Conichalcite
Conichalcite is CaCu(AsO4)(OH), hardness 4.5, density 4.33, with a green streak. Density is what separates it from duftite, which looks the same at 6.40.
Conichalcite is a calcium copper arsenate, CaCu(AsO4)(OH), orthorhombic, hardness 4.5, measured density 4.33, with a green streak. Crystals reach only about 3 mm; the usual form is a radial fibrous, botryoidal or reniform crust in grass-green to pistachio-green. It belongs to the adelite group and occurs in the oxidised zone of copper deposits.
In short
- The streak is green. That is unusual and it is the first test to run: most green crusts on an oxidised copper specimen streak white or pale green. Conichalcite, paratacamite and mottramite are among the few that give a genuinely coloured streak.
- Hardness will not separate the green arsenate crusts. Conichalcite, cornwallite and duftite are all 4.5, and austinite is 4 to 4.5. Four species, one hardness value.
- Density does separate them, and duftite is the outlier. Conichalcite 4.33, cornwallite 4.17, austinite 4.13 — then duftite at 6.40, which is 48 per cent heavier and reflects the lead in its formula.
- It is typically an alteration product of enargite, which is the sort of detail that tells you where to look in a dump: conichalcite marks ground that carried copper-arsenic sulphides rather than plain chalcopyrite.
- Britain has it at two ends of the country. The Handbook names Wheal Kendall at St Hilary and the Hingston Down quarry at Calstock in Cornwall, and the Caldbeck Fells in Cumbria.
| Species | Formula | Hardness | Density | Streak | Colour |
|---|---|---|---|---|---|
| Conichalcite | CaCu(AsO4)(OH) | 4.5 | 4.33 | Green | Grass-green to yellowish green, pistachio-green |
| Duftite | PbCu(AsO4)(OH) | 4.5 | 6.40 | Pale green to white | Bright olive-green to grey-green |
| Cornwallite | Cu5(AsO4)2(OH)4 | 4.5 | 4.17 | Not stated | Verdigris-green to blackish green |
| Austinite | CaZn(AsO4)(OH) | 4 to 4.5 | 4.13 | Not stated | Colourless, white to pale yellowish white, green |
| Mottramite | PbCu(VO4)(OH) | 3 to 3.5 | about 5.9 | Yellowish green | Grass-green, olive-green, blackish brown |
| Olivenite | Cu2(AsO4)(OH) | 3 | 4.38 | Olive-green to brown | Olive-green to yellow, brown |
The green streak, and why it is the test to run first
Most of what you can do to a botryoidal crust on a matrix is destructive. It is too small to weigh accurately, too irregular to measure, and too thin to scratch without taking a piece off. The streak plate is the exception: one light drag on an inconspicuous edge leaves a mark and takes almost nothing.
Conichalcite streaks green. So does olivenite, olive-green to brown, but olivenite is hardness 3 rather than 4.5. Mottramite streaks yellowish green and is 3 to 3.5. Duftite — the species conichalcite is most often confused with, because they are the same formula with lead in place of calcium — streaks pale green to white. That is a real difference and it is visible.
The same approach works on the whole oxidised-copper suite and is the method our page on identifying a mineral specimen puts first for crust material. Streak, then colour, then weight if the specimen is big enough to weigh — in that order, because each step costs more than the last.
Duftite at 6.40, and the 48 per cent gap
Conichalcite is CaCu(AsO4)(OH). Duftite is PbCu(AsO4)(OH). The structures are closely related — both sit in the space group P212121 with almost identical cell dimensions — and the only change is calcium out, lead in.
That single substitution takes the measured density from 4.33 to 6.40, a 48 per cent increase. It is the clearest illustration in this part of the mineral kingdom of why density is worth measuring: two minerals that look the same, scratch the same at hardness 4.5 and occur in the same places differ by half their own weight.
The practical method is hydrostatic weighing — weigh the specimen in air, weigh it suspended in water, and the ratio of the two gives the density. It needs the crust to be separable from the matrix, which is often the obstacle. Where it is not separable, the streak difference above is what you have. Do not attempt to infer density from how heavy a specimen feels: the matrix dominates the mass and the crust is a film on top of it.
An alteration product of enargite, and what that tells you about the ground
The occurrence line is more useful than it looks: an uncommon secondary mineral in the oxidized zone of copper deposits, typically an alteration product of enargite.
Enargite is a copper arsenic sulphide. So conichalcite is a marker for copper deposits that carried arsenic in the primary sulphides, rather than copper deposits generally. That is why the locality list reads the way it does — Tsumeb and the Guchab mine in Namibia, the Ojuela mine at Mapimí in Durango, Bisbee in Arizona, the Tintic district and the Gold Hill mine in Utah, Collahuasi in Chile, and the type locality at Hinojosa de Córdoba in Andalusia. These are the classic arsenic-rich oxidised zones.
The associated species named by the Handbook make the same point: austinite, olivenite, clinoclase, libethenite, chenevixite, brochantite, malachite, azurite and jarosite. If you are working a dump and finding olivenite and clinoclase, conichalcite is a reasonable thing to be looking for. Our Ojuela and Tsumeb notes cover two of the productive localities.
British conichalcite: Cornwall and the Caldbeck Fells
Two British areas appear in the Handbook's distribution list. In Cornwall: Wheal Kendall at St Hilary, and the Hingston Down quarry at Calstock. In Cumbria: the Caldbeck Fells.
Neither produces display material. Conichalcite crystals reach 3 mm at the outside anywhere in the world, and British occurrences are crusts and radial fibrous aggregates measured in millimetres across. This is micromount and thumbnail territory, and it needs a stereo microscope to be worth owning. Our page on micromounts covers that size class, and choosing a microscope covers the instrument.
The Caldbeck Fells occurrence is the more interesting of the two in context, because that area is already the reference British locality for copper and lead secondaries. A Caldbeck suite that runs to the uncommon arsenates rather than stopping at linarite and the other well-known species is a considerably better collection, and it is assembled by asking rather than by browsing. Our Caldbeck Fells notes set out the ground, and specific species-and-locality briefs belong on a wanted list.