Segnitite
Segnitite is PbFe3(AsO4)(AsO3OH)(OH)6, approved 1991 from Broken Hill: beudantite with a second arsenate for sulphate. Hardness about 4, density 4.77 calc.
Segnitite is a lead ferric iron arsenate, PbFe3+3(AsO4)(AsO3OH)(OH)6, approved by the IMA in 1991 (1991-017) from Broken Hill, New South Wales. It is beudantite with the sulphate replaced by a second, protonated arsenate: hardness about 4, calculated density 4.77, brown rhombohedral crystals to 5 mm. It is 19.83% arsenic by mass against beudantite's 10.53%.
In short
- Segnitite is one corner of a two-by-two grid. Four entries on the September 2026 IMA-CNMNC Master List share the skeleton PbFe3+3(TO4)(TO4)(OH)6: beudantite (arsenic with sulphate), corkite (phosphorus with sulphate), segnitite (arsenic, no sulphate) and kintoreite (phosphorus, no sulphate). The grid is set out on beudantite.
- Sulphate is the decision rule between segnitite and beudantite. Ideal beudantite is 11.25% SO3 and 16.15% As2O5; ideal segnitite is 0% SO3 and 30.42% As2O5. The Handbook's Broken Hill segnitite analysis reads 0.57% SO3 and 27.89% As2O5.
- The Handbook and the IMA write the formula differently, and both are right. HPbFe3(AsO4)2(OH)6 and PbFe3(AsO4)(AsO3OH)(OH)6 contain the same atoms; both give a formula mass of 755.62. The IMA form shows where the extra hydrogen sits.
- Density will not separate it at home. The calculated figure is 4.77 against beudantite's 4.49, a 6.2% gap, but segnitite's measured density is printed only as greater than 4.2.
- One Cornish mine is on the sheet: the Penberthy Croft mine, St Hilary. Of the 176 sheets in this site's reference set, four name Penberthy Croft, and corkite, segnitite's phosphate-sulphate neighbour in the grid, is one of them.
| Species | Formula (IMA) | Anion pair | Hardness | Density (measured) | As by mass | SO3 (ideal) | IMA record |
|---|---|---|---|---|---|---|---|
| Segnitite | PbFe3+3(AsO4)(AsO3OH)(OH)6 | Arsenate + protonated arsenate | About 4 | Greater than 4.2 (4.77 calculated) | 19.83% | 0% | Approved 1991-017, Australia |
| Beudantite | PbFe3+3(AsO4)(SO4)(OH)6 | Arsenate + sulphate | 3.5 to 4.5 | 4.48 | 10.53% | 11.25% | Redefined 1987, special procedure, Germany |
| Corkite | PbFe3+3(SO4)(PO4)(OH)6 | Phosphate + sulphate | 3.5 to 4.5 | 4.295 | 0% | 11.99% | Redefined 1987, special procedure, Ireland |
| Kintoreite | PbFe3+3(PO4)(PO3OH)(OH)6 | Phosphate + protonated phosphate | No sheet consulted | No sheet consulted | 0% | 0% | Approved 1992-045, Australia |
Is it segnitite or beudantite? Sulphate decides
Physically they are close to indistinguishable. Both are brown to yellow and rhombohedral, segnitite pseudo-octahedral and beudantite pseudocubic, hardness around 4, with a good {0001} cleavage and refractive indices near 1.95 to 1.98: segnitite ω 1.975 and ε 1.955, beudantite ω 1.957 and ε 1.943. The segnitite sheet even names beudantite–corkite among its Broken Hill associates.
A worked comparison of the two ideal formulas, on standard atomic weights. Beudantite, PbFe3(AsO4)(SO4)(OH)6, formula mass 711.75, is 11.25% SO3 and 16.15% As2O5. Segnitite, formula mass 755.62, is 0% SO3 and 30.42% As2O5: the arsenate nearly doubles because it takes the sulphate's place. By element, arsenic is 19.83% of segnitite and 10.53% of beudantite, 1.88 times as much.
The decision rule: an analysis with sulphate near zero and As2O5 near 30% is segnitite; one with SO3 near 11% and As2O5 near 16% is beudantite. The Handbook's Broken Hill segnitite, an average of ten microprobe analyses, reads 0.57% SO3 and 27.89% As2O5, with small zinc, aluminium and barium. Real specimens sit between the ideals, so a label without an analysis is an attribution. The same point for beudantite and corkite is made on the beudantite page.
Why the Handbook and the IMA formulas look different
The Handbook writes segnitite as HPbFe3+3(AsO4)2(OH)6; the September 2026 IMA list writes PbFe3+3(AsO4)(AsO3OH)(OH)6. Count the atoms: one lead, three iron, two arsenic, fourteen oxygen and seven hydrogen in both. On standard atomic weights both come to 755.62, and the Handbook's ideal column, PbO 29.54, Fe2O3 31.70, As2O5 30.42 and H2O 8.34, matches that formula to the second decimal.
The difference is where the seventh hydrogen is written. The Handbook puts it at the front; the IMA attaches it to one arsenate, making an AsO3OH group. The charge arithmetic shows why: lead and three ferric irons give +11 and six hydroxyls −6, so the two anions must supply −5. Arsenate plus sulphate is −3 −2 = −5; two arsenates would be −6, and the extra proton brings it back to −5. Kintoreite, PbFe3+3(PO4)(PO3OH)(OH)6, does the same with phosphate.
The Handbook sheet's reference is the 1992 description in American Mineralogist, which called segnitite a new mineral in the lusungite group. The IMA master list records no group membership, so this page describes the family from the formulas, as the beudantite page does.
Segnitite crystals: small, brown and usually clustered
The Handbook records rhombohedral or pseudo-octahedral crystals to 5 mm, but typically spherical clusters, drusy crusts and spongy aggregates. Colour runs greenish brown to yellowish brown and dark brown, with a pale yellow streak and a vitreous to adamantine lustre. It is brittle with an uneven fracture.
At hardness about 4 and 5 mm at most, segnitite is a loupe and microscope species, and the Handbook singles out Tsumeb for large crystals. See what is a micromount.
Where segnitite is found, including a Cornish mine
The sheet calls it of rare occurrence in the oxidised zone of a Pb–Zn orebody at Broken Hill, and records it in a dolostone-hosted hydrothermal polymetallic deposit at Tsumeb. The Broken Hill associates are beudantite–corkite, adamite–olivenite, duftite, bayldonite, carminite, mimetite, hidalgoite, pharmacosiderite, mawbyite, agardite-(Y), spessartine, quartz, coronadite and goethite; at Tsumeb, olivenite and duftite.
The full distribution: Broken Hill, New South Wales, where its type material came from, now at Museum Victoria, Melbourne; Coppin Pool, about 41 km east-southeast of Mt Tom Price, Western Australia; large crystals from Tsumeb; the Centennial Eureka mine in the Tintic district, Utah; and the Penberthy Croft mine, St Hilary, Cornwall.
Penberthy Croft is named on four of the 176 sheets in this site's reference set (method: string search over the pdftotext output): atacamite, chalcoalumite, corkite and duftite. Segnitite's sheet is not in the set, but it makes the mine a place where two corners of the PbFe3 grid are both recorded. The rest of the arsenates are indexed at the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.