Five electron-microprobe analyses were carried out using VEGA TS 5130MM SEM equipped with EDX analyzer (INCA Si(Li) detector), at an operating voltage of 20 kV and a beam current of 0.6 nA. The program INCA Energy 200 was used for the analytical data calculation. Attempts to use the WDS mode with higher beam currents were unsuccessful because of the instability of the mineral containing significant amounts of volatile components. H2O was determined by gas chromatography of products of ignition at 1200C with a Vario MICRO cube analyser (Elementar GmBH, Germany). CO2 was not analysed because of the absence of absorption bands corresponding to vibrations of C–O bonds in the IR spectrum. Analytical data are given in Table 1. Contents of F, Na, Mg, Si, P, K, Ca, Mn, Cu, Zn, As, Br, Sr, Pb are below detection limits.
The empirical formula (based on 14 anions pfu and OH/H2O ratio calculated from charge balance) is Al1.96Fe3+0.06(SO4)0.98Cl0.98(OH)3.12·5.98H2O. The simplified formula is Al2(SO4)(OH)3Cl·6H2O, which requires Al2O3 29.59, SO3 23.23, Cl 10.29, H2O 39.21, O = Cl -2.32, total 100.00 wt%.
The new mineral vendidaite was discovered in the abandoned open pit La Vendida, near Sierra Gorda, Antofagasta Region, Atacama desert, Chile. Associated minerals are aubertite, magnesioaubertite, belloite, alunite, kaolinite and halloysite. Vendidaite forms colourless platy crystals up to 0.01 0.3 0.3 mm in size and their clusters up to 1 mm across. Vendidaite is brittle, with Mohs hardness of 2 and perfect cleavage on (010). Dmeas = 1.97(1) g/cm3, Dcalc = 1.974 g/cm3. The new mineral is biaxial (+), α = 1.522(2), β = 1.524(2), γ = 1.527(2), 2V (meas.) = 75(15)º, 2V (calc.) = 79º. Infra-red spectrum contains the following lines: ????. 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)0.98Cl0.98(OH)3.12·5.98H2O. The crystal structure was solved using single-crystal X-ray diffraction data (R = 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)º, V = 1163.4(3) Å3, 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 diffraction pattern [d, Å (I, %) (hkl)] are: 6.78 (59) (11-1), 4.849 (94) (021), 4.366 (80) (13-1), 4.030 (75) (040, 111), 3.855 (100) (31-1), 3.285 (59) (131), 2.435 (52) (26-1). Type material is deposited in the collections of the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow, Russia, registration number 4335/1.
This work was financially supported by the Russian Foundation for Basic Research (grants nos. 11-05-00397-a, 11-05-00407-a). SVK and APC were supported in this work by the Russian Federal Grant-in-Aid Program ‘Cadres’ (state contract 16.740.11.0490). X-ray diffraction studies were performed at the SPbSU X-ray diffraction Resource Centre. The authors are grateful to P.C. Piilonen, J. Schlüter, H. Friis, and L.A. Groat for valuable comments.