Bassetite
Bassetite is an iron uranyl phosphate, hardness 2.5, density 3.40 to 3.63, in olive to bronzy tablets to 3 mm, first described from Cornwall's Basset mines.
Bassetite is a ferrous iron uranyl phosphate, Fe2+(UO2)2(PO4)2 with 8 or 10 water depending on the source, monoclinic but pseudotetragonal, hardness 2.5, density 3.40 to 3.63, and radioactive. It forms thin olive-green to bronzy yellow tablets to 3 mm. It is named for the Basset mine group near Illogan, Cornwall, where it was first described in 1915.
In short
- A Cornish type species, and the Handbook says so in its Name field: for the Basset mine group, Cornwall, England, from which it was first described. The IMA list gives the year as 1915 and the country as the United Kingdom.
- Colour and size separate it from the familiar uranium micas. Olive-green, olive-brown and bronzy yellow tablets to 3 mm, where torbernite is emerald-green to 2.5 cm and saléeite lemon-yellow to 2.5 cm. Bassetite crystals top out at about an eighth of that size.
- The two primary references disagree on its water, and the analyses fit neither exactly. The Handbook writes 8 waters, which is 15.50% H2O; the IMA list writes 10, which is 18.65%. The sheet's Wheal Basset analysis found 14.49% and its Sardinian analysis 14.20%.
- Six species in this site's reference set are recorded as radioactive, and bassetite is one. Six of the 176 sheets say Radioactive, one per species. No dose figures or health advice here — see how to store radioactive mineral specimens.
- The sheet records no fluorescence for it, which puts it with the copper uranyl phosphates rather than with calcium-bearing autunite or magnesium-bearing saléeite.
| Species | Formula (IMA) | Colour | Largest crystals on the sheet | Hardness | Density (measured) | Fluorescence on the sheet |
|---|---|---|---|---|---|---|
| Bassetite | Fe2+(UO2)2(PO4)2(H2O)10 | Olive-green, olive-brown, bronzy yellow | 3 mm | 2.5 | 3.40 to 3.63 | None recorded |
| Saléeite | Mg(UO2)2(PO4)2(H2O)10 | Lemon-yellow, straw-yellow, greenish yellow | 2.5 cm | 2 to 3 | 3.27 | Bright lemon-yellow in LW UV, pale yellow in SW |
| Torbernite | Cu(UO2)2(PO4)2(H2O)4·8H2O | Emerald-green to apple-green | 2.5 cm | 2 to 2.5 | 3.22 | None recorded |
| Metatorbernite | Cu(UO2)2(PO4)2(H2O)4·4H2O | Pale green to dark green | 2 cm | 2.5 | 3.52 to 3.70 | None recorded |
| Autunite | Ca(UO2)2(PO4)2(H2O)11 | Lemon-yellow to pale green; may be dark green | 2 cm | 2 to 2.5 | 3.05 to 3.2 | Strong yellow-green |
| Meta-autunite | Ca(UO2)2(PO4)2·6H2O | Lemon-yellow to dark green | Size not given | 2 to 2.5 | 3.35 to 3.55 | Yellowish green |
How do you tell bassetite from torbernite and saléeite?
Three things, in the order a collector can use them.
Colour. The Handbook of Mineralogy gives bassetite as olive-green, olive-brown, yellowish brown, bronzy yellow and yellow, with a bronzy lustre on {010}. Torbernite is the bright emerald-to-apple-green species; saléeite and autunite are the lemon-yellow ones. A brownish or bronzy uranium tablet is the one to look at twice.
Size. Bassetite crystals reach 3 mm. Torbernite and saléeite reach 2.5 cm, metatorbernite and autunite 2 cm. A bassetite is a microscope or hand-lens specimen, typically in parallel or fan-like groups of thin tablets, with twinning at roughly 90° described as common.
Ultraviolet. The saléeite sheet records bright lemon-yellow fluorescence in longwave UV, and autunite and meta-autunite both fluoresce. The bassetite and torbernite sheets record none. A yellow uranium mica that stays dark under a longwave lamp is not saléeite or autunite by the Handbook's account, though absence from a sheet is weaker evidence than presence, and a lamp is no substitute for analysis. Lamps are covered on the best UV lamp for mineral collecting.
None of the three is definitive. The Handbook's powder pattern for bassetite is from Cornish material, and diffraction or chemistry is how a label becomes certain.
Eight waters or ten: the two references disagree
This is the page's worked number, and the reason bassetite's formula is quoted twice above.
The Handbook sheet writes bassetite as Fe2+(UO2)2(PO4)2·8H2O. The September 2026 IMA-CNMNC Master List writes Fe2+(UO2)2(PO4)2(H2O)10. Using standard atomic weights, the 8-water formula has a mass of 929.96 and is 15.50% water, which matches the Handbook's own ideal column exactly. The 10-water formula is 965.99 and 18.65% water.
The two analyses printed on the sheet found 14.49% H2O at Wheal Basset and 14.20% at Arcu su Linnarbu in Sardinia. Both sit below even the 8-water figure. Uranium runs 51.19% by mass on the Handbook formula and 49.28% on the IMA one.
We do not resolve this, and it would be wrong to pretend to. Old wet analyses of a hydrated species can read low if the material lost water before it was weighed, and a modern formula can rest on structural work the old analyses could not do. What a collector can take from it is narrower: quote the IMA formula on a label, and treat any single water count for bassetite as provisional. The general question of what a uranium mineral does on a shelf is on is my mineral radioactive.
Where bassetite has been found
The Handbook calls it a rare secondary mineral in the oxidized zone of some uranium-bearing hydrothermal mineral deposits. At the Basset mines it is recorded with uranospathite, torbernite, uraninite and pyrite; in Sardinia with meta-autunite and torbernite; at the Sue mine in Arizona with saléeite and metanováčekite.
In England the sheet names the Basset group of mines, Illogan, and the South Terras mine, St Stephen-in-Brannel, both in Cornwall. Elsewhere: Arcu su Linnarbu, Capoterra, near Cagliari in Sardinia; Daverdisse in Belgium; the Los Ratones and Pedro Alvaro mines in Salamanca Province, Spain; La Crouzille in Haute-Vienne, France; and in the USA Overland Mountain in Colorado, the Denise No. 1 and Fuemrol mines in Utah, the Sue and Red Bluff mines and the Dripping Springs quartzite in Gila County, Arizona, and the Merry Widow mine in New Mexico.
Counted across the 176 sheets in this site's reference set, the word Basset appears on three sheets: bassetite, saléeite and torbernite.
One more Cornish thread from the IMA list: uranospathite shares bassetite's first reference, Mineralogical Magazine 17 (1915), 221, and its country, and the Handbook records the two together at the Basset mines. 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.