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The new mineral riotintoite was discovered in the abandoned La Vendida mine, near Sierra Gorda, Antofagasta Region, Atacama desert, Chile. Associated minerals are vendidaite, eriochalcite, Mg-rich aubertite, magnesioaubertite, belloite, alunite, kaolinite, and halloysite. Riotintoite forms colorless platy crystals up to 0.1x0.4x0.4 mm in size and exhibits the forms {010} (the major form), {001}, {100}, {110}, {01(1) over bar}, {1(2) over bar 0}, {2(1) over bar 0}, {123}, {(1) over bar 23}, {(1) over bar 31}, and {1(3) over bar 1}. Riotintoite is brittle, with Mohs’ hardness of 21/2. There are three good cleavages: {010}, {001}, and {201}. D-meas = 2.13(2) g/cm(3), D-calc = 2.129 g/cm(3). The new mineral is optically biaxial (-), alpha 1.513(2), beta 1.522(2), gamma 1.526(2), 2V (meas.) = 62(1)degrees, 2V (calc.) = 67 degrees. The infrared spectrum is given. The chemical composition (electron microprobe, H2O by gas chromatography) is (in wt.%): Al2O3 24.36, SO3 40.69, H2O 34(2), total 99.05. The empirical formula is: Al-0.93(SO4)(0.99)(OH)(0.81)center dot 3.25H(2)O. The crystal structure was solved using single-crystal X-ray diffraction data (R-1 = 0.0398). Riotintoite is triclinic, space group P (1) over bar, a 5.600(2) angstrom, b 7.450(3) angstrom, c 7.671(3) angstrom, alpha 74.785(7)degrees, beta 86.042(8)degrees, gamma 75.810(7)degrees, V 299.4(2) angstrom(3), Z = 2. The structure is based on isolated clusters which are formed by pairs of edge-shared Al(H2O)(3)(OH)(2)O octahedra (Alempty set(6) octahedra link via common OH-OH edge and O belongs to the SO42-anion) and SO4 tetrahedra. The clusters are linked by strong hydrogen bonds between water molecules and non-bridging oxygen atoms of SO4 tetrahedra. The strongest lines of the powder X-ray diffraction pattern [d, angstrom (I, %) (hkl)] are: 6.975 (100) (010), 4.459 (40) (111), 4.391 (72) (101), 3.766 (31) ((11) over bar1), 3.695 (29) (002, 012), 3.491 (24) (021, 020), 2.552 (26) (201, (2) over bar 01, 013).