Chalcopyrite
Chalcopyrite is CuFeS2, hardness 3.5 to 4, density 4.1 to 4.3, crystals to 10 cm. Hardness alone separates it from pyrite at 6 to 6.5 and from gold at 2.5 to 3.
Chalcopyrite is copper iron sulphide, CuFeS2, tetragonal, hardness 3.5 to 4 with a density of 4.1 to 4.3 and crystals to 10 cm. It is brass-yellow with a greenish-black streak and a metallic lustre, and occurs with sphalerite and galena. A steel point scratches it, which separates it at once from pyrite at hardness 6 to 6.5.
In short
- Hardness settles it in five seconds. Chalcopyrite is 3.5 to 4 and a steel point cuts it. Pyrite is 6 to 6.5 and a steel point slides off. Gold is 2.5 to 3 and deforms rather than powdering. Those three responses are unmistakable and no lighting condition changes them.
- The streak is greenish black. Gold's streak is golden and it smears rather than powders, because gold is malleable and the sulphides are brittle. This is the test that has stopped more false gold claims than any other.
- Crystals reach 10 cm and are equant, tetrahedral-looking sphenoids. The large {112} faces are dull and striated parallel to [110]; the small opposing faces are bright. That dull-large, bright-small contrast is diagnostic when you can see it.
- Iridescent tarnish can be natural or induced. The Handbook of Mineralogy records chalcopyrite as brass-yellow and possibly tarnished and iridescent, so the colour on its own does not tell you which. Bulk-sold rainbow material has usually been acid-treated.
- It is the main copper ore of the Cornish and Devon mines, and the primary sulphide from which most of the copper secondaries on this site — azurite, malachite, olivenite, clinoclase — were derived by oxidation.
| Mineral | Hardness | Streak | Density | The decisive test |
|---|---|---|---|---|
| Chalcopyrite | 3.5–4 | Greenish black | 4.1–4.3 | A steel point cuts it; brittle, powders |
| Pyrite | 6–6.5 | Greenish to brownish black | 5.0 | Steel will not scratch it; strikes sparks |
| Marcasite | 6–6.5 | Greenish to brownish black | 4.9 | Paler, tin-white when fresh; spear-shaped twins |
| Gold | 2.5–3 | Golden yellow | 19.3 | Malleable — it dents and smears, never powders |
| Pyrrhotite | 3.5–4.5 | Dark greyish black | 4.6 | Bronze, and magnetic to a varying degree |
| Bornite | 3–3.25 | Greyish black | 5.1 | Copper-red on a fresh break under the tarnish |
The three-test sequence for a brass-yellow mineral
Run these in order. Each one costs seconds and the sequence resolves almost every case without equipment.
1. Scratch it with steel. A knife point, a needle or a scriber is about hardness 5.5. Chalcopyrite at 3.5 to 4 is cut easily and gives a powder. Pyrite and marcasite at 6 to 6.5 are not cut at all; the steel skids and may leave a grey smear that wipes off. This single test separates the two commonest confusions.
2. Take a streak. On unglazed porcelain, chalcopyrite gives greenish black. Pyrite gives greenish to brownish black — similar, which is why the streak is the second test and not the first. Gold gives a golden streak and, crucially, smears instead of powdering, because gold is malleable and every sulphide here is brittle. If the mark is metallic and continuous rather than dusty, stop and get it looked at.
3. Weigh it in the hand. Chalcopyrite is 4.1 to 4.3, pyrite 5.0. That difference is detectable on a fist-sized piece once you have handled both, though not reliably on a small one. Gold at 19.3 is in a different world and a gold-bearing specimen feels wrong immediately.
Crystal form is the fourth check where crystals exist: chalcopyrite's sphenoids look tetrahedral, pyrite's cubes and pyritohedra do not. The full sequence for unknowns is on how to identify a mineral specimen.
Peacock ore, and what the iridescence actually is
“Peacock ore” is a trade name, not a species. It is applied to two different things: to bornite, Cu5FeS4, which tarnishes to blue and purple naturally, and to chalcopyrite that has been given an iridescent surface.
The Handbook of Mineralogy records chalcopyrite's colour as brass-yellow, may be tarnished and iridescent, so natural iridescent chalcopyrite is real and does occur. The problem is that the material sold in bulk — uniform, saturated, evenly coloured across every surface including fresh breaks — is usually chalcopyrite treated with acid to produce the film.
The tell is uniformity and the break. Natural tarnish is patchy, follows the exposure history of the specimen, and stops where the crystal was buried in matrix. An induced film is even, covers faces that were never exposed, and often runs over a broken surface, which is physically impossible for a tarnish that formed underground. A treated specimen is also frequently ex-mine crushed ore rather than a crystallised specimen.
None of this makes a treated piece worthless, but it does make it a different object, and the honest position is that undisclosed treatment is the problem, not the treatment itself. We go through the general question on how to tell if a crystal is fake.
The primary ore behind the secondary species
Chalcopyrite is a primary mineral in hydrothermal veins, stockworks, disseminations, and massive replacements, with sphalerite, galena, tetrahedrite and pyrite as its usual company. It is, by a wide margin, the world's most important copper ore mineral.
For a British collector its significance is indirect. The copper secondaries that make Cornwall and Devon famous — azurite, malachite, olivenite, clinoclase, liroconite, cornwallite — are oxidation products of primary sulphide ore, and that ore was overwhelmingly chalcopyrite. The arsenic in the arsenates came from arsenopyrite in the same lodes. Understanding the primary ore explains why the secondaries occur where they do and why they stop at the depth the oxidised zone stops.
Crystallised chalcopyrite specimens worth collecting are a different matter from ore. The Handbook picks out large crystals from the Groundhog mine at Vanadium in Grant County, New Mexico; crystals from the Rossie lead mines in St Lawrence County, New York; French Creek in Pennsylvania; Joplin in Missouri; Bisbee in Arizona; Cananea in Sonora; Huaron in Peru; the Kidd Creek mine near Timmins and Sudbury in Ontario; the Noranda mine in Quebec; Baňská Štiavnica in Slovakia; Freiberg in Saxony; the Ani and Arakawa mines in Akita Prefecture, Japan; and large crystals from the Nababiep mine in South Africa. Our Cornwall and Devon pages cover the British side.
Storage: a sulphide with the usual sulphide problem
Chalcopyrite is a sulphide, and sulphides in damp air are the standard conservation problem in a mineral collection. The Natural Sciences Collections Association's guidance is explicit that above 60 per cent relative humidity iron pyrite reacts with atmospheric water to form hydrated iron sulphate and sulphur trioxide, which can form a weak sulphuric acid, and that other sulphide species can deteriorate by similar reactions at high humidity.
In practice chalcopyrite is more robust than pyrite and far more robust than marcasite, and a normally heated room will not damage it. The situations that do are an unheated garage, a cellar, a conservatory and an outbuilding — anywhere the temperature falls overnight, because relative humidity rises as air cools even though nothing has been added to it. A reading of 55 per cent taken at 20 degrees in the afternoon is above 90 per cent at 12 degrees the same night. That calculation is worked through on storing a collection in a damp house.
The second risk is the label. Sulphide decay produces acid, and the acid attacks paper long before the specimen looks obviously bad. If you keep sulphides, keep the labels out of the box with them. Copper sulphide material we are looking for is listed on the wanted list.