Adamite
The yellow-green arsenate that made Mapimí famous. Bright under shortwave ultraviolet, soft, and easy to confuse with three other Ojuela species.
Adamite is zinc arsenate, Zn2(AsO4)(OH), hardness 3.5, forming yellow to green crystals and crusts in the oxidised zone of zinc orebodies. It was described in 1866 from Chañarcillo in Chile and named for Gilbert-Joseph Adam. The Ojuela mine at Mapimí, Durango produces the specimens most collectors own.
In short
- Adamite is Zn2(AsO4)(OH), hardness 3.5, orthorhombic, and sits in the olivenite group — the zinc end of a series running to copper olivenite.
- Ojuela, at Mapimí in Durango, Mexico, is the benchmark: yellow crystals on brown limonite, the same matrix that carries legrandite and aurichalcite.
- Colour tracks chemistry. Copper gives green to blue-green, cobalt gives pink to violet, and iron staining gives brown.
- Yellow adamite fluoresces bright green-yellow under shortwave ultraviolet. Green cuprian material usually does not, because copper quenches it.
- It is an arsenate at hardness 3.5. No acid, no ultrasonic cleaner, no dry brushing, and wash your hands afterwards.
| Species | Formula | Typical colour | Hardness |
|---|---|---|---|
| Adamite | Zn2(AsO4)(OH) | Yellow, green, sometimes pink or violet | 3.5 |
| Paradamite | Zn2(AsO4)(OH) | Pale yellow to near colourless | 3.5 |
| Olivenite | Cu2(AsO4)(OH) | Olive-green to brown | 3 |
| Zincolivenite | CuZn(AsO4)(OH) | Green to blue-green | 3.5 |
| Legrandite | Zn2(AsO4)(OH)·H2O | Golden yellow prisms | 4.5 |
| Austinite | CaZn(AsO4)(OH) | Colourless to pale green crusts | 4–4.5 |
| Aurichalcite | (Zn,Cu)5(CO3)2(OH)6 | Pale blue-green tufts | 1–2 |
| Hemimorphite | Zn4Si2O7(OH)2·H2O | Colourless, white, blue | 4.5–5 |
What adamite is, and why the colour moves
Adamite is a basic zinc arsenate and a member of the olivenite group. Olivenite itself is the copper analogue, Cu2(AsO4)(OH), and the two form a series with an approved intermediate species, zincolivenite, sitting between them. That matters commercially, because the green material sold as "cuprian adamite" occupies exactly that middle ground and cannot be assigned by eye. If you are paying for a name rather than for a specimen, ask what the label is based on.
Pure adamite is close to colourless or pale yellow. Almost everything a collector sees owes its colour to something substituting into the structure or staining it from outside. Copper pushes it green through blue-green. Cobalt gives the pink to violet material that turns up in small quantity at Ojuela and is the scarcest of the common colours. Iron from the surrounding limonite stains crystals brown at the base and leaves the tips clean, which is why so many Ojuela pieces look as though they were dipped.
Habit is usually radiating sprays or druses of small wedge-shaped crystals, rarely more than a few millimetres each, forming rosettes and crusts across a plate. Single free-standing crystals over a centimetre are unusual and are priced accordingly.
Ojuela: the limonite matrix and the arsenate suite
The Ojuela mine at Mapimí in Durango has been worked since the 1500s, and its deeply oxidised orebody is the reason a single locality dominates a species. The brown, porous, iron-oxide matrix that carries almost every Ojuela specimen is limonite — the residue left after the primary sulphides broke down — and it is both the reason the arsenates formed and the reason they display so well. A saturated yellow spray against warm brown reads across a room.
Adamite is not alone in there. The same pockets produce legrandite (the golden prisms that made the mine's reputation), paradamite, austinite, conichalcite, aurichalcite, hemimorphite, wulfenite and mimetite. Learning to tell them apart on a plate is most of what buying Ojuela material consists of, and the table above is where to start.
Two practical checks. Limonite is friable, so ask whether the matrix has been consolidated — stabilising a crumbly plate is common and legitimate, but should be disclosed. And look at the back: a plate sawn flat is easier to display and perfectly acceptable, but it is not the same object as a natural-backed pocket piece.
Lavrion, Tsumeb and the type locality
Adamite was described in 1866 from Chañarcillo in the Atacama region of Chile, and named for the French mineralogist Gilbert-Joseph Adam. Type-locality material is scarce and is bought for what it is rather than for how it looks.
The Lavrion district in Greece is the second name every collector learns: green to blue-green cuprian material, generally in small crusts and sprays, out of an orefield worked since antiquity and reworked for specimens in modern times. Tsumeb in Namibia produced adamite too, in smaller quantity than its famous species but with the lustre Tsumeb material tends to have. Cap Garonne in France and the Hilarion workings at Lavrion supply micromount-grade material that is far better than its size suggests.
None of these displaces Ojuela for cabinet specimens. If you want one good adamite, it will almost certainly be Mexican.
Fluorescence, and what it does not prove
Yellow adamite fluoresces a bright lemon to green-yellow under shortwave ultraviolet, and the response is strong enough that a dealer's tray of Ojuela plates under a lamp is one of the more persuasive sights in the trade. The activator is usually attributed to trace uranyl in the structure; the quantity involved is far too small to make the specimen a radiation concern, and fluorescence and radioactivity are unrelated properties in any case.
Green cuprian adamite generally does not fluoresce. Copper is an efficient quencher, so the greener the piece, the weaker the glow — which makes a lamp a rough field test for how much copper is in the material, and a useful one when a label says "adamite" and the crystal is emphatically green.
What fluorescence does not do is make a poor specimen good. It is a property, not a grade. Buy the plate you would want in daylight.
Handling: an arsenate at hardness 3.5
Adamite is soft, brittle and arsenic-bearing. All three point the same way: leave it alone. Loose dust and a dry brush are the only realistic route to an arsenic exposure in a domestic collection, so do not brush it dry, do not saw or grind it, keep it out of the kitchen, and wash your hands after handling. As a solid specimen on a shelf it is inert.
Cleaning is limited to a rinse in lukewarm water and a very soft brush, used wet. Acid attacks the arsenate directly. An ultrasonic cleaner will shear crystal sprays off friable limonite in seconds. If the matrix is iron-stained, that stain is part of the specimen.
Store it in its own box or compartment. A hardness-3.5 rosette will not survive contact with a quartz or fluorite specimen sliding against it in a drawer.