Libethenite
Libethenite is hardness 4, density 3.97, in olive-green crystals to 3 cm. Olivenite is the arsenate of the same structure — denser, softer, and the same colour.
Libethenite is a copper phosphate, hardness 4 and measured density 3.97, in wedge-topped prismatic crystals to 3 cm, olive-green to blackish green, from the oxidised zone of copper deposits. Olivenite is the arsenate of the same structure and the same colour, at hardness 3 and density 4.46 — softer and 12% denser at once.
In short
- Libethenite and olivenite are the phosphate and the arsenate of one structure. Cu2(PO4)(OH) against Cu2(AsO4)(OH). Same habit, same olive-green colour, same oxidised copper zone, same mines.
- The two tests point in opposite directions, which is what makes the pair useful. The arsenate is denser (4.46 against 3.97) and softer (3 against 4). Get one result and the other confirms it.
- The reason is the atom being swapped. Arsenic is 74.92 to phosphorus's 30.97 in the same crystal site, so the arsenate must be heavier. A 12% density gap on identical-looking crystals is unusually large.
- Crystals reach 3 cm, wedge-topped prisms or equant pseudo-octahedra, often striated, and also in sprays of bladed to acicular crystals and as druses.
- Olivenite was described in Britain in 1820 and libethenite in Slovakia in 1823, three years apart — and the two occur together at Cornish mines, where the British species is the commoner of the pair.
| Property | Libethenite | Olivenite | Does it separate them? |
|---|---|---|---|
| Formula | Cu2(PO4)(OH) | Cu2(AsO4)(OH) | Yes, but only by analysis |
| Density | 3.97 | 4.46 | Yes — a 12% gap |
| Hardness | 4 | 3 | Yes, and in the opposite direction |
| Colour | Light to dark olive-green | Olive-green, greenish brown | No |
| Crystal system | Monoclinic, pseudo-orthorhombic | Monoclinic, pseudo-orthorhombic | No |
| Largest crystals | 3 cm | 2.5 cm | No |
| Country of first description | Slovakia, 1823 | United Kingdom, 1820 | — |
Why a density test is genuinely diagnostic here
Most of the time, density is one line of evidence among several. On this pair it is close to decisive, and the reason is worth understanding because it generalises to every phosphate-arsenate pair a collection holds.
Libethenite and olivenite have the same structure and differ by one atom: phosphorus in one site, arsenic in the other. Arsenic has an atomic weight of 74.92 against phosphorus's 30.97 — well over twice as heavy — occupying essentially the same volume in essentially the same cell. The density has no choice but to rise, and the Handbook of Mineralogy records exactly that: 3.97 for the phosphate, 4.46 for the arsenate.
A 0.49 gap is large by the standards of this kind of test. Our note on measuring specific gravity at home shows that on a scale reading to a hundredth of a gram, a 5 g specimen already measures to within about 0.03 of the true value — six times better than the gap needs.
Hardness confirms it, and it runs the other way. Libethenite is 4 and olivenite 3. That opposition is the useful part: a specimen that weighs light and resists a copper coin is libethenite on two independent tests, and a specimen that weighs heavy and is marked by one is olivenite. Getting one of each is a signal that the specimen is mixed or that one of the measurements is wrong.
The same pair, in the same mines, under two names
Both species occur as uncommon secondary minerals in the oxidised zone of copper deposits, and their Handbook association lists overlap almost completely — pseudomalachite, malachite, azurite, chrysocolla and iron oxides for libethenite; a longer list for olivenite that includes clinoclase, conichalcite, cornwallite, brochantite and scorodite.
In Cornwall they occur together. The Handbook names good libethenite crystals at Wheal Phoenix, around Calstock and at Illogan; olivenite is a Cornish species in the fullest sense, with the IMA-CNMNC list giving the United Kingdom as its country of first description in 1820. Our Cornwall note covers the district and the oxidised copper suite it produced.
An old Cornish label reading “olivenite” on olive-green prisms is therefore likelier to be right than wrong, because olivenite is much the commoner of the two there — but the point of this page is that the check is cheap and the two are genuinely indistinguishable by eye. The determination that matters is the one nobody made, not the one on the card.
Elsewhere: Lʹubietová near Banská Bystrica in Slovakia, the type locality, known in the older literature as Libethen — which is also on the chrysocolla distribution list; Nizhni Tagil in the Urals; the Miguel Vacas mine near Vila Viçosa in Portugal; very large crystals from the Rokana mine near Kitwe and at Chingola in Zambia; Kakanda, Mindouli and Likasi in Katanga; and the Burra Burra mine in South Australia.
Reading the habit, which is the free test
Before weighing anything, the shape of the crystals narrows the field, and libethenite has a distinctive one.
The Handbook describes wedge-topped prismatic crystals, elongated along [100] or [001], with {110}, {011} and {111} dominant and often striated, and also equant to pseudo-octahedral forms. Add sprays of bladed to acicular crystals and druses. Olivenite by contrast is commonly curved, lamellar, matted fibrous, globular and reniform as well as crystallised, and its fibrous form is grey-green to straw-yellow rather than olive.
So the rule of thumb runs: sharp wedge-topped prisms and pseudo-octahedra suggest libethenite; matted fibres, globules and reniform crusts suggest olivenite. It is a tendency rather than a test, and it fails on exactly the specimens that matter — the sharp, well-crystallised ones, which both species produce.
Lustre is a small additional clue. Libethenite is vitreous on faces and greasy on fracture surfaces, which is a specific enough observation to be worth checking on a broken edge under a lens. See our note on identifying a specimen for the order to run these checks in.
What a collector should actually do with this
The honest summary of this page is that you can settle libethenite against olivenite yourself, at home, in about ten minutes, with a scale, a glass of water and a thread — and that almost nobody does, which is why so much material in old collections carries whichever of the two names was fashionable at the mine.
What it costs is the part usually left out. A scale reading to 0.01 g is not expensive, but the measurement needs a clean, single-species fragment of at least a few grams, and that is the constraint that defeats most specimens: a druse of tiny crystals on matrix cannot be weighed hydrostatically without breaking it, and breaking it costs more than the determination is worth. For those, the answer is to record the uncertainty on the label rather than to guess.
Our note on labelling and cataloguing sets out the three certainty states a determination can be recorded in. “Libethenite or olivenite, not distinguished” is a better catalogue entry than a confident wrong one, and it is one a future owner can act on.
We keep no stock list and nothing here is offered. If you hold Cornish copper-phosphate or arsenate material with old labels, the wanted list sets out what we look for, and our species notes cover the rest of the Cornish oxidised copper suite.