wash your hands after handling. Keep it out of reach of anyone likely to put it in their mouth.