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Joteite, Ca 2 CuAl[AsO 4 ][AsO 3 (OH)] 2 (OH) 2 ·5H 2 O, a new arsenate with a sheet structure and unconnected acid arsenate groups

2013; Cambridge University Press; Volume: 77; Issue: 6 Linguagem: Inglês

10.1180/minmag.2013.077.6.08

1471-8022

Anthony R. Kampf, Stuart J. Mills, R. M. Housley, George R. Rossman, Barbara P. Nash, Maurizio Dini, Robert A. Jenkins,

Crystal structures of chemical compounds

Abstract Joteite (IMA2012-091), Ca 2 CuAl[AsO 4 ][AsO 3 (OH)] 2 (OH) 2 ·5H 2 O, is a new mineral from the Jote mine, Tierra Amarilla, Copiapó Province, Atacama, Chile. The mineral is a late-stage, low-temperature, secondary mineral occurring with conichalcite, mansfieldite, pharmacoalumite, pharmacosiderite and scorodite in narrow seams and vughs in the oxidized upper portion of a hydrothermal sulfide vein hosted by volcanoclastic rocks. Crystals occur as sky-blue to greenish-blue thin blades, flattened and twinned on {001}, up to ~300 μm in length, and exhibiting the forms {001}, {010}, {1 0}, {2 0} and {111}. The blades are commonly intergrown in wheat-sheaf-like bundles, less commonly in sprays, and sometimes aggregated as dense crusts and cavity linings. The mineral is transparent and has a very pale blue streak and vitreous lustre. The Mohs hardness is estimated at 2 to 3, the tenacity is brittle, and the fracture is curved. It has one perfect cleavage on {001}. The calculated density based on the empirical formula is 3.056 g/cm 3 . It is optically biaxial (–) with α = 1.634(1), β = 1.644(1), γ = 1.651(1) (white light), 2V meas = 78(2)° and 2V calc = 79.4°. The mineral exhibits weak dispersion, r < v . The optical orientation is X ≈ c* ; Y ≈ b* . The pleochroism is Z (greenish blue) > Y (pale greenish blue) > X (colourless). The normalized electron-microprobe analyses (average of 5) provided: CaO 15.70, CuO 11.22, Al 2 O 3 8.32, As 2 O 5 46.62, H 2 O 18.14 (structure), total 100 wt.%. The empirical formula (based on 19 O a.p.f.u.) is: Ca 1.98 Cu 1.00 Al 1.15 As 2.87 H 14.24 O 19 . The mineral is slowly soluble in cold, concentrated HCl. Joteite is triclinic, P , with the cell parameters: a = 6.0530(2), b = 10.2329(3), c = 12.9112(4) Å, α = 87.572(2), β = 78.480(2), γ = 78.697(2)°, V = 768.40(4) Å 3 and Z = 2. The eight strongest lines in the X-ray powder diffraction pattern are [ d obs Å (I)(hkl) ]: 12.76(100)(001), 5.009(23)(020), 4.206(26)(120,003,121), 3.92(24)(022,0 2, 02), 3.40(25)( 13), 3.233(19)(031,023,123,0 3), 2.97(132,201) and 2.91(15)( 22, 13). In the structure of joteite ( R 1 = 7.72% for 6003 F o > 4σ F ), AsO 4 and AsO 3 (OH) tetrahedra, AlO 6 octahedra and Cu 2+ O 5 square pyramids share corners to form sheets parallel to {001}. In addition, 7- and 8-coordinate Ca polyhedra link to the periphery of the sheets yielding thick slabs. Between the slabs are unconnected AsO 3 (OH) tetrahedra, which link the slabs only via hydrogen bonding. The Raman spectrum shows features consistent with OH and/or H 2 O in multiple structural environments. The region between the slabs may host excess Al in place of some As.