Torbernite
Torbernite is a copper uranyl phosphate, hardness 2 to 2.5, density 3.22, crystals to 2.5 cm. It is radioactive, and in dry air it dehydrates to metatorbernite.
Torbernite is copper uranyl phosphate, Cu(UO2)2(PO4)2 with 8 to 12 water, tetragonal, hardness 2 to 2.5 with a measured density of 3.22 and tabular crystals to 2.5 cm. It is radioactive. The Handbook of Mineralogy records that its water content varies with relative humidity, dulling as it dehydrates to metatorbernite.
In short
- Torbernite is one of the few minerals that changes species in your cabinet. The Handbook states that its water content varies with relative humidity and that the lustre becomes dull on dehydration to metatorbernite. Store it too dry and the label stops being correct.
- That puts it in direct conflict with the storage rule for iron sulphides, which want 45% relative humidity or lower. A collection holding both cannot be kept at one humidity, and the resolution is per-specimen microclimates, not a compromise number.
- It is radioactive and it is a uranium mineral, not a curiosity. Handle with gloves and a dust mask, minimise handling time, keep it out of living space, and have the collection checked with a counter.
- Hardness 2 to 2.5 with perfect micaceous cleavage on {001}. A fingernail marks it, and the plates split. It should never be picked up by a crystal, only by the matrix.
- It belongs to the autunite group and sits with metatorbernite, autunite and zeunerite. Emerald to apple-green square or octagonal plates with a pale green streak are the look; the Cornish mines, Jáchymov, Saxony and Katanga are the names.
| Group | Example species | What goes wrong | Direction of risk | Documented threshold |
|---|---|---|---|---|
| Hydrated uranyl phosphates | Torbernite, autunite | Loses water, dulls, converts to the meta- phase | Too dry | Water content varies with RH (Handbook of Mineralogy) |
| Hydrated copper sulphates | Chalcanthite, CuSO4·5H2O | Crumbles to a pale blue powder | Too dry | Below 35% RH (NatSCA/Icon) |
| Iron sulphides | Pyrite, marcasite | Efflorescence, acid, disintegration | Too damp | Above 60% RH; store at 45% or lower (NatSCA/Icon) |
| Soluble halides | Halite, NaCl | Deliquesces and dissolves | Too damp | Soluble in water (Handbook of Mineralogy) |
| General mixed geological store | Everything else | Little, within reason | Neither | 45–60% RH, 16–18 °C (NatSCA/Icon) |
The mineral that dehydrates on the shelf
Most species are inert in a cabinet. Torbernite is not. The Handbook of Mineralogy’s physical properties entry carries the line H2O content varies with relative humidity, and its lustre description ends becoming dull on dehydration to metatorbernite.
What that means in practice is that a bright, transparent emerald-green torbernite kept in a dry room will gradually lose water, lose transparency, and become metatorbernite — a different species, with four water molecules rather than eight to twelve. The change is not a defect and it is not anyone’s fault. It is the mineral doing what its structure allows.
The collector consequence is that a torbernite label records a determination made at a moment, on a specimen that has since been stored somewhere. Old Cornish torbernites in old collections are very often metatorbernite now. This is not a reason to relabel a drawer — the distinction is a hydration state and needs proper determination, not a guess — but it is a reason to record the date and the storage conditions alongside the name. The three-state convention on labelling and cataloguing a collection handles it.
The humidity conflict, and how to resolve it
Advice to keep a mineral collection dry is close to universal, and for sulphides it is right. The NatSCA and Icon leaflet Care and Conservation of Geological Specimens recommends 45 to 60% relative humidity for a mixed geological store, and for pyrite-bearing material not above 60% and preferably 45% or lower.
Torbernite wants the opposite, and so does chalcanthite, which the same leaflet says will lose water and crumble to a pale blue powder below 35% RH. Put a torbernite in a cabinet dried to protect the pyrite and you are actively converting it.
There is no single number that serves both, and the right answer is not a compromise — it is to stop treating the collection as one environment. The leaflet describes the method: a well-sealed box, a specimen packed in a Plastazote cut-out, and a small pierced zip-lock bag of oven-dried silica gel to hold the interior dry. The same technique run the other way, with conditioned rather than dried gel, holds a stable middle humidity for the species that need one. Each specimen gets the environment its chemistry requires, and the room can be whatever the room is.
The arithmetic of room humidity, and why a cold outside wall is the real problem in a British house, is worked through on storing a collection in a damp house.
Radioactivity, and handling it sensibly
Torbernite is a uranium mineral and the Handbook lists it plainly as radioactive. It is not dangerous to own and it is not something to be casual about either.
The conservation guidance is specific and worth following: check the collection with a counter to find out what you actually have; contact a Radiation Protection Adviser if the collection holds radioactive specimens; wear gloves and a dust mask when handling, because radon decay products attach to dust which can be inhaled; wash your hands afterwards; and minimise the time you spend holding the specimen. Store it lidded and away from where people sit.
Two practical additions for torbernite specifically. It is soft, at hardness 2 to 2.5 with perfect micaceous cleavage, so it sheds — which is precisely the dust the guidance is concerned with. And it should not be stored in a sealed container with no ventilation in a living room, because radon accumulates. A lidded box in an outbuilding or a rarely occupied room is the sensible compromise. Our page on whether a mineral is radioactive covers testing, and toxic minerals in a collection covers the rest of the hazardous drawer.
Localities, and the Cornish material
Torbernite occurs as a secondary mineral in the oxidised zone of uranium-bearing copper deposits, with metatorbernite, autunite, zeunerite and uraninite. It is found at many localities but rarely in quantity.
The historic names are Jáchymov in the Czech Republic, and Schneeberg and Johanngeorgenstadt in Saxony. France produced fine examples at Margabal in Aveyron and Les Bois Noirs in Puy-de-Dôme. In England the Handbook names a number of Cornish mines, Wheal Basset and Stenna Gwyn among them. The largest crystals came from the Musonoi mine near Kolwezi, with rich specimens from Shinkolobwe in Katanga; also Richelle in Belgium, Sabugal in Portugal, Mt Painter and the South Alligator Valley in Australia, and Chalk Mountain in North Carolina and the Majuba Hill mine in Nevada.
The species is named for Torbern Olof Bergmann, 1735 to 1784, the Swedish chemist and mineralogist, Professor of Chemistry at Uppsala. Cornish uranium secondaries with firm mine-level provenance are genuinely scarce and always on our wanted list; our Cornwall notes and the full species reference cover the ground they come from.