Abstract

The new mineral vendidaite was discovered in the abandoned La Vendida mine, near Sierra Gorda, Antofagasta Region, Atacama desert, Chile. Associated minerals are aubertite, magnesioaubertite, belloite, eriochalcite, alunite, kaolinite, and halloysite. Vendidaite forms colorless platy crystals up to 0.01 × 0.3 × 0.3 mm in size; these occur in clusters up to 0.5 mm across. Vendidaite is brittle, with estimated Mohs’ hardness of 2–2½ and perfect cleavage on (010). Dmeas = 1.97(1) g/cm3, Dcalc = 1.974 g/cm3. The new mineral is optically biaxial (+), α = 1.522(2), β = 1.524(2), γ = 1.527(2), 2V (meas.) = 75(15)°, 2V (calc.) = 79°. Infra-red spectrum is given. The chemical composition (electron microprobe, H2O by gas chromatography) is (in wt.%): Al2O3 28.51, Fe2O3 1.39, SO3 22.38, Cl 9.87, H2O 38.8, O = Cl −2.23, total 98.72. The empirical formula is: Al1.96Fe3+0.06(SO4)0.98 Cl0.98(OH)3.12 5.98H2O. The crystal structure was solved using single-crystal X-ray diffraction data (R1 = 0.044). Vendidaite is monoclinic, space group C2/c, a 11.9246(16), b 16.134(2), c 7.4573(9) Å, β 125.815(2)°, V 1163.4(3) Å3, and Z = 4. The structure is based upon [Al2(OH)3(H2O)6]3+ chains of edge- and corner-sharing [Al(OH,H2O)] octahedra running parallel to the a axis. The chains are linked by hydrogen bonds to (SO4) tetrahedra and Cl anions to form pseudolayers parallel to the (010) plane. The strongest lines of the powder X-ray diffraction pattern [d, Å (I, %) (hkl)] are: 6.78 (59) (111¯), 4.849 (94) (021), 4.366 (80) (131¯), 4.030 (75) (040, 111), 3.855 (100) (311¯), 3.285 (59) (131), 2.435 (52) (261¯). Type material is deposited in the collections of the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow, Russia, registration number 4335/1.

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