Cerussite
Cerussite is PbCO3, hardness 3 to 3.5, density 6.55, adamantine and very brittle. Two separate twin laws make the reticulated and heart-shaped groups.
Cerussite is lead carbonate, PbCO3, orthorhombic, hardness 3 to 3.5 with a measured density of 6.55 and crystals recorded to 0.6 m. It is adamantine, very brittle, and may fluoresce yellow under long-wave ultraviolet. Two twin laws produce the reticulated and the heart-shaped groups collectors buy it for.
In short
- Density 6.55 at hardness 3 to 3.5 is the identification. Almost nothing else in an oxidised lead zone is that heavy and that soft at once. A colourless, adamantine crystal that feels like metal in the hand is cerussite before you have done a single test.
- There are two separate twin laws and they make two different specimens. Contact twins on {110} build the stellate pseudohexagonal and reticulated lattices; contact twins on {130} build the heart-shaped groups. The Handbook records that both laws may occur in one aggregate.
- It is very brittle, with good cleavage on {110} and {021}. Reticulated cerussite is the most easily damaged habit in common commerce after crocoite, because the lattice is a frame of thin blades with nothing supporting them.
- It may fluoresce yellow under long-wave ultraviolet, which is a useful confirmation on a crusty specimen where the crystal form is not visible.
- It is a lead mineral and conservation guidance names it. The NatSCA and Icon leaflet lists cerussite among the lead minerals to be handled with gloves and stored in lidded boxes, alongside arsenic and antimony species.
| Species | Formula | Hardness | Density | Lustre | How to separate it |
|---|---|---|---|---|---|
| Cerussite | PbCO3 | 3–3.5 | 6.55 | Adamantine | Effervesces in dilute acid; reticulated or heart-shaped twins |
| Anglesite | PbSO4 | 2.5–3 | 6.38 | Adamantine to resinous | A sulphate: no effervescence. Softer |
| Barite | BaSO4 | 3–3.5 | 4.50 | Vitreous to resinous | Much lighter in the hand; tabular blades |
| Phosgenite | Pb2(CO3)Cl2 | 2–3 | 6.12–6.15 | Adamantine | Tetragonal, stubby prisms; sectile, and it also fluoresces yellow |
| Calcite | CaCO3 | 3 | 2.71 | Vitreous | Effervesces too, but less than half the weight |
| Mimetite | Pb5(AsO4)3Cl | 3.5–4 | 7.24 | Resinous to subadamantine | Heavier still; hexagonal, yellow to orange |
Weight is the first test, and on cerussite it is nearly conclusive
Cerussite’s measured density is 6.55. For comparison, calcite — which it superficially resembles, shares a carbonate chemistry with, and sits beside in many collections — is 2.71. Barite, the heavy mineral most collectors have a feel for, is 4.50. Cerussite is half again as dense as barite.
Because density perception is relative rather than absolute, the way to use this is comparative: pick up a piece of calcite of about the same size, then the unknown. The difference is not subtle and it does not need a balance. Combined with a hardness of 3 to 3.5 — a copper coin marks it, a steel point marks it easily — and an adamantine lustre, the determination is close to secure.
What weight will not do is separate cerussite from anglesite, at 6.38, or phosgenite, at 6.13. For those, the acid test works: cerussite is a carbonate and effervesces in cold dilute hydrochloric acid; anglesite is a sulphate and does not. Test a detached fragment on a watch glass rather than the specimen, for the reasons set out on identifying a mineral specimen.
The two twin laws, and what each one makes
Cerussite is collected almost entirely for its twins, and the Handbook of Mineralogy is precise about there being two distinct laws.
Contact twins on {110}, simple or cyclic, produce the stellate pseudohexagonal groups and the open reticulated lattices — the snowflake and trellis specimens from Tsumeb and Mibladen that are the species’ signature. Contact twins on {130} produce heart-shaped composites, the V-shaped and butterfly pairs that Touissit is known for. Both laws may occur in one aggregate, which is why a single specimen can show a reticulated mesh and a heart in the same view.
Knowing which law you are looking at is a practical skill rather than pedantry, because it tells you what to check. On a reticulated group the question is how many blade intersections are intact; on a heart the question is whether the re-entrant angle is complete and undamaged. Those are different condition reports, and a seller’s photographs should show whichever applies. Our guidance on buying minerals online covers the photographs to ask for.
Untwinned cerussite is also common and often overlooked: tabular on {010}, equant, elongated, or pseudohexagonal dipyramidal, with crystals recorded to 0.6 m.
Tsumeb, Mibladen, Broken Hill and the British occurrences
Cerussite is common in the oxidised zone of lead deposits, where it may itself be an ore, associated with anglesite, smithsonite, malachite, azurite, phosgenite, pyromorphite and the galena it forms from.
The Handbook’s list of localities for fine crystals is essentially a list of the great lead districts. Superb crystals and twinned groups at Tsumeb and at the Kombat mine 49 km to its south, in Namibia. Mibladen in Morocco, with fine twins from the Touissit mine near Oujda. Broken Hill in New South Wales, and the Magnet and other mines near Dundas in Tasmania. Also Friedrichssegen near Braubach, Příbram, Mežica in Slovenia, Monteponi and Montevecchio in Sardinia, Nerchinsk in Siberia, the Nakhlak mine in Iran, and in the USA the Wheatley mine in Pennsylvania, Bunker Hill in Idaho, Leadville in Colorado, the Mammoth-St Anthony mine in Arizona and large twins from the Stevenson-Bennett mine in New Mexico.
Britain contributes two names to that list: Leadhills in Lanarkshire, and the Frankmills mine at Christow in Devon. Neither produces Tsumeb-scale material, and both are worth having for exactly that reason — our Leadhills and Wanlockhead and Devon notes cover them.
The name is from the Latin cerussa, white lead — the synthetic lead carbonate used as a pigment for two thousand years before anyone connected it with the mineral.
Handling, storage and the condition questions worth asking
Cerussite is a lead mineral, and the conservation guidance names it explicitly. The NatSCA and Icon leaflet Care and Conservation of Geological Specimens lists arsenic, antimony and some lead minerals — cerussite among them — as common toxic specimens to be handled with gloves and stored in lidded clear polystyrene boxes lined with Plastazote. It also makes the useful point that galena is not toxic in the same way, because it is insoluble. Cerussite is a carbonate and is not.
Physically it is very brittle with good cleavage on {110} and {021}, which in a reticulated group means that the thin blades snap rather than bend. It should be supported from underneath and never gripped across the lattice. A cut foam nest in a lidded box is the right storage; the options are compared on specimen boxes and stands.
The condition questions on any twinned cerussite are: how many blade or arm terminations are complete, whether the group is still on its original matrix, and whether anything has been reattached. Repairs on cerussite are common and often good, which is a reason to check rather than a reason to worry — the method is on how to tell if a specimen is repaired.
Fine twinned cerussite from any of the classic districts is a standing entry on our wanted list, and the rest of the lead suite is covered across the species reference.