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 0.1 × 0.4 × 0.4 mm in size and exhibits the forms {010} (the major form), {001}, {100}, {110}, {01forumla}, {1forumla0}, {2forumla0}, {123}, {forumla23}, {forumla31}, and {1forumla1}. Riotintoite is brittle, with Mohs' hardness of 2½. There are three good cleavages: {010}, {001}, and {201}. Dmeas = 2.13(2) g/cm3, Dcalc = 2.129 g/cm3. The new mineral is optically biaxial (–), α 1.513(2), β 1.522(2), γ 1.526(2), 2V (meas.) = 62(1)°, 2V (calc.) = 67°. 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: Al0.93(SO4)0.99(OH)0.81·3.25H2O. The crystal structure was solved using single-crystal X-ray diffraction data (R1 = 0.0398). Riotintoite is triclinic, space group Pforumla⁠, a 5.600(2) Å, b 7.450(3) Å, c 7.671(3) Å, α 74.785(7)°, β 86.042(8)°, γ 75.810(7)°, V 299.4(2) Å3, Z = 2. The structure is based on isolated clusters which are formed by pairs of edge-shared Al(H2O)3(OH)2O octahedra (AlØ6 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, Å (I, %) (hkl)] are: 6.975 (100) (010), 4.459 (40) (111), 4.391 (72) (101), 3.766 (31) (⁠forumlaforumla1), 3.695 (29) (002, 012), 3.491 (24) (021, 020), 2.552 (26) (201, forumla01, 013).

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