Langite
Langite is a hydrated copper sulphate, hardness 2.5 to 3, crystals to 7 mm. It can form on mine walls and dumps after the workings closed, so it postdates them.
Langite is a hydrated copper sulphate, monoclinic, hardness 2.5 to 3 and density 3.28 to 3.34, in deep blue crystals to 7 mm and in wavy crusts. The Handbook of Mineralogy records that it may be of post-mine formation — so a Cornish langite can be younger than the mine it came from.
In short
- It can be younger than the mine. The Handbook of Mineralogy states that langite may be of post-mine formation. It grows on exposed sulphide in abandoned workings and on dump material, which means a specimen labelled with an eighteenth-century mine may have crystallised in the twentieth.
- Britain is the type country. The IMA-CNMNC list records langite as a grandfathered species of 1864 with the United Kingdom as the country of first description, and the Handbook's published analysis is Cornish.
- It cannot be told from its neighbours by eye. Wroewolfeite, posnjakite, serpierite, devilline and brochantite are all blue to blue-green hydrated copper sulphates from the same settings. Langite is dimorphous with wroewolfeite — same formula, different structure — so those two are inseparable without analysis.
- Crystals reach only 7 mm and are commonly skeletal. Twinning on {110} is repeated, producing snowflake-like or star-shaped groupings, which is the habit the species is collected for.
- Hardness 2.5 to 3, two cleavages, and a hydrated structure. This is a species to leave alone: do not wash it, do not brush it, and do not keep it in very dry air.
| Species | Formula | Hardness | Relationship to langite | Separation |
|---|---|---|---|---|
| Langite | Cu4(SO4)(OH)6·2H2O | 2.5–3 | — | — |
| Wroewolfeite | Cu4(SO4)(OH)6·2H2O | 2.5 | Dimorph — identical composition | X-ray or Raman only |
| Posnjakite | Cu4(SO4)(OH)6·H2O | ≈ 2 | One less water molecule | Analysis; habit is a weak guide |
| Serpierite | Ca(Cu,Zn)4(SO4)2(OH)6·3H2O | 2.5 | Contains calcium and zinc | Acicular habit; needs analysis to confirm |
| Devilline | CaCu4(SO4)2(OH)6·3H2O | 2.5 | Contains calcium | Platy habit; needs analysis to confirm |
| Brochantite | Cu4(SO4)(OH)6 | 3.5–4 | Anhydrous equivalent | Harder; greener; commoner |
Post-mine formation, and what it does to a locality label
The Handbook of Mineralogy gives langite's occurrence as an uncommon but widespread secondary mineral in the oxidation zone of copper sulfide deposits, and then adds five words that change how the species should be read: may be of post-mine formation.
What that means physically is straightforward. Mining exposes fresh sulphide to air and water. Oxidation begins immediately and continues for as long as the exposure lasts. Blue copper sulphates crystallise on the walls of abandoned adits, on the surfaces of dump material, and in the puddles and seeps of a worked-out mine, and they can do it over years rather than over geological time.
The consequence for a collector is precise and worth stating carefully. A label reading “langite, Botallack mine, Cornwall” is a statement about where the specimen was found, not about when it formed. Both can be entirely true and the specimen can still be recent. That is not a fault in the specimen and it is not a reason to doubt the label — it is a fact about the species.
It does mean langite is a poor choice if what you want from a specimen is antiquity. It is an excellent choice if what you want is the crystallography, because post-mine material is often better crystallised than the deep primary oxidation-zone material. The wider question of what a locality proves is on is a specimen without a locality worth buying.
The identification problem, stated honestly
Langite is dimorphous with wroewolfeite. Same formula, Cu4(SO4)(OH)6 with two waters; different structure. There is no hardness difference worth using, no density difference you could measure at home, no colour difference and no reliable habit difference. They are separated by X-ray diffraction or Raman spectroscopy and by nothing else.
The Handbook's association list makes it worse before it makes it better: wroewolfeite, posnjakite, serpierite, devilline, chalcophyllite, connellite, brochantite, malachite, gypsum. Every one of the first five is a blue to blue-green secondary copper sulphate from the same oxidised copper ground, and several of them occur on the same specimen.
Three things genuinely help. Habit: langite's repeated {110} twinning produces distinctive snowflake-like or star-shaped groupings, and a good rosette is suggestive. Hardness against brochantite: brochantite at 3.5 to 4 is measurably harder than langite at 2.5 to 3, and brochantite is far commoner, so a harder, greener blue crust is more likely to be brochantite. Provenance: material from a locality where langite has been confirmed analytically carries more weight than material from one where it has not.
The Handbook closes langite's distribution list with a sentence that should be read as a warning: Many older localities require modern confirmation. That applies to the label on an old Cornish blue crust as much as to anything on this site.
The British localities
The IMA-CNMNC list records the United Kingdom as the country of first description, in 1864, and the Handbook's analysis (1) is from Cornwall, England. The British localities it names are:
Cornwall — a number of mines, with the Fowey Consols mine at Tywardreath and the Botallack mine at St Just named specifically. Devon — large crystals at the Bedford United mine, Tavistock, which is the British locality for crystallised material and also a clinoclase locality. Cheshire — the Engine Vein mine at Alderley Edge, a copper locality quite unlike the Cornish ones. Cumbria — several mines in the Caldbeck Fells. And across the Irish Sea, the Dooneen mine at Allihies in County Cork.
Outside Britain: L'ubietová near Baňská Bystrica in Slovakia, which supplies the reference X-ray powder pattern; the Roua copper mines north of Nice and Mazega in Aveyron, France; the Clara mine in the Black Forest and the Richelsdorf Mountains in Hesse; Schneeberg and Freiberg in Saxony; Klausen in Alto Adige; Tsumeb; Paddy's River in the Australian Capital Territory; the Silver King mine in Nevada, the Grandview mine in the Grand Canyon and Bisbee in Arizona; and El Teniente in Chile.
The species is named for Victor von Lang, 1838 to 1921, physicist and crystallographer, Professor of Physics at the University of Vienna. Type material is at the Natural History Museum in Vienna, number A.a.4353. Our Cornwall and Caldbeck Fells pages cover the British ground, and specific copper secondary material is on the wanted list.
Keeping a hydrated copper sulphate alive
This is the part that costs people specimens. Langite carries structural water, has hardness 2.5 to 3, and has cleavage on both {001} and {010}. It is near the fragile end of everything in a normal collection.
Do not wash it. Not in water, not in detergent, not in an ultrasonic cleaner. Hydrated copper sulphates are the group where cleaning does the most damage for the least gain — see how to clean mineral specimens.
Do not let the air get very dry. The Natural Sciences Collections Association's guidance for geological collections notes that deep blue copper sulphate crystals lose water and crumble to a pale blue powder below 35 per cent relative humidity, and recommends store rooms at 45 to 60 per cent relative humidity and 16 to 18 degrees Celsius. A sealed case with silica gel, or a specimen kept beside a radiator in a modern centrally heated house, can go below that in winter. This is the one fault where a dehumidifier makes things worse, and it is covered on why a specimen grows a white crust.
Do not display it in strong light or handle the crystals. A skeletal 7 mm rosette is destroyed by a fingertip. Keep it in a lidded clear box, lined, and look at it through the lid. Placement rules for a mixed collection are on how to display a mineral collection.