Leydetite, monoclinic Fe(UOb>2b>)(SOb>4b>)b>2b>(Hb>2b>O)b>11b> (IMA 2012–065), is a new supergene uranyl sulfate from Mas d'Alary, Lodève, Hérault, France. It forms yellow to greenish, tabular, transparent to translucent crystals up to 2 mm in size. Crystals have a vitreous lustre. Leydetite has a perfect cleavage on (001). The streak is yellowish white. Mohs hardness is ~2. The mineral does not fluoresce under long- or short-wavelength UV radiation. Leydetite is colourless in transmitted light, non-pleochroic, biaxial, with α = 1.513(2), γ = 1.522(2) (further optical properties could not be measured). The measured chemical composition of leydetite, FeO 9.28, MgO 0.37, Alb>2b>Ob>3b> 0.26, CuO 0.14, UOb>3b> 40.19, SOb>3b> 21.91, SiOb>2b> 0.18, Hb>2b>O 27.67, total 100 wt.%, leads to the empirical formula (based on 21 O a.p.f.u.), (Feb>0.93b>Mgb>0.07b>Alb>0.04b>Cub>0.01b>)b>Σ1.05b>(Ub>1.01b>Ob>2b>)(Sb>1.96b>Sib>0.02b>)b>Σ1.98b>Ob>8b>(Hb>2b>O)b>11b>. Leydetite is monoclinic, space group C2/c, with a = 11.3203(3), b = 7.7293(2), c = 21.8145(8) Å, β = 102.402(3)°, V = 1864.18(10) Å3, Z = 4, and Db>calcb> = 2.55 g cm−3. The six strongest reflections in the X-ray powder diffraction pattern are [db>obsb> in Å (I) (hkl)]: 10.625 (100) (002), 6.277 (1) (1İ11), 5.321 (66) (004), 3.549 (5) (006), 2.663 (4) (008), 2.131 (2) (0 0 10). The crystal structure has been refined from single-crystal X-ray diffraction data to Rb>1b> = 0.0224 for 5211 observed reflections with [I > 3σ(I)]. Leydetite possesses a sheet structure based upon the protasite anion topology. The sheet consists of UOb>7b> bipyramids, which share four of their equatorial vertices with SOb>4b> tetrahedra. Each SOb>4b> tetrahedron, in turn, shares two of its vertices with UOb>7b> bipyramids. The remaining unshared equatorial vertex of the bipyramid is occupied by Hb>2b>O, which extends hydrogen bonds within the sheet to one of a free vertex of the SOb>4b> tetrahedron. Sheets are stacked perpendicular to the c direction. In the interlayer, Fe2+ ions and Hb>2b>O groups link to the sheets on either side via a network of hydrogen bonds. Leydetite is isostructural with the synthetic compound Mg(UOb>2b>)(SOb>4b>)b>2b>(Hb>2b>O)b>11b>. The name of the new mineral honours Jean Claude Leydet (born 1961), an amateur mineralogist from Brest (France), who discovered the new mineral.
