Bayldonite
Bayldonite, Cu3PbO(AsO3OH)2(OH)2: monoclinic green copper lead arsenate, hardness 4.5, density 5.24 to 5.65, from Cornwall, Caldbeck Fells and Tsumeb.
Bayldonite is a copper lead arsenate, Cu3PbO(AsO3OH)2(OH)2 on the IMA list (G, 1865, United Kingdom): monoclinic, usually green mammillary crusts, rarely crystals to 1 cm, hardness 4.5, density 5.24 to 5.65. It has three copper per lead; the St. Day, Cornwall, analysis gives 2.88. The Handbook names large crystals from Tsumeb, Namibia.
In short
- The three-to-one check. The September 2026 IMA-CNMNC Master List writes Cu3PbO(AsO3OH)2(OH)2 and the Handbook of Mineralogy sheet PbCu3O(AsO3OH)2(OH)2: three copper per lead either way. Worked from the printed oxide weights, the St. Day, Cornwall, analysis gives 2.88 and agrees with its printed recast, Pb1.03Cu2.97.
- Documented failure: the Kayrakty column. The sheet’s Kayrakty, Kazakhstan, analysis prints the recast Pb1.00Cu3.00, but its own weights, CuO 31.70 and PbO 32.60, work out at 2.73 copper per lead. Recompute the atoms from the weights before trusting a printed recast; the rendered sheet shows the same figures as the text.
- The Cu-per-Pb ladder. A script over the IMA list finds 7 rows built only of lead, copper, arsenic, oxygen and hydrogen. Copper per lead: duftite 1; heimite, metaheimite and thometzekite 2; bayldonite 3; plumboagardite 6; zanelliite 9. Three to one is bayldonite’s alone among them.
- Where. In Cornwall the Distribution field names the Penberthy Croft mine, St. Hilary; Wheal Carpenter, near Gwinear; Wheal Gorland, Gwennap; and Wheal Alfred, Phillack; and many mines at Caldbeck Fells, Cumbria. Large crystals come from Tsumeb, Namibia. The sheet marks no type locality.
- Lead and arsenic. Worked from the formula, bayldonite is 28.47% lead and 20.59% arsenic by mass. The general position is on toxic minerals in a collection and is galena toxic; this page is not safety or health advice.
| Species or analysis | IMA formula or source | Cu per Pb | Hardness | Density (meas.) | 20.00 g in water | Streak |
|---|---|---|---|---|---|---|
| Duftite | PbCu(AsO4)(OH) | 1 | 4.5 | 6.40 | 16.88 g | Pale green to white |
| Heimite | PbCu2(AsO4)(OH)3(H2O)2 | 2 | No sheet | — | — | — |
| Metaheimite | PbCu2(AsO4)(OH)3 | 2 | No sheet | — | — | — |
| Thometzekite | PbCu2+2(AsO4)2(H2O)2 | 2 | No sheet | — | — | — |
| Bayldonite | Cu3PbO(AsO3OH)2(OH)2 | 3 | 4.5 | 5.24 to 5.65 | 16.18 to 16.46 g | Not printed |
| Bayldonite, St. Day, Cornwall | Sheet analysis (1) | 2.88 | — | — | — | — |
| Bayldonite, Kayrakty, Kazakhstan | Sheet analysis (2) | 2.73 (printed recast 3.00) | — | — | — | — |
| Plumboagardite | PbCu6(AsO4)2(AsO3OH)(OH)6(H2O)3 | 6 | No sheet | — | — | — |
| Zanelliite | PbCu9[AsO3.5(OH)0.5]2(AsO4)2(OH)9(H2O)3 | 9 | No sheet | — | — | — |
What is bayldonite? A green copper lead arsenate
Bayldonite is Cu3PbO(AsO3OH)2(OH)2 on the September 2026 IMA list, status G, 1865, country United Kingdom. The list’s reference is American Mineralogist 66 (1981), 148, the structure and chemistry paper the Handbook of Mineralogy bayldonite sheet cites as Sumín de Portilla and co-authors. The Handbook writes the formula lead first, PbCu3O(AsO3OH)2(OH)2, and says the name honours John Bayldon, the English physician who collected the original specimens.
On the sheet. Monoclinic, point group 2/m, space group C2/c. Rare as crystals, to 1 cm; usually mammillary crusts, powdery or fine-grained massive; twinned as pseudoscalenohedral trillings with composition plane (-311), read from the rendered page. Fracture conchoidal to irregular; no cleavage and no streak are printed. Hardness 4.5; density 5.24 to 5.65 measured, 5.707 calculated. Siskin-green, apple-green, yellow-green and grass-green, dark green to black in crystals, with a resinous lustre. Biaxial (+), α 1.95, β 1.97, γ 1.99.
Its chemistry, worked. On standard atomic weights the formula is 26.20% copper, 28.47% lead and 20.59% arsenic by mass. As oxides it returns the sheet’s ideal column (3) to within 0.01: CuO 32.79, PbO 30.67, As2O5 31.58 against 31.59, H2O 4.95.
Why is three copper per lead the useful number?
Because it is what separates bayldonite from its formula neighbours. By script, the IMA list has 7 rows built only of Pb, Cu, As, O and H, and their copper per lead is 1, 2, 3, 6 or 9. Bayldonite is the one row at 3. Duftite, PbCu(AsO4)(OH), at 1, is the nearest with a Handbook sheet; that sheet names bayldonite as an associate at the Cap Garonne mine, France.
What the analyses give. Worked from the printed weights, CuO 30.88 and PbO 30.13 at St. Day give 2.88 copper per lead, matching the printed recast Pb1.03Cu2.97. At Kayrakty, CuO 31.70 and PbO 32.60 give 2.73, while the sheet prints Pb1.00Cu3.00. A ratio of two weights does not depend on how the analysis was normalised, so normalisation cannot explain the difference. The page reports both and does not choose.
How do you tell bayldonite from the other green arsenates?
Hardness and weight narrow it; an analysis decides. On their sheets conichalcite and cornwallite share hardness 4.5 but are lighter, 15.38 and 15.20 g in water for 20.00 g; olivenite is hardness 3 and 15.52 g. Bayldonite reads 16.18 to 16.46 g, duftite 16.88 g, and mimetite, pale to bright yellow on its sheet, 17.24 g. The method is on how to measure specific gravity at home.
Where the tests stop. A powdery green crust is too thin to weigh cleanly, and colour overlaps across the whole group. Bayldonite’s printed refractive indices, 1.95 to 1.99, are a laboratory reading, not a hand test. For a crust, the copper-per-lead ratio from an analysis is the reliable reading.
Where is bayldonite found? Cornwall, Caldbeck Fells and Tsumeb
The Distribution field names, in England, the Penberthy Croft mine, St. Hilary; Wheal Carpenter, near Gwinear; Wheal Gorland, Gwennap; and Wheal Alfred, Phillack, all in Cornwall; and many mines at Caldbeck Fells, Cumbria. Also the Tynagh mine, Co. Galway, Ireland; Cap Garonne, Var, La Rabasse, Hérault, and Les Ardillats, Rhône, France; large crystals from Tsumeb, Namibia; Kayrakty, Kazakhstan; Broken Hill, New South Wales, the Anticline prospect, Western Australia, and the Mt. Malvern mine, South Australia; and in the USA the Killie mine, Nevada, Bisbee and the Frijole prospect, Arizona, and the Black Pine mine, Montana.
Tsumeb associates. The sheet lists mimetite, olivenite, azurite, malachite, tsumebite, cuprian adamite, duftite, keyite, schulténite and philipsbornite at Tsumeb. The sheet marks no type locality, so this page claims none; the IMA country column reads United Kingdom, and the term is explained on what is a type locality. The sheet says nothing about gem use, and neither does this page.
The rest of the reference section is at the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.