Bournonite
Bournonite is PbCuSbS3, hardness 2.5 to 3, density 5.83, crystals to 11 cm. Repeated twinning on 110 makes the cogwheels, and the type locality is Cornish.
Bournonite is lead copper antimony sulphosalt, PbCuSbS3, orthorhombic, hardness 2.5 to 3 with a measured density of 5.83 and crystals to 11 cm across. Repeated twinning on {110} produces the cross and cogwheel aggregates it is collected for. The IMA records the United Kingdom as the country of first description, in 1805.
In short
- The cogwheel is the identification. Repeated contact twinning on {110} builds crosses and wheel-like aggregates that no other common sulphosalt makes. A steel-grey mineral in cogwheel groups is bournonite until proved otherwise.
- The type locality is British. The IMA-CNMNC list gives the United Kingdom and 1805 for the first description, and the Handbook of Mineralogy names Wheal Boys, St Endellion, Cornwall as the type locality, with exceptional crystals from the Herodsfoot mine at Lanreath.
- It forms a series with seligmannite, the arsenic analogue. Nothing you can do at a table separates the two ends of that series, so a bournonite label on material from an arsenic-rich locality is an assumption unless it has been analysed.
- Hardness 2.5 to 3 and density 5.83. It is soft enough to mark with a fingernail at the low end and heavy enough that the weight is obvious in the hand — an unusual pairing that is itself a clue.
- It is a vein mineral of medium temperatures, and it keeps predictable company: galena, tetrahedrite, sphalerite, chalcopyrite, pyrite, stibnite, siderite, quartz, rhodochrosite, dolomite and barite.
| Species | Formula | Hardness | Density | Streak | Diagnostic habit |
|---|---|---|---|---|---|
| Bournonite | PbCuSbS3 | 2.5–3 | 5.83 | Steel-grey to iron-black | Cross and cogwheel twins on {110} |
| Tetrahedrite group | Cu6(Cu4X2)Sb4S13 | 3–4.5 | 4.97 | Black to brown | Tetrahedra, never wheels |
| Galena | PbS | 2.5–2.75 | 7.58 | Lead-grey | Cubes with perfect cubic cleavage |
| Stibnite | Sb2S3 | 2 | 4.63 | Lead-grey | Long striated prisms, often bent |
| Seligmannite | PbCuAsS3 | 2.5–3 | 5.38 | Chocolate-brown to purple-black | Identical to bournonite in every other respect |
The cogwheel twin, and what to look for
Bournonite crystals are short prismatic to tabular, typically striated, and reach 11 cm across. On their own they are unremarkable steel-grey to iron-black. What makes the species is the twinning: repeated contact twins on {110}, commonly forming cross or cogwheel aggregates.
The effect is a stack of interpenetrating plates whose edges step around the group like gear teeth. Under a loupe the striations on adjacent segments run at right angles to one another, which is the giveaway even on a small or partly buried group. If the striations all run one way, you are looking at a single crystal, not a cogwheel, and the identification is less secure.
The German miners’ name for the species, Rädelerz — wheel ore — is the same observation, made by people who had no crystallography and did not need any. Our guide to identifying a mineral specimen sets out how far habit can be trusted on its own.
Wheal Boys, Herodsfoot, and the British claim
The IMA-CNMNC master list gives bournonite’s status as grandfathered, the year as 1805, and the country of first description as the United Kingdom. The Handbook of Mineralogy names the type locality: Wheal Boys, St Endellion, Cornwall.
The specimens that made the species famous came from elsewhere in the same county. The Herodsfoot mine at Lanreath, near Liskeard, produced what the Handbook calls exceptional crystals, and Herodsfoot bournonite remains the material against which everything else is judged. It is a nineteenth-century product; the classic groups in collections have been changing hands for well over a century, and old labels are part of what they are. Our page on what ex coll. means on a label covers how that provenance is recorded, and the Cornwall locality notes set the mine in context.
Outside Britain the species is widespread and sometimes superb: Silvermines in Co. Tipperary; Horhausen, Clausthal, Neudorf and Wolfsberg in Germany; Příbram; Baia Sprie, Cavnic and Săcărîmb in Romania; Pontgibaud and St Laurent Le Minier in France; Machacamarca and Oruro in Bolivia; Quiruvilca in Peru; Park City, Utah; Noche Buena and Naica in Mexico; Broken Hill in New South Wales; the Chichibu and Nakaze mines in Japan; and the Yaogangxian mine in Hunan.
The seligmannite problem
Bournonite forms a series with seligmannite, PbCuAsS3, in which arsenic substitutes for antimony. The two are isostructural. Hardness is shared at 2.5 to 3, the habit is shared, the colour is shared, and the densities — 5.83 against 5.38 — are too close to judge in the hand on a specimen with matrix attached.
One test does separate them, and it is the one people skip: the streak. The Handbook gives bournonite a steel-grey to iron-black streak and seligmannite a chocolate-brown to purple-black one. That is a real difference on unglazed porcelain, and it costs a fragment of the specimen’s underside to find out. It is not a substitute for analysis on a valuable piece, but it is far better than assuming.
For most collections the question does not arise, because the great majority of labelled bournonite comes from antimony-dominant localities and is bournonite. It matters at localities where both are present — Lengenbach in Switzerland is the classic seligmannite ground — and there the honest label records what the specimen was identified by. A determination from habit, streak and locality is a reasonable determination; it is not an analysis, and saying so on the label costs nothing.
This is the same distinction we recommend everywhere on this site, and it has a shape: as written, determined, or undetermined. The method is on labelling and cataloguing a collection.
Condition, tarnish and what to ask about
At hardness 2.5 to 3 bournonite marks easily, and the metallic lustre shows every mark. The Handbook describes the lustre as brilliant to dull, which is a polite way of saying that specimens range from mirror-bright to grey and lifeless depending on how they have been kept. A dulled bournonite is usually surface oxidation and is not reversible by anything a collector should be doing.
The realistic condition questions on an old Herodsfoot group are: how many cogwheel terminations are complete, is the group still attached to its original matrix, and has anything been reassembled. That last one is common on nineteenth-century sulphosalt specimens and is not a scandal if it is disclosed — see how to tell if a specimen is repaired.
The species is named for Jacques Louis de Bournon, 1751 to 1825, the French crystallographer and mineralogist who spent his emigration in England and worked on exactly this material. Classic Cornish bournonite is on our wanted list permanently; the wider species reference covers the vein minerals that accompany it.