Riotintoite, Al(SO (sub 4) )(OH).3H (sub 2) O, a new mineral from La Vendida copper mine, Antofagasta region, Chile

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 X 0.4 X 0.4 mm in size and exhibits the forms {010} (the major form), {001}, {100}, {110}, {011}, {120}, {210}, {123}, {123}, {131}, and {131}. Riotintoite is brittle, with Mohs' hardness of 2 (super 1) / (sub 2) . There are three good cleavages: {010}, {001}, and {201}. D (sub meas) = 2.13(2) g/cm (super 3) , D (sub calc) = 2.129 g/cm (super 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, H (sub 2) O by gas chromatography) is (in wt.%): Al (sub 2) O (sub 3) 24.36, SO (sub 3) 40.69, H (sub 2) O 34(2), total 99.05. The empirical formula is: Al (sub 0.93) (SO (sub 4) ) (sub 0.99) (OH) (sub 0.81) .3.25H (sub 2) O. The crystal structure was solved using single-crystal X-ray diffraction data (R (sub 1) = 0.0398). Riotintoite is triclinic, space group P1, a 5.600(2) A, b 7.450(3) A, c 7.671(3) A, alpha 74.785(7) degrees , beta 86.042(8) degrees , gamma 75.810(7) degrees , V 299.4(2) A (super 3) , Z = 2. The structure is based on isolated clusters which are formed by pairs of edge-shared Al(H (sub 2) O) (sub 3) (OH) (sub 2) O octahedra (AlO (sub 6) octahedra link via common OH-OH edge and O belongs to the SO (sub 4) (super 2-) anion) and SO (sub 4) tetrahedra. The clusters are linked by strong hydrogen bonds between water molecules and non-bridging oxygen atoms of SO (sub 4) tetrahedra. The strongest lines of the powder X-ray diffraction pattern [d, A (I, %) (hkl)] are: 6.975 (100) (010), 4.459 (40) (111), 4.391 (72) (101), 3.766 (31) (111), 3.695 (29) (002, 012), 3.491 (24) (021, 020), 2.552 (26) (201, 201, 013).