Overview
Beta-uranophane is a hydrated calcium uranyl silicate and one of the most common secondary uranium minerals. It is the rarer dimorph of uranophane, meaning the two share the same chemical formula but differ in crystal structure. In 2022 the International Mineralogical Association revised its nomenclature, and the species is now formally recognised as parauranophane; the older names beta-uranophane and uranophane-β remain in wide use among collectors and in historical literature. Like all uranyl minerals it is strikingly coloured, typically forming bright yellow crusts, fibres and acicular sprays that fluoresce and glow against darker host rock.
Composition & structure
The mineral is built from sheets of uranyl (UO2)2+ polyhedra linked by silicate groups, with calcium cations and water molecules occupying the interlayer space. The beta polymorph differs from ordinary uranophane in the way these uranyl-silicate sheets are stacked and bonded, which gives it a distinct monoclinic geometry. Because the structure is delicate, the mineral can partially invert toward uranophane when crushed or heated.
| Formula | Ca(UO2)2(SiO3OH)2·5H2O |
| Crystal system | Monoclinic |
| Mohs hardness | ~2.5 |
| Lustre | Vitreous to silky, sometimes pearly |
| Colour | Bright yellow to greenish-yellow |
| Type locality | Jáchymov (Joachimsthal), Bohemia, Czech Republic |
Formation & occurrence
Beta-uranophane is a supergene mineral: it forms when primary uranium minerals such as uraninite weather and oxidise in the presence of silica- and calcium-bearing groundwater. The dissolved uranyl ions recombine with silica and calcium to precipitate these yellow coatings on fracture surfaces, in vugs and along grain boundaries. It is found in oxidised zones of uranium deposits, granitic pegmatites and some sandstone-hosted ores. It was first described from Jáchymov in the Czech Republic and is now reported from many localities, including sites in Germany, France, Brazil, Namibia, Canada and several US states.
Identification & similar species
Its vivid yellow colour, fibrous to acicular habit and bright green-yellow response under ultraviolet light are good first clues, but visual identification is rarely conclusive. Beta-uranophane is essentially indistinguishable by eye from uranophane itself and resembles other yellow uranyl minerals such as autunite, boltwoodite and uranopilite. Reliable separation requires X-ray diffraction or detailed optical work. Strong radioactivity confirms the uranyl nature of the mineral but does not distinguish the species.
Radioactivity & handling
This is a uranium mineral and is genuinely radioactive, emitting alpha, beta and gamma radiation. The main practical hazard for collectors is ingestion or inhalation of dust, plus prolonged close contact. Sensible practice is to store specimens in a sealed container away from living and sleeping areas, avoid handling or breaking pieces unnecessarily, never lick or taste specimens, and wash hands afterward. Treat it as a display piece kept at arm's length rather than a stone to carry or wear.
Notable localities & collecting
Classic material comes from the historic silver-uranium veins of Jáchymov, where the species was named. Fine specimens are also associated with uranium districts in Germany's Black Forest, pegmatites in Brazil and Namibia, and uranium occurrences across the western United States. Because it is closely tied to uranium ore, good crystallised examples are uncommon; most specimens are thin crusts or fibrous aggregates valued chiefly as a representative uranyl silicate and for their intense fluorescent colour.