Pseudomalachite
Pseudomalachite is a copper phosphate, hardness 4.5 to 5, named from the Greek for false because it resembles malachite. The tests that separate the two.
Pseudomalachite is a copper phosphate, hardness 4.5 to 5, named from the Greek for false because it looks like malachite and is not. The decisive test is cold dilute acid: malachite is a carbonate and fizzes, pseudomalachite is a phosphate and does not. Crystals are rare and reach 6 mm.
In short
- Two tests separate it from malachite, and both are quick. Cold dilute acid: malachite effervesces, pseudomalachite does not. Hardness: 3.5 to 4 against 4.5 to 5, so a steel point marks malachite and struggles on pseudomalachite.
- The colour is darker than malachite's — dark emerald, blackish green or bluish green — and the streak is pale green to pale bluish green rather than malachite's brighter green.
- Crystals are genuinely rare, reaching about 6 mm and usually rough and rounded. Almost all pseudomalachite is compact radiating and spherical aggregate, botryoidal crust or a paint-like film.
- It is the phosphate analogue of cornwallite, the arsenate, which is why the two turn up together in Cornish copper ground and why old labels confuse them.
- Caldbeck Fells and a number of Cornish mines are the British occurrences; the type locality is the Virneberg mine at Rheinbreitbach in Germany.
| Species | Formula | Hardness | Cold dilute acid | Streak |
|---|---|---|---|---|
| Pseudomalachite | Cu5(PO4)2(OH)4 | 4.5 to 5 | No effervescence | Pale green to pale bluish green |
| Malachite | Cu2(CO3)(OH)2 | 3.5 to 4 | Effervesces strongly | Light green |
| Cornwallite | Cu5(AsO4)2(OH)4 | 4.5 | No effervescence | Green. The arsenate analogue; needs analysis to separate |
| Libethenite | Cu2(PO4)(OH) | 4 | No effervescence | Olive-green. Associated, and sharply crystallised |
| Atacamite | Cu2Cl(OH)3 | 3 to 3.5 | No effervescence | Apple-green |
| Chrysocolla | Hydrated copper silicate | 2 to 4 | No effervescence | White to pale green. Much softer, and blue-leaning |
The name is the identification problem
Pseudomalachite takes its name from the Greek for false, because it closely resembles malachite. The resemblance is real: both are green secondary copper minerals from oxidised copper deposits, both are commonly botryoidal, both form radiating fibrous aggregates, and both occur as crusts and films on the same kinds of matrix. A great deal of material in old collections labelled malachite is pseudomalachite, and the reverse is also true.
Two things resolve it, and neither requires equipment. Acid is decisive: malachite is a copper carbonate and effervesces briskly under a drop of cold dilute hydrochloric acid, while pseudomalachite is a phosphate and does nothing. As always this is destructive, so use it on an inconspicuous point of ordinary material and not at all on a fine specimen. Hardness is the non-destructive alternative and it is unusually clean here: malachite at 3.5 to 4 is marked by a steel point, while pseudomalachite at 4.5 to 5 mostly is not.
Colour is a genuine hint even though it is not evidence. Pseudomalachite tends darker — dark emerald to blackish green, sometimes distinctly bluish green — where malachite is a brighter, more yellow-leaning green. And the streaks differ: pale green to pale bluish green against malachite's light green.
Habit, and why crystals matter here
Crystals of pseudomalachite are rare and small. The Handbook of Mineralogy gives them as reaching 6 mm, typically rough and rounded rather than sharply faced, with a monoclinic form and twinning on {100}. What you will normally meet is compact radiating and spherical aggregate, fibrous material, botryoidal surfaces, and paint-like crusts and films — that last term is exact, because thin pseudomalachite on a fracture surface really does look as though someone has applied dark green paint to the rock.
The consequence for collecting is that a crystallised pseudomalachite is a significant specimen and a crust is an ordinary one, and the gap between them is larger than for most species. Where the specimen is a crust, the interest shifts to the association and the locality: pseudomalachite with libethenite, or on a Cornish matrix with cornwallite and olivenite, is worth more attention than a better-looking crust with no context.
Cleavage is perfect on {100} and the fracture is splintery, so despite a respectable hardness the fibrous aggregates are not robust. Box it individually.
Cornwallite, and the arsenate-phosphate pairing
Pseudomalachite is the phosphate analogue of cornwallite, Cu5(AsO4)2(OH)4 — the same structure with arsenic in place of phosphorus. Cornwallite was described from Cornwall, hence the name, and the two species occur together in the same Cornish copper ground.
This is a recurring pattern in the oxidised copper zone and it is worth learning as a pattern rather than as a list, because it explains a great deal of label confusion. Olivenite is the arsenate paired with libethenite, the phosphate. Cornwallite is the arsenate paired with pseudomalachite. In each case the two members look alike, occur together, and cannot be separated without analysis — and in each case the Cornish mines produced both.
What follows practically: on Cornish material, be sceptical of a confident assignment between these pairs on an old label, keep the label anyway because it is provenance, and treat analysed material as meaningfully more valuable than unanalysed. The same reasoning we apply to old collection material in general applies with extra force here.
Localities and what to look for
The type locality is the Virneberg mine at Rheinbreitbach in Rhineland-Palatinate, Germany, where the name was introduced in 1813, with further rich German material from Ehl near Linz. Other significant occurrences are Łubietová in Slovakia; Nizhni Tagil and Bogoslovsk in the Urals; Kakanda and M'sesa near Kambove in Katanga and Mindouli in the Republic of the Congo; the Bwana Mkubwa, Ndola and Nchanga mines in Zambia; the Perkiomen and Ecton mines in Montgomery County, Pennsylvania; the Empire-Nevada mine at Yerington and several Arizona localities; Chuquicamata in Chile; and the West Bogan mine near Tottenham, New South Wales.
In Britain it occurs in the Caldbeck Fells and at a number of mines in Cornwall, generally in small amounts.
When buying, three questions do most of the work. Is it crystallised or a crust? — the difference is substantial. Has it been distinguished from malachite and from cornwallite, and how? — a seller who says the acid test was done, or that it was analysed, is telling you something worth having. What is the mine name? — on Congolese and Zambian material the country is often all that is offered, and on Cornish material a bare county is not enough.
Handling is easy: copper minerals are among the least hazardous in a collection, and normal handwashing suffices. If you are looking for crystallised material or a documented British piece, tell us on the wanted list. We hold no stock; the useful part is asking the three questions above before anything is committed.