Dagenaisite, Zn3Te6+O6, is a new mineral from the Gold Chain mine, Tintic district, Juab County, Utah, U.S.A. It is a late-stage secondary phase formed by the oxidative alteration of earlier Te- and Zn-bearing minerals. It is associated with cinnabar, dugganite, eurekadumpite, and gold in vugs in a matrix composed of quartz and dolomite. The mineral occurs as tiny light greenish-gray platelets, generally intermixed with amorphous material, forming porous masses that are apparently replacements of earlier phases. The streak is white, the luster is pearly, and crystals are transparent to translucent. The hardness could not be measured, but appears to be <2 (Mohs). The tenacity is flexible, the fracture is irregular, and cleavage was not observed. The calculated density is 6.00 g/cm3 for the empirical formula. At room temperature, the mineral is slowly soluble in dilute HCl and rapidly soluble in concentrated HCl. Optical properties could not be determined. Electron-microprobe analyses gave the empirical formula (Zn2.39Cu0.36Ca0.06Mn0.03As0.03Si0.02)Σ2.89Te1.02O6. The mineral is monoclinic, space group C2/c, with cell parameters a 14.87(2), b 8.88(2), c 10.37(2) Å, β 93.33(2)°, V 1367(4) Å3, and Z = 12. The five strongest lines in the X-ray powder diffraction patterns are [dobs Å(I)(hkl)]: 4.311(30)(310), 3.029(44)(222), 2.744(68), 2.539(100), and 1.6568(48). The mineral is the natural counterpart of synthetic Zn3Te6+O6, which has a structure based on an approximate close packing of O atoms in an hhchhc sequence along .