Paulscherrerite, UO2(OH)2, is a newly named mineral of the schoepite subgroup of hexavalent uranium hydrate/hydroxides. It is monoclinic, but no space group has been determined because no single-crystal study has been done. Paulscherrerite occurs as a canary yellow microcrystalline powdery product with a length of ~500 nm. It forms by the weathering and ultimate pseudomorphism of uranium-lead bearing minerals such as metaschoepite. The type locality for paulscherrerite is the Number 2 Workings, Radium Ridge near Mount Painter, North Flinders Ranges, South Australia, an area where radiogenic heat has driven hydrothermal activity for millions of years. It is named for Swiss physicist Paul Scherrer, co-inventor of the Debye-Scherrer X-ray powder diffraction camera. Study of paulscherrerite and related minerals is important for understanding the mobility of uranium around mining sites, as well as designing successful strategies for the storage of nuclear weapons and the containment of nuclear waste. Introduction The schoepite subgroup of the fourmarierite group: schoepite, metaschoepite, paraschoepite, and “dehydrated schoepite”, are closely related hexavalent uranium (uranyl) oxide hydrates/hydroxides. Schoepite was first described by T. L. Walker in 1923 and the determination of the relationship between the various subgroups has since been ongoing. Detailed X-ray powder diffraction and single crystal studies have led to a better understanding of the natural dehydration process of schoepite that result in the rest of the subgroup. “Dehydrated schoepite” has now been formally described as a mineral species by a team of geologists led by Joel Brugger of the University of Adelaide, Australia and given the name paulscherrerite, with the formula UO3•1.02H2O. Composition The empirical formula for paulscherrerite is UO3 · 1.02H2O. The formulas for the rest of the schoepite group are: schoepite (UO2)8O2(OH)12 · 12H2O and metaschoepite UO3•1-2H2O. Electron microprobe 20 point analyses showed that it is an almost pure uranyl oxide-hydroxide/hydrate, with less than ~1 wt% of minor elements such as Al, Ba, and Pb. The simplified structural formula is UO2(OH)2, which requires the presence of water: UO3 93.96, H2O 6.04, Total 100.00 wt%. Table 1 shows an analysis of the chemical composition. Because paulscherrerite always exists in powder form, mixed with substantial amounts of metaschoepite, thermogravimetric analysis (TGA) is the best method of water measurement. Structure Paulscherrerite is monoclinic (pseudo-orthrombic), with a = 4.288(2), b = 10.270(6), c = 6.885(5)Å, β = 90.39(4) = 90.39(4)o, V = 303.2(2)Å3, and Z = 4. No space group determination has been made, as no single-crystal study