Cyanotrichite
Cyanotrichite is a blue copper aluminium sulphate in acicular tufts to 3 cm. Its hardness has never been determined. Density 2.74 to 2.95. Look-alikes and care.
Cyanotrichite is a hydrated copper aluminium sulphate forming sky-blue to azure acicular crystals to 3 cm, usually as radiating, plush-like tufts on matrix. Its density is 2.74 to 2.95. The Handbook of Mineralogy records its hardness as not determined, which tells you most of what you need to know about handling it.
In short
- Its hardness has never been determined. The Handbook of Mineralogy prints Hardness = n.d. for cyanotrichite. That is not an omission: the crystals are hair-fine acicular needles and there is no surface on which a scratch test means anything.
- Density 2.74 to 2.95 — light, for a copper mineral. Compare olivenite at 4.46 or azurite at 3.77. The tufts sit on matrix and contribute almost nothing to the weight of a specimen.
- Sky-blue to azure-blue with a pale blue streak and a silky lustre. The name is from the Greek for blue and hair, and the habit is exactly that: acicular crystals to 3 cm, cottonball-like, in radially fibrous and plush-like tufted aggregates forming incrustations.
- It is one of several blue fibrous copper secondaries that cannot be separated by eye — carbonatecyanotrichite, langite, ktenasite and others share the colour and the habit. A label without an analysis is a probability.
- The classic localities are the Grandview mine in the Grand Canyon, Wheal Gorland and the Phoenix United mine in Cornwall, Old Potts Gill in the Caldbeck Fells, Cap Garonne in France, Moldova Nouă in Romania and Laurium in Greece.
| Species | Composition class | Habit | Whether you can call it by eye |
|---|---|---|---|
| Cyanotrichite | Cu-Al sulphate | Acicular to 3 cm, radiating plush-like tufts, silky | No. Needs analysis to separate from carbonatecyanotrichite |
| Carbonatecyanotrichite | Cu-Al carbonate-sulphate | Visually identical tufts | No. Distinguished only by analysis |
| Langite | Cu sulphate | Tabular to bladed crusts, greener blue | Sometimes. Habit is platy rather than hairlike |
| Ktenasite | Cu-Zn sulphate | Bladed, blue-green | Sometimes, from the greener tone and platy habit |
| Azurite | Cu carbonate | Blocky, tabular or acicular, vitreous | Yes. Effervesces in acid, much deeper blue, density 3.77 |
| Chalcanthite | Cu sulphate | Blue prismatic; water-soluble | Yes. It dissolves in water, which no other entry here does |
A species with no hardness, and what that means in practice
Open the Handbook of Mineralogy at cyanotrichite and the Physical Properties line reads, in full: Hardness = n.d. D(meas.) = 2.74–2.95 D(calc.) = 2.88. There is no cleavage listed, no fracture, no tenacity. For a mineral described in the scientific literature since the nineteenth century, that is a striking blank.
It is not an oversight. Mohs hardness is measured by scratching a surface, and cyanotrichite does not present one. The crystals are acicular needles elongated on [001], and the aggregates the Handbook describes as cottonball-like and radially fibrous and plushlike tufted. A scratch test on a tuft of hair-fine needles measures how easily the tuft is crushed, which is not the same property and is not comparable to a Mohs value for a solid crystal.
The working translation is simple: treat cyanotrichite as having no mechanical strength at all. A brush destroys it. A puff of compressed air destroys it. Water destroys the tufted structure even where it does not dissolve the mineral, because surface tension mats the needles down and they do not spring back. This is a species you handle by the matrix, never by the mineral, and preferably do not handle at all.
One quotable line for a label or a catalogue: cyanotrichite is the mineral that has no hardness because there is nothing solid enough to scratch.
The identification problem: blue fibrous copper minerals
Cyanotrichite has a near-twin. Carbonatecyanotrichite is the carbonate-bearing analogue, it forms in the same environments, and it produces visually identical sky-blue fibrous tufts. There is no colour difference, no habit difference and no practical field test. A great deal of material in older collections labelled cyanotrichite has never been checked.
Beyond that pair, the oxidised zones of copper deposits produce a whole family of blue and blue-green secondary sulphates that overlap in appearance: langite, posnjakite, ktenasite, serpierite, devilline. Some are separable on habit — langite and serpierite tend to be platy or bladed rather than hairlike — but the separation is a matter of experience and probability, not of a test.
What is reliably distinguishable is the group as a whole from the things it is confused with by non-specialists. Azurite is a carbonate, effervesces in acid, is a deeper blue and is far denser at 3.77. Chalcanthite is water-soluble, which nothing else here is, and dissolving a specimen is a decisive if regrettable test. Linarite is a lead-copper sulphate, deep royal blue, and much heavier.
Our page on identifying a mineral specimen sets out the order to run tests in. On this family, the honest answer is that the sequence stops early: locality plus habit gives a strong probability, and analysis gives an identification, and there is nothing useful in between. If you want an analysed cyanotrichite rather than a probable one, that is a specific thing to ask for on a wanted list.
Where it comes from, and the British occurrences
Cyanotrichite is an uncommon secondary mineral in the oxidised portions of copper sulphide deposits. Its associates are the other copper secondaries of that zone: brochantite, spangolite, chalcophyllite, olivenite, tyrolite, parnauite, azurite and malachite.
The Handbook names the Grandview mine — also called the Last Chance — in Grand Canyon National Park, Arizona, for exceptional specimens, and the published X-ray powder pattern for the species is taken from that locality. It is the reference occurrence.
Britain contributes two occurrences that matter. In Cornwall, the Handbook lists Wheal Gorland at St Day, the Phoenix United mine at Linkinhorne, and Lanivet. Wheal Gorland is one of the most important copper-arsenate localities in the world and is the type locality for several species; cyanotrichite from there is a genuine classic and almost entirely confined to old collections. In Cumbria, the Old Potts Gill mine in the Caldbeck Fells is the recorded occurrence.
Elsewhere: Moldova Nouă in Romania, the Cap Garonne mine near le Pradet in Var, France, the Sa Duchessa mine in Sardinia, Laurium in Greece, the Mednorudyanskoye copper deposit near Nizhni Tagil in the Urals, Akchagyl in Kazakhstan, and the Springbok mine in Namaqualand, South Africa.
Because the Cornish material is old-collection material, provenance carries unusual weight on this species — a Wheal Gorland cyanotrichite with a nineteenth-century label is a different object from an unattributed tuft. Our guide to reading collection labels covers how to weigh those claims.
Keeping one alive
Everything about cyanotrichite storage follows from the habit rather than the chemistry.
Never clean it. Not with water, not with a brush, not with an air blower, not ultrasonically. If a cyanotrichite specimen is dusty, it stays dusty. There is no recoverable state to return it to.
Immobilise it in a rigid, sealed container. A clear lidded polystyrene box with the specimen seated in a Plastazote cut-out is the standard, and it does two jobs: it keeps dust off, and it lets the specimen be looked at without being picked up. Reducing handling is the entire game.
Mind the static. A plastic lid lifted off a dry box carries enough charge to lift and mat hair-fine needles. Earth the box against your hand before opening, or better, do not open it.
Keep it dry and out of strong light. As a hydrated sulphate it is more comfortable in a stable, moderately dry environment than a damp one, and the general recommendation for a mixed geological store is 45 to 60% relative humidity at a stable 16–18 °C. Avoid rapid swings. Our handling and storage guide covers building microclimates for species with requirements that differ from the room.
Because it cannot be cleaned or repaired, condition on arrival is condition forever. That makes cyanotrichite one of the species where how a dealer packs a specimen is worth asking about before you commit, not after.