The new mineral batoniite, [Al8(OH)14(H2O)18](SO4)5·5H2O, was discovered in the Cetine di Cotorniano Mine, Chiusdino, Siena, Tuscany, Italy. It occurs as hemispherical aggregates composed of brittle tabular crystals, up to 1mm in size, white to colorless in color, with a white streak and a vitreous to greasy luster. Batoniite is biaxial negative, with αCombining double low line1.4833(6), βCombining double low line1.4948(6), γ=1.5019(5) (589nm), and 2V(meas.)Combining double low line71(1). Electron microprobe analysis, affected by the dehydration of batoniite under the chamber vacuum, gave (in wt%) the following: Al2O3 33.48, Fe2O3 0.05, SO3 33.00, and H2Ocalc 44.41, total 110.94. It corresponds to the chemical formula (Al7.98Fe0.013+)ς7.99(SO4)5.01(OH)13.95(H2O)18·5H2O. Batoniite is triclinic, belonging to space group P1ĝ€3/4, with aCombining double low line9.1757(6), bCombining double low line12.0886(9), cCombining double low line20.9218(15)Å, αCombining double low line82.901(3), βCombining double low line87.334(3), 3Combining double low line86.999(2)°, VCombining double low line2297.8(3)Å3, and ZCombining double low line2. The crystal structure was refined to R1Combining double low line0.0916 for 8118 unique reflections with Fo>4σ(Fo) and 811 refined parameters and 60 restraints. Batoniite is characterized by isolated [Al8(OH)14(H2O)18]10+ polyoxocations, H-bonded to five interstitial (SO4)2- and five H2O groups. In type material, it is associated with gypsum and a poorly crystallized Al-Fe sulfate. The crystallization of batoniite is probably due to the action of H2SO4 on Al-bearing rocks of Paleozoic age cropping out in the Garibaldi Tunnel, the lowest mining level of the Cetine di Cotorniano Mine.