Olivenite
Olivenite is a copper arsenate, hardness 3, from the oxidised copper zones of Cornwall. Habit, the series with adamite, wood copper, and Wheal Gorland material.
Olivenite is the commonest secondary copper arsenate in oxidised copper deposits, hardness 3, density 4.46, and named for its olive-green colour. Crystals reach 2.5 cm. Cornwall is the classic ground: Wheal Gorland, Wheal Unity and the Carharrack mine in the Gwennap copper district.
In short
- Cornwall is the reference locality in every practical sense. The Gwennap and St Day copper mines produced the material that defines the species for collectors, and none of them has worked for well over a century.
- It forms a series with adamite, copper to zinc, with zincolivenite as the approved intermediate. Copper-bearing adamite and zinc-bearing olivenite cannot be told apart by eye.
- Colour runs olive-green to blackish green in crystals and almost white in the finely fibrous form, historically called wood copper for its woody texture. The streak is olive-green to brown.
- Habit is variable and often unhelpful: prismatic or acicular crystals, but also curved, lamellar, matted fibrous, globular and reniform, granular and earthy.
- It is an arsenate. Solid specimens are safe; the dust is not.
| Species | Formula | Hardness | Colour | Habit that identifies it |
|---|---|---|---|---|
| Olivenite | Cu2(AsO4)(OH) | 3 | Olive-green to blackish green; white if fibrous | Prisms, and the fibrous wood copper form |
| Clinoclase | Cu3(AsO4)(OH)3 | 2.5 to 3 | Dark blue-green to almost black | Bladed crystals in rosettes |
| Liroconite | Cu2Al(AsO4)(OH)4·4H2O | 2 to 2.5 | Sky blue to blue-green | Flattened lens-shaped crystals. Cornwall's rarity |
| Cornwallite | Cu5(AsO4)2(OH)4 | 4.5 | Dark green | Crusts and botryoidal masses; arsenate analogue of pseudomalachite |
| Adamite | Zn2(AsO4)(OH) | 3.5 | Yellow, green, colourless | The zinc end-member of olivenite's series |
Wheal Gorland and the Gwennap copper zone
The oxidised zones of the St Day and Gwennap copper mines produced a secondary arsenate suite that is found in this combination almost nowhere else — liroconite, clinoclase, olivenite, cornubite and connellite together. Olivenite is the common member of that suite, which is precisely why it matters: it is the species most likely to carry an authentic old Cornish label, and it is the anchor around which a Cornish secondary suite is usually built.
The Handbook of Mineralogy lists the English occurrences as the Carharrack mine, Wheal Gorland, Wheal Unity and others in Gwennap and Linkinhorne, with further material from Alston Moor in Cumbria and Tavistock in Devon. Wheal Gorland is the name that carries weight on a label, for the same reason it does on our Cornwall page: it was the source of the finest liroconite, and a mine with that reputation attracted careful collecting and careful labelling.
None of this ground is producing. South Crofty, the last working Cornish tin mine, closed in 1998, and the copper mines had gone generations earlier. Every Cornish olivenite on the market has been in somebody's cabinet, usually for a very long time, which makes condition and provenance the two things worth examining — see buying old collection material.
The adamite series, and what cannot be determined by eye
Olivenite is Cu2(AsO4)(OH) and adamite is Zn2(AsO4)(OH). The two form a series, and the intermediate composition has been given its own approved name, zincolivenite. This has a practical consequence that collectors meet constantly: material sold as cuprian adamite — the attractive green Ojuela material especially — sits somewhere in this range, and where exactly is not a question a loupe can answer.
Colour is misleading in both directions. Copper drives olivenite's green, and adding copper to adamite drives it green too, so a green arsenate from a zinc-rich locality and a green arsenate from a copper-rich locality can look identical. The useful discriminator in practice is the geological setting: an arsenate on a copper orebody with cuprite, malachite and chalcophyllite around it is olivenite country; the same arsenate on a zinc orebody with smithsonite, hemimorphite and aurichalcite is adamite country. Association identifies more specimens than any physical test.
Wood copper, and the habits that confuse
Olivenite's crystals can be handsome — elongated prisms with many forms, up to about 2.5 cm — but most olivenite is not crystallised at all. It comes as curved, lamellar or matted fibrous aggregates, as globular and reniform crusts, and as granular or earthy masses. The finely fibrous variety is pale, almost white or straw-yellow, silky rather than adamantine, and looks so unlike a copper arsenate that it acquired its own name: wood copper, Holzkupfer in the old German literature, from its resemblance to weathered timber.
Two identification points follow. First, the streak is olive-green to brown even where the specimen is pale, which is the single most useful cheap test on ambiguous fibrous material. Second, lustre tracks habit: adamantine to vitreous on crystals, pearly to silky on the fibrous forms, and a specimen showing both on one piece is common and is not two minerals.
Cleavage is indistinct and the fracture conchoidal to irregular, so at hardness 3 olivenite chips rather than cleaves. It is not fragile in the way a bladed mineral is, but it marks easily and should not share a tray with anything harder.
Beyond Cornwall, and what to look for when buying
Fine olivenite is not a British monopoly. The Handbook lists large crystals from Tsumeb; fine examples from the Mammoth and American Eagle mines in the Tintic district, Juab County, Utah, and from the Majuba Hill mine in Nevada; the Cap Garonne mine in Var, France; the Clara Mine in the Black Forest; the Kamariza mine at Laurium in Greece; Touissit near Oujda in Morocco; Chuquicamata, Collahuasi and the Copiapó district in Chile; and Ashburton Downs in Western Australia.
When buying, look at three things in order. Crystal quality, because the difference between a crust and a crystallised plate is the whole difference in interest. Association, because an olivenite with clinoclase or cornwallite on the same matrix is worth more than the sum of two specimens and is also self-authenticating for locality. And the label, because on Cornish material the mine name is most of what you are buying — a piece labelled only Cornwall tells you very little, and our notes on old labels explain what a good one carries.
Handling is the standard arsenate rule from our toxic minerals page: safe as a solid, never sawn, ground or dry-brushed, and hands washed afterwards. Well-labelled Cornish material appears rarely. If it is what you want, say so on the wanted list; we hold no stock and cannot sell you one today, but we can watch for it.