Copper
Native copper is the metal itself, hardness 2.5 to 3 and a density of 8.95. Crystals reach 15 cm and masses of hundreds of tons have been found. Habit and care.
Native copper is the metal itself, uncombined: cubic, hardness 2.5 to 3, with a measured density of 8.95 that is higher than any common sulphide. Crystals reach 15 cm and masses weighing hundreds of tons have been recovered. A fresh surface is pale rose and darkens within minutes.
In short
- The colour you buy is not the colour it was. The Handbook of Mineralogy records native copper as pale rose on a fresh surface, darkening quickly to copper-red and then to a dull metallic film. Every specimen you will ever see is already tarnished.
- Hardness 2.5 to 3, and malleable and ductile with it. Copper does not chip, it bends. A crystal that has been dropped is deformed rather than broken, and that deformation cannot be reversed without making it worse.
- Density 8.95 is the field test. Nothing else that looks like copper is that heavy — a small specimen feels wrong in the hand, which is the first thing an experienced collector checks.
- Twinning on {111} produces the contact and penetration twins and the cyclic groups that give the best Keweenaw crystals their character. Wire, filiform and arborescent habits are the other classic forms.
- The classic localities are the Keweenaw Peninsula of Michigan, Bisbee and Ajo in Arizona, the Turinsk mine in the Urals, Onganja and Tsumeb in Namibia, Broken Hill in New South Wales, Corocoro in Bolivia, and many Cornish mines.
| Habit | What it looks like | What it implies |
|---|---|---|
| Crystallised | Cubes, dodecahedra, tetrahexahedra; rarely octahedra. Commonly flattened on {111} or elongated along [001] | The collectable form. Sharp faces are uncommon because copper deforms so readily |
| Twinned | Contact and penetration twins on {111}, and cyclic groups | The Keweenaw speciality. Cyclic groups are what the best Michigan material is bought for |
| Wire and filiform | Twisted, hairlike strands, sometimes matted | Extremely fragile. This is the habit that arrives damaged if it is packed like a rock |
| Arborescent | Branching, tree-like plates | Usually flattened in a fracture, so it has a front and a back — check both before buying |
| Massive | Solid metal, no crystal form | Industrially important, historically interesting, collectable mainly for size or locality |
| Coarse powder | Disseminated grains | Not a specimen habit. Mentioned because it turns up in matrix and can be mistaken for alteration |
What native copper actually is, and why that matters for handling
Most copper minerals are compounds — the carbonates azurite and malachite, the oxide cuprite, the sulphides chalcocite and bornite. Native copper is the element on its own, cubic, point group 4/m 3 2/m, with a cell edge of 3.615 Å. It occurs where oxidising conditions in a copper deposit reduce the metal out of solution, typically in porous zones in mafic extrusive rocks, less often in sandstones and shales, and in the oxidised zone of large disseminated copper deposits.
Being a metal rather than a compound changes everything about how it behaves on a shelf. It has no cleavage; the fracture is hackly, the way torn metal tears. It is highly malleable and ductile, which sounds like a curiosity until you realise what it means for a specimen: a crystal that takes a knock does not lose a corner, it acquires a flat. A dealer's photograph taken before a journey and your specimen after it can differ in a way that no amount of cleaning will address.
The other consequence is weight. At a measured density of 8.95 — against 7.24 for mimetite, 4.46 for olivenite and 2.65 for quartz — a thumbnail of solid copper is startlingly heavy. Density is the quickest non-destructive check there is, and on this species it is decisive.
Why every copper specimen you see is the wrong colour
The Handbook of Mineralogy is precise about this and most descriptions are not: the colour is pale rose on a fresh surface, quickly darkening to copper-red, then metallic and shining. The bright salmon-pink of a freshly broken face lasts minutes in air. What you buy is an oxide film.
This creates a standing problem in the trade, because a tarnished copper specimen can be brightened, and a brightened specimen looks more like the mineral than an untouched one does. Dilute acid, ammonia, proprietary metal polish and simple abrasion all work. None of them is detectable at a glance afterwards, and all of them remove metal along with the film.
The defensible position for a collector is the same one that applies to repair and restoration: cleaning is normal and acceptable, and undisclosed cleaning is not. Ask. A dealer who says a piece was cleaned with ammonia has told you something useful; a dealer who is vague about a suspiciously bright wire group has told you something else. If you are assembling a copper suite and want untouched surfaces specifically, say so on your wanted list, because it is a real constraint that changes which pieces are suitable and it is much easier to apply before a piece is bought than after.
One quotable rule: on native copper, brightness is a claim about the last owner, not about the mine.
The localities, and what each is actually known for
The Keweenaw Peninsula in Michigan — Keweenaw and Houghton counties — is the locality the species is measured against, and the Handbook singles it out for remarkably large masses and excellent, large crystals. Keweenaw material is also the source of most of the classic copper-in-calcite and copper-with-silver “halfbreed” specimens.
Arizona contributes several porphyry copper deposits: the New Cornelia mine at Ajo in Pima County, the Copper Queen and other mines at Bisbee in Cochise County, and Ray in Gila County. Santa Rita in New Mexico works the same way. From Russia, the Turinsk copper mine at Bogoslovsk in the Urals produced large crystals. Namibia gives Onganja, 60 km northeast of Windhoek, and Tsumeb. Germany has Rheinbreitbach in North Rhine-Westphalia and the Friedrichssegen mine near Bad Ems. Australia has Broken Hill, Chile has Andacollo near Coquimbo, and Bolivia has Corocoro.
Britain's contribution is Cornwall, where the Handbook records fine specimens from many mines without singling one out. Cornish native copper is genuinely scarce on the market compared with the county's copper secondaries — the arsenates and carbonates that made the orefield famous — and an old-label Cornish copper is a harder thing to find than its price history suggests.
A note on labels: “Michigan” is not a locality. The Keweenaw mines are individually named and the difference between them matters to anyone building a serious suite. Our guide to reading mineral labels covers how much weight to put on a vague one.
Storing copper, and the one association that causes trouble
Copper itself is chemically undemanding by the standards of a mineral collection. It does not deliquesce, it does not dehydrate, and it is not light-sensitive. The tarnish film that forms is stable and, once formed, largely self-limiting.
Two things do cause trouble. The first is acid in the air of a drawer. Copper is attacked by the sulphuric acid that decaying pyrite throws off, and by acids released from unsuitable card, chipboard and some adhesives. If you keep decaying pyrite in the same cabinet, the copper will develop green corrosion products — and unlike tarnish, those are not a film, they are a loss of metal.
The second is the matrix, not the copper. Keweenaw specimens frequently carry calcite, datolite, epidote and prehnite. Those minerals have their own requirements and they are the ones that will fail first. Judge the storage conditions by the most demanding thing in the box, which on a copper specimen is rarely the copper.
The general environment recommended for mixed geological collections is 45 to 60% relative humidity at a stable 16–18 °C, which suits copper perfectly well. The detail is in our handling and storage guide. Where copper differs from almost everything else is mechanical: because it deforms rather than breaks, packing matters more than for a harder mineral of the same size, and the wire and arborescent habits need to be immobilised rather than merely cushioned. See packing and shipping specimens.