Chalcocite
Chalcocite is Cu2S, hardness 2.5 to 3, density 5.5 to 5.8, crystals to 25 cm. Much material labelled chalcocite is really djurleite. Habit, tests, localities.
Chalcocite is copper sulphide, Cu2S, monoclinic and pseudo-orthorhombic, hardness 2.5 to 3, measured density 5.5 to 5.8. Prismatic crystals reach 25 cm and tabular ones 12 cm across. Twinning on {110} gives the pseudohexagonal stellate groups Cornwall is known for. Colour and streak are blackish lead-grey.
In short
- A large share of specimen-grade “chalcocite” is not chalcocite. The copper sulphides form a tight compositional cluster — chalcocite Cu2S, djurleite Cu1.94S, digenite Cu9S5 — and the Handbook of Mineralogy's own X-ray powder pattern for chalcocite, from Bristol, Connecticut, is noted as close to djurleite.
- Nothing you can do by hand separates them. Same colour, same streak, same lustre, overlapping hardness and density. The distinction is made by X-ray diffraction or by electron microprobe, and by nothing else.
- Hardness 2.5 to 3, brittle but somewhat sectile. It marks with a fingernail-hard point, and a knife cuts rather than shatters it. That combination is diagnostic against the harder sulphides.
- Twinning on {110} yields pseudohexagonal stellate forms — the star-shaped groups that the Cornish and Bristol, Connecticut material is collected for. Twinning on {032} and {112} also occurs.
- The named crystal localities are Bristol in Connecticut, Cornwall at St Just, St Ives, Camborne and Redruth, Bisbee and the Magma mine in Arizona, Butte in Montana, Nababiep West and Messina in South Africa, Kolwezi in Katanga, and Telfer in Western Australia.
| Species | Formula | What a hand test shows | How it is actually identified |
|---|---|---|---|
| Chalcocite | Cu2S | Blackish lead-grey, metallic, H 2.5–3, D 5.5–5.8 | XRD or microprobe. No field test distinguishes it from djurleite |
| Djurleite | Cu1.94S | Indistinguishable from chalcocite by eye | XRD. Very common in material sold as chalcocite |
| Digenite | Cu9S5 | Blue-black tinge sometimes visible; often intergrown | XRD; the blue cast is suggestive, never conclusive |
| Covellite | CuS | Indigo-blue, iridescent, H 1.5–2, micaceous | Colour and the low hardness. This one you can call by eye |
| Bornite | Cu5FeS4 | Bronze on fresh surface, iridescent tarnish, H 3–3.25 | Fresh colour plus hardness. Distinguishable in the field |
| Chalcopyrite | CuFeS2 | Brass-yellow, H 3.5–4, greenish-black streak | Colour and streak. Not really confusable with chalcocite |
The identification problem, stated honestly
Most species pages tell you how to identify the mineral. This one has to start by telling you that you cannot, and that the people who say otherwise are guessing.
Chalcocite is Cu2S. Djurleite is Cu1.94S. That is a difference of three per cent in copper content, and it produces no difference in colour, streak, lustre, tenacity or habit that a person can see. Both are blackish lead-grey with a blackish lead-grey streak and a metallic lustre. Both are brittle but somewhat sectile. Their densities overlap. Their hardnesses overlap.
The evidence that this is a real problem and not a pedantic one is in the Handbook of Mineralogy's own entry: the X-ray powder pattern it publishes for chalcocite is taken from Bristol, Connecticut — the world's classic crystal locality for the species — and the entry annotates it close to djurleite. If the reference pattern for the species is ambiguous at the type-grade locality, a dealer's label is not going to resolve it.
The rule worth remembering: on the copper sulphides, a species name written without an analysis is a habit description with a species name attached. That is not an accusation of dishonesty. It is the normal state of the trade, and the correct response is to price and catalogue accordingly rather than to argue about it.
What to do about it as a collector
Three positions are defensible and one is not.
Accept the label as a group name. Buy the specimen for its crystal form, its locality and its condition, record it as “chalcocite” because that is what the label says, and understand privately that the analysis has not been done. This is what most collections do and there is nothing wrong with it provided you know that is what you are doing.
Buy analysed material specifically. Some museum-deaccessioned and some research-derived specimens carry an actual XRD or microprobe determination. These are uncommon and worth seeking out if the species matters to you as a species rather than as an object. Ask for the analysis, not for reassurance.
Have it analysed. Powder X-ray diffraction on a few milligrams is a routine service at many university departments and some mineralogical societies arrange it. The USGS primer on X-ray powder diffraction explains what the method does and what it needs. It is destructive at the milligram scale, so it suits a specimen with loose material rather than a single perfect crystal.
The position that is not defensible is relabelling on suspicion — writing “djurleite?” on a label because a piece looks slightly blue, or upgrading a djurleite to a chalcocite because the crystal is good. Both destroy information. If you are building a Cornish copper suite where this distinction matters, say so when you put a piece on your wanted list: analysed material is findable, but only if someone is looking for it on your behalf.
Habit, twinning and what the good crystals look like
Where chalcocite is unambiguous is in its crystals, and they are genuinely remarkable for a sulphide of this softness. The Handbook records short prismatic crystals elongated on [001] and thick to tabular on {001} reaching 12 cm across, and prismatic crystals on [100] reaching 25 cm long. The {001} faces are striated parallel to [100], which is a useful orientation cue on a well-formed crystal.
Twinning is the collector's interest. Twins on {110} produce pseudohexagonal stellate forms — six-pointed star groups — and these are the classic Cornish and Bristol, Connecticut specimens. Twinning also occurs on {032} and {112}, and lamellar twinning shows up in polished section.
Beyond the crystals, chalcocite is massive, compact or fine powdery, and in that form it is an ore rather than a specimen. The cleavage is indistinct on {110} and the fracture is conchoidal, so a broken face on massive material is smooth and dark rather than stepped.
One practical warning about the sectility: chalcocite takes a mark from anything harder than a fingernail, including a steel probe, a tweezer tip and a grain of quartz in a packing box. A scratch on a chalcocite crystal is bright against the dark surface and does not fade. Pack it in isolation. See packing and shipping mineral specimens.
Occurrence and the localities that matter
Chalcocite is an uncommon primary hydrothermal mineral but an important secondary one. It forms in or below the zone of oxidation in hydrothermal veins and in large low-grade porphyry copper orebodies — the supergene enrichment blanket, which is where the economic copper often is. Its associates are pyrite, chalcopyrite, covellite, bornite, molybdenite and other sulphides and their alteration products.
For specimens, the Handbook names a short list. Bristol, Hartford County, Connecticut produced exceptional crystals and is the reference locality in every sense. In England, fine crystals come from Cornwall at St Just, St Ives, Camborne and Redruth — the same mining districts that produced the county's copper arsenates and native copper. Russia gives the Turinsk copper mine at Bogoslovsk in the Urals.
In the United States beyond Connecticut: Bisbee in Cochise County and the Magma mine at Superior in Pinal County, Arizona; the United Verde Extension mine in Yavapai County; Butte in Silver Bow County, Montana; the Flambeau mine in Rusk County, Wisconsin; and Kennicott in the Copper River district of Alaska. Southern Africa gives large crystals from the Nababiep West mine in Cape Province and Messina in the Transvaal, plus M'Passa in the Congo Republic and the Mashamba West mine at Kolwezi in Katanga. Australia contributes fine crystals from Telfer in Western Australia.
Because chalcocite tarnishes and because the good crystals are large and soft, condition varies more between specimens of this species than between specimens of most others. Buying it well is mostly about reading the listing carefully and asking for raking-light photographs of every face.