Beudantite
Beudantite is PbFe3(AsO4)(SO4)(OH)6, hardness 3.5 to 4.5, density 4.48. Corkite matches its hardness and nearly its density; arsenic versus phosphorus decides.
Beudantite is a lead ferric iron arsenate-sulphate, PbFe3+3(AsO4)(SO4)(OH)6, a secondary mineral of oxidised polymetallic deposits. It is hexagonal but pseudocubic, hardness 3.5 to 4.5, density 4.48, with crystals to 5 mm in yellow, green, brown or black. Corkite has the same hardness and a density of 4.295; only chemistry separates the two reliably.
In short
- Beudantite and corkite are one anion apart. Beudantite carries arsenate plus sulphate; corkite carries phosphate plus sulphate. The Handbook gives both hardness 3.5 to 4.5 and both good-to-perfect basal cleavage, and their densities differ by 4.3%, 4.48 against 4.295. By eye and by hand they are the same mineral.
- Four entries on the September 2026 IMA list share the formula skeleton PbFe3+3(TO4)(TO4)(OH)6: beudantite, corkite, segnitite and kintoreite. They form a two-by-two grid of arsenic or phosphorus, with or without sulphate. That grid is the table below.
- The quick separation that does work is against pharmacosiderite, the other small yellow-to-green cube of arsenic-rich gossans. Beudantite at 4.48 is 60% denser than pharmacosiderite at 2.797, and harder, 3.5 to 4.5 against 2.5.
- It is a British species of note. The Handbook names many mines in Caldbeck Fells, Cumbria and Cornwall, alongside superb crystals at Tsumeb.
- It contains lead and arsenic, 29.11% and 10.53% by mass. No health advice here; the general approach is on toxic minerals in a collection.
| Species | Formula (IMA) | Hardness | Density (measured) | Crystals on the sheet | Refractive index | Separating feature |
|---|---|---|---|---|---|---|
| Beudantite | PbFe3+3(AsO4)(SO4)(OH)6 | 3.5 to 4.5 | 4.48 | Pseudocubes to 5 mm | omega 1.957 | Arsenic and sulphur both present |
| Corkite | PbFe3+3(SO4)(PO4)(OH)6 | 3.5 to 4.5 | 4.295 | Pseudocubic rhombohedra to 2 mm | n 1.93 to 1.96 | Phosphorus instead of arsenic |
| Segnitite | PbFe3+3(AsO4)(AsO3OH)(OH)6 | No sheet in set | No sheet in set | No sheet in set | No sheet in set | Arsenic, no sulphate; approved 1991 |
| Kintoreite | PbFe3+3(PO4)(PO3OH)(OH)6 | No sheet in set | No sheet in set | No sheet in set | No sheet in set | Phosphorus, no sulphate; approved 1992 |
| Pharmacosiderite | KFe3+4(AsO4)3(OH)4·6-7H2O | 2.5 | 2.797 | True cubes to 2 cm, diagonally striated | n 1.687 to 1.704 | Cubic, soft and light |
| Scorodite | Fe3+AsO4·2H2O | 3.5 to 4 | 3.27 | Pyramidal, tabular or prismatic, to 5 cm | alpha 1.741 to 1.784 | No lead; pyramidal crystals, not cubes |
| Alunite | KAl3(SO4)2(OH)6 | 3.5 to 4 | 2.6 to 2.9 | Pseudocubic to 1 cm | omega 1.572 | Aluminium, no lead; white streak |
How do you tell beudantite from corkite?
Honestly: not by anything a collector can do at home.
Both are hexagonal, pseudocubic, lead ferric iron minerals from the oxidised zones of base-metal deposits. The Handbook of Mineralogy gives both hardness 3.5 to 4.5. Beudantite's cleavage is good on {0001}, corkite's perfect on {0001}. Colours overlap across yellow, green and brown. Refractive indices overlap too: beudantite omega 1.957, corkite 1.93 to 1.96.
Density is the closest thing to a physical separator, and it is not close enough. Beudantite 4.48, corkite 4.295: a 4.3% difference, which is inside the error of a home specific-gravity measurement on a millimetre-sized crystal on matrix. The method and its limits are on how to measure specific gravity at home.
The decision rule, then, is chemical: arsenic says beudantite, phosphorus says corkite. The Handbook's ideal columns put beudantite at 16.15% As2O5 and corkite at 10.63% P2O5, and the Dernbach corkite analysis on its sheet carries only 0.29% As2O5. A label that says beudantite without an analysis behind it is an attribution, not a determination.
The two-by-two grid on the IMA list
Method: every formula on the September 2026 IMA-CNMNC Master List (text extraction of the PDF) was searched by regular expression for the prefix PbFe3+3 and the ending (OH)6. Four entries match, and they fill a grid:
Arsenic with sulphate is beudantite, redefined in 1987 by special procedure, country Germany. Phosphorus with sulphate is corkite, also recorded as redefined by special procedure in 1987, country Ireland. Arsenic with a protonated arsenate in place of sulphate is segnitite, approved 1991, Australia. Phosphorus with a protonated phosphate is kintoreite, approved 1992, Australia.
Near relatives sit just outside the grid: plumbojarosite, Pb0.5Fe3+3(SO4)2(OH)6, with two sulphates and half a lead; gallobeudantite, PbGa3(AsO4)(SO4)(OH)6, with gallium in place of iron; and hidalgoite and hinsdalite, the aluminium analogues of beudantite and corkite. Alunite, KAl3(SO4)2(OH)6, is the potassium-aluminium-sulphate member everyone has heard of, and the list records it as redefined by special procedure in 1987 as well.
Nomenclature, stated carefully. The Handbook assigns beudantite and corkite to the beudantite group and alunite to the alunite group. The IMA master list records no group membership at all, so the family resemblance here is read from the formulas, not quoted from a group report we have checked.
Beudantite or pharmacosiderite: the heft test that works
Beudantite and pharmacosiderite each appear among the other's associates on their Handbook sheets, and both sheets name Cornwall and the Caldbeck Fells. Both can be small yellow, green or brown cubes. Here the physical tests do separate them.
Symmetry. Pharmacosiderite is genuinely cubic, with cubes striated diagonally to 2 cm. Beudantite is hexagonal and only pseudocubic, to 5 mm. Hardness. Pharmacosiderite 2.5, beudantite 3.5 to 4.5: a steel point that marks one may not mark the other, as explained on how to test mineral hardness properly. Density. 4.48 against 2.797, so beudantite is about 60% denser, which a clean fragment of either will show.
Scorodite, the third member of the usual assemblage, is harder than pharmacosiderite at 3.5 to 4, contains no lead, and grows as pyramidal, tabular or prismatic crystals rather than cubes.
Where beudantite is found, and what grows with it
The Handbook's occurrence line is short: a secondary mineral in the oxidized zone of polymetallic deposits. Its associates are carminite, scorodite, mimetite, dussertite, arseniosiderite, pharmacosiderite, olivenite, bayldonite, duftite, anglesite, cerussite and azurite — the classic arsenate gossan suite.
Named localities: in Germany, the Louise mine at Horhausen, the Schöne Aussicht mine near Dernbach, Reichenbach, and the Clara mine and elsewhere in the Black Forest; in France, Cap Garonne and Blond; in England, many mines in the Caldbeck Fells and in Cornwall; Laurium in Greece, including the Kamariza mine and the slags; superb crystals at Tsumeb; the Ojuela mine at Mapimí and La Mur in Sonora, Mexico; the Mohawk mine in California and Bisbee in Arizona; and the Belvedere mine in Western Australia.
Corkite's sheet adds large crystals at the Mexico mine, Caldbeck Fells, and three Cornish mines, which is one more reason a Caldbeck pseudocube needs an analysis before it gets a name. The rest of this section is indexed at the species index. There is no catalogue here and nothing here is offered for sale; the wanted list is the only commercial route.