Telluromandarinoite, a tellurite, is a new mineral species from the Wendy open pit, Tambo mine, El Indio-Tambo mining property, Coquimbo Province, Chile. The ideal endmember telluromandarinoite formula is Fe3+2Te4+3O9·6H2O and it is the Te4+ analogue of the selenite mineral mandarinoite, Fe3+2Se4+3O9·6H2O. These deposits are located in rhyolitic and dacitic pyroclastic volcanic rocks of Tertiary age (8–11 Ma) that are strongly hydrothermally altered. The mineralization in the Tambo area is characterized by high-level epithermal veins and breccias located along roughly east–west structures. Hydrothermal breccias consisting of silicified clasts of dacite tuffs cemented by a silica/barite/alunite matrix are common at the occurrence. In fact, all studied specimens containing tellurite mineralization are associated with alunite. Telluromandarinoite is translucent, pale green, with a white streak and vitreous luster. It forms as individual platy crystals, 0.2 mm or less in size, but more commonly as aggregates of platy crystals. The crystals are too small to allow a Mohs hardness determination; they are brittle with an uneven fracture and no observed cleavage or parting. Telluromandarinoite is biaxial positive with α = 1.750(3), β = 1.807(3), and γ = 1.910(5), with a calculated 2V = 76.9°. The optical orientation is Y = b, c ˆ Z = 10° in obtuse β. No dispersion was noted and no pleochroism was observed. An average of 10 electron microprobe analyses gave SeO2 22.91, TeO2 44.30, Fe2O3 26.43, and H2O (calc.) 17.59, total 111.23 wt.%. The mineral loses H2O in vacuum, so the high totals obtained were expected. The empirical formula (based on 15 O atoms) is Fe3+2.03(Te4+1.71Se4+1.27)Σ2.98O9·6H2O with Z = 4, and Dcalc = 3.372 g/cm3. Spot analyses gave stoichiometries that range from telluromandarinoite Fe3+2.03(Te4+2.12Se4+0.86)Σ2.98O9·6H2O to mandarinoite Fe3+2.07(Se4+1.64Te4+1.31)Σ2.95O9·6H2O. A crystal-structure analysis shows the mineral to be monoclinic, space group P21/c, with a 16.9356(5), b 7.8955(3), c 10.1675(3) Å, β 98.0064(4)°, and V 1346.32(13) Å3. The strongest lines in the X-ray powder pattern [d in Å,(I),(hkl)] are: 8.431(44)(200), 7.153(100), 3.5753(41), 3.4631(21), 2.9964(34), 2.8261(19)(412). The crystal structure of telluromandarinoite is similar to that of emmonsite, Fe3+2Te4+3O9·2H2O.