Chalcanthite
Chalcanthite is CuSO4 pentahydrate, hardness 2.5, density 2.286, crystals to 2 cm. It dissolves in water, dehydrates in dry air, and is commonly post-mine.
Chalcanthite is hydrated copper sulphate, hardness 2.5, density 2.286, triclinic, with natural crystals to 2 cm and Berlin-blue to sky-blue colour. It is soluble in water and dehydrates in dry air. The Handbook of Mineralogy records it as a secondary mineral of oxidised copper sulphide zones, commonly of post-mining formation.
In short
- It dissolves in water. Not slowly, not partially — the Handbook of Mineralogy lists chalcanthite as soluble in H2O with a metallic taste. One rinse under a tap will visibly damage a specimen and a soak will destroy it.
- It dehydrates in dry air, losing water from the structure and going chalky. A collection held below roughly 35 per cent relative humidity is a collection that will lose its chalcanthite, which is the opposite of the problem pyrite has.
- Most large blue chalcanthite on sale is laboratory-grown, not mined. Natural crystals reach about 2 cm; the vivid blue clusters several centimetres across, often on a rough matrix, are grown from solution. This is not a secret in the trade, but it is frequently not stated on the label.
- Much natural chalcanthite is post-mine. The Handbook says so explicitly — it forms on mine timbers, walls and dumps after working stopped, which makes it a genuine mineral but a young one, and explains why so much of it is a crust rather than a crystal.
- Do not taste it. The old determinative tables give the taste as diagnostic. Copper sulphate is toxic and the test has no place in a modern collection.
| Species | Formula | Hardness | The failure mode | What it needs |
|---|---|---|---|---|
| Chalcanthite | Cu(SO4)(H2O)4 · H2O | 2.5 | Dissolves in water; dehydrates and goes chalky in dry air | Sealed box, humidity held above about 35 per cent, never washed |
| Halite | NaCl | 2 | Dissolves in water; deliquesces in humid air | Sealed box with desiccant — the opposite requirement |
| Torbernite | Cu(UO2)2(PO4)2(H2O)4 · 8H2O | 2 to 2.5 | Water content varies with humidity; dulls on dehydration to metatorbernite | Stable humidity, and see the radioactive-storage note |
| Langite | Cu4(SO4)(OH)6 · 2H2O | 2.5 to 3 | Hydrated copper sulphate; unstable if dried hard | Sealed box, no direct heat |
| Pyrite | FeS2 | 6 to 6.5 | Decays in damp air | Below 60 per cent relative humidity, preferably 45 or lower |
The grown-crystal problem, and how to recognise it
Chalcanthite is trivially easy to crystallise from solution — it is the standard school demonstration — and the results are large, vivid and geometric in a way natural material almost never is. The Handbook of Mineralogy gives natural crystals as uncommon and reaching about 2 cm; the commonest natural form is a stalactitic or reniform crust, cross-vein fibrous, massive or granular.
So the arithmetic is straightforward. A cluster of sharp blue crystals several centimetres across, uniform in size, sitting on a rough dark rock that looks unrelated to them, is grown material mounted on a matrix. A blue crust or fibrous vein filling, with crystals under a centimetre if there are any at all, is much more likely to be natural.
Grown chalcanthite is not a fake in the sense that matters — nobody is passing off one mineral as another, and the substance really is copper sulphate pentahydrate. The problem is only the label. A grown specimen described as from a named mine, with no note that it was crystallised rather than collected, is a misdescription, and it is the kind of thing our page on telling a fake crystal from a real one exists to cover. Ask; a straight dealer will tell you straight away.
Post-mine formation, and why that is not a criticism
The Handbook records chalcanthite as forming in the oxidised portions of copper sulphide deposits, commonly of post-mining formation, and rarely as a fumarolic deposit. In plain terms: a lot of it grew after the mine shut, on timbers, on walls and in the dumps, out of copper-rich water that had nowhere else to go.
That does not make it less of a mineral. It is a naturally formed crystalline substance with a fixed composition, and the IMA has it on the master list with 1853 as the year of description. It does mean two practical things. First, chalcanthite from a famous mine may postdate the famous production by decades, so a locality alone tells you little about the age of the piece. Second, post-mine material is often associated with other soluble sulphates — melanterite, epsomite, goslarite, pickeringite — which have the same storage requirements.
The same qualifier appears against crocoite and langite, which is one of the reasons the phrase ex-mine on a label carries less information than people assume. We cover that on what ex-mine means on a specimen label.
Storage: the species that proves one humidity does not fit a collection
Chalcanthite dehydrates in dry air. Pyrite decays in damp air. Those two sentences are the whole of the problem, and a single cabinet cannot satisfy both.
The NatSCA and Icon leaflet on the care and conservation of geological specimens sets the sulphide requirement at not above 60 per cent relative humidity and preferably 45 per cent or lower. Chalcanthite wants the air above roughly 35 per cent and would rather it were higher. The overlap is real but narrow, and a British house that is heated in winter and unheated in summer will leave it in both directions over a year.
The resolution is the one the leaflet itself describes and the one our torbernite page reached from the other side: per-specimen microclimates rather than one room-wide number. A sealed polypropylene box with a small quantity of conditioned silica gel holds its own humidity regardless of what the room does, and you can set different boxes to different values. It is more work than a dehumidifier and it is the only approach that does not require you to choose which half of the collection to sacrifice. Our note on storing a collection in a damp house sets out the method.
Where natural chalcanthite comes from
Chalcanthite is widespread, and the Handbook notes that commercial deposits — it was once worked for copper — occur in arid regions, which follows directly from its solubility. Anywhere it rains, it washes away.
The named occurrences include the island of Cyprus; Vesuvius in Campania; Rammelsberg and Goslar in the Harz; Rio Tinto in Huelva, Spain, where it is abundant; and in Chile Chuquicamata, Quetena near Calama, and Copaquire in Antofagasta. In the United States it is recorded at the United Verde mine and Clifton-Morenci in Arizona, the Blue Bird mine in New Mexico, in commercial amounts at the Bluestone mine in the Yerington district of Nevada, at Butte in Montana, and at Ducktown in Tennessee. The analysed material in the Handbook is from Luishia in the Congo.
The name comes from an older Latin word, chalcanthum, for flowers of copper. Note what is not in that list: no British locality of any consequence. Chalcanthite occurs in Cornish and Welsh mines as an efflorescence underground, but it is not material that survives collection, which is the honest reason it is absent from British cabinets.