Bornite
Bornite is Cu5FeS4, hardness 3 to 3.25, density 5.06, crystals to 6 cm. It tarnishes iridescent by itself, which is why real peacock ore is rarely bornite.
Bornite is copper iron sulphide, Cu5FeS4, orthorhombic and pseudocubic, hardness 3 to 3.25 with a measured density of 5.06 to 5.08 and crystals to 6 cm. Fresh surfaces are copper-red to pinchbeck-brown and tarnish iridescent purple on their own. The streak is pale greyish black.
In short
- Bornite tarnishes iridescent without help. The Handbook of Mineralogy records fresh surfaces as copper-red to pinchbeck-brown, rapidly tarnishing iridescent purplish. That is the species behaving normally, not a treatment, and it is the reason the collector name attached itself to bornite in the first place.
- Most material sold as peacock ore is chalcopyrite, not bornite. Chalcopyrite is brass-yellow at hardness 3.5 to 4; bornite is copper-red at 3 to 3.25. The two are separated on a fresh break in a few seconds, and the fresh break is the only test that works once a film is on the specimen.
- Hardness 3 to 3.25 makes it a soft specimen for a sulphide. A copper coin at roughly 3.5 will mark bornite. It should not be stored where it can rub against harder species, and it should not be cleaned with anything abrasive.
- Crystals are the rare form. The species is overwhelmingly granular, compact or disseminated. Pseudocubic and dodecahedral crystals to 6 cm, commonly penetration-twinned on {111}, are what makes a bornite specimen worth keeping rather than worth smelting.
- It is a thermometer as well as an ore. Bornite is stable below about 200 degrees C, so its presence in a vein says something about the temperature at which that part of the orebody formed.
| Species | Colour on a fresh break | Hardness | Density | Streak | What the iridescence is |
|---|---|---|---|---|---|
| Bornite | Copper-red to pinchbeck-brown | 3–3.25 | 5.06–5.08 | Pale greyish black | Natural, and forms within days of the break |
| Chalcopyrite | Brass-yellow | 3.5–4 | 4.1–4.3 | Greenish black | May be natural; on commercial peacock ore it is usually induced |
| Chalcocite | Blackish lead-grey | 2.5–3 | 5.5–5.8 | Blackish lead-grey | Dull black tarnish, not iridescent |
| Covellite | Indigo-blue | 1.5–2 | 4.6–4.76 | Lead-grey | Commonly iridescent, but the fresh colour is already blue |
| Pyrite | Pale brass-yellow | 6–6.5 | 5.018 | Greenish to brownish black | Darkening, occasionally iridescent; far too hard to confuse by feel |
What peacock ore actually is
“Peacock ore” is a trade description of a surface, not a mineral name, and two entirely different species arrive under it. One is bornite, which does the thing by itself. The other is chalcopyrite that has been treated with acid to produce a film, and that is the material sold in quantity.
The distinction matters because the two behave differently in a cabinet. A natural bornite tarnish is the outer skin of the specimen and continues to develop; an induced film on chalcopyrite sits on top of whatever surface it was applied to and can be uneven, glassy and improbably uniform across a fresh break. Our page on chalcopyrite sets out the same problem from the other side. The issue is never that a specimen is treated. It is that a treated specimen is often not described as one.
The honest position is that a bright, wall-to-wall purple-and-green film on a cheap, lump-shaped piece of sulphide is almost never bornite. Bornite tarnish is patchier, more copper-toned, and sits on material that looks like an ore mineral rather than like a souvenir.
The fresh-break test, and why nothing else works
Once a tarnish film is present, colour is useless: the film is the colour. Every reliable test therefore has to reach fresh material.
Find a hidden edge — the back, the underside, a broken corner — and look at unfilmed mineral under good light. Bornite is copper-red to bronze. Chalcopyrite is brass-yellow. That single observation settles it, and it does not require a scratch.
Where no fresh surface exists, hardness will do the same job with a small sacrifice. Bornite at 3 to 3.25 is marked by a copper coin; chalcopyrite at 3.5 to 4 is not. Do the test on the base or on matrix, never on a display face, and confirm with the streak: bornite gives pale greyish black, chalcopyrite greenish black. The method is set out in full on testing mineral hardness.
The weight test that people reach for does not work here. Bornite at 5.06 to 5.08 and chalcopyrite at 4.1 to 4.3 differ by about a fifth, which is not something a hand can judge on an irregular lump with matrix attached.
Where crystallised bornite comes from
The Handbook of Mineralogy lists the localities for fine crystals, and it is a short list. Butte, Silver Bow County, Montana and Bristol, Hartford County, Connecticut in the USA; the Carn Brea mine, Illogan, and elsewhere in Cornwall in England; large crystals from the Mangula mine, Lomagundi district, Zimbabwe; Frossnitz Alpe in eastern Tirol, Austria; Dzhezkazgan in Kazakhstan; and the N’ouva mine, Talate, Morocco.
As an ore rather than as crystals it is far more widespread — the Magma mine at Superior and Bisbee in Arizona, Kennicott in Alaska, Ookiep in Namaqualand and the Messina mine in the Transvaal, Mt Lyell in Tasmania and Olympic Dam in South Australia. A locality on a bornite label is doing most of the work, because the species is common and the crystallised material is not. Our Cornwall locality notes cover the Carn Brea ground, and the rest of the species reference covers the copper minerals that keep company with it.
Bornite is a contact and hydrothermal species, associated with chalcopyrite, pyrite, garnet, calcite, wollastonite and quartz, and also found disseminated in mafic igneous rocks and cupriferous shales.
Keeping a bornite specimen
Assume the surface will change. A bornite that arrives copper-bronze will not stay that way, and a bornite that arrives iridescent purple will go on darkening. Nothing stops this, and attempts to stop it — lacquer, wax, polish — are worse than the tarnish, because they are irreversible and they are the sort of intervention that has to be disclosed for the rest of the specimen’s life.
What you can do is keep it away from abrasion. At hardness 3 to 3.25 it is softer than most of what it will sit next to, so it wants its own compartment: a lidded box with a cut foam nest rather than a shared drawer. The reasoning behind that is on specimen boxes and stands, and the general handling rules are on handling and storage.
Bornite is named for Ignaz Edler von Born, 1742 to 1791, the Austrian mineralogist. If you are looking for crystallised material from one of the named localities rather than massive ore, our wanted list is the place to tell us what you are after.