Joteite, Ca_2CuAl[AsO_4][AsO_3(OH)]_2(OH)_2-5H_2O, a new arsenate with a sheet structure and unconnected acid arsenate groups

摘要

Joteite (IMA2012-091), Ca_2CuAl[AsO_4][AsO_3(OH)]_2(OH)_2-5H_2O, is a new mineral from the Jote mine, Tierra Amarilla, Copiapo 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 urn in length, and exhibiting the forms {001}, {010}, {llO}, {210} 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/cm3. It is optically biaxial (—) with a = 1.634(1), (3 = 1.644(1), y = 1.651(1) (white light), 2V_(meas )= 78(2)° and 2V_(calc) = 79.4°. The mineral exhibits weak dispersion, r Y (pale greenish blue) > X (colourless). The normalized electron-microprobe analyses (average of 5) provided: CaO 15.70, CuO 11.22, A1_2O_3 8.32, As_2O_5 46.62, H_2O 18.14 (structure), total 100 wt.%. The empirical formula (based on 19 O a.p.f.u.) is: Ca_(1.98)Cu_(1.Oo)Al_(1.15)As_(2 87)H_(14.24)O_(19). The mineral is slowly soluble in cold, concentrated HC1. Joteite is triclinic, PI, with the cell parameters: a = 6.0530(2), b = 10.2329(3), c = 12.9112(4) A, a = 87.572(2), P = 78.480(2), y = 78.697(2)°, V= 768.40(4) A3 and Z = 2. The eight strongest lines in the X-ray powder diffraction pattern are [d_(obs) A(T)(hkl)]: 12.76(100)(001), 5.009(23)(020), 4.206(26)(120,003,121), 3.92(24)(022,022,102), 3.40(25)(113), 3.233(19)(031,023,123,023), 2.97(132,201) and 2.91(15)(122,ll3). In the structure of joteite (R_1= 7.72% for 6003 F_o > 4aF), AsO_4 and AsO_3(OH) tetrahedra, A1O_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 AsC>3(OH) tetrahedra, which link the slabs only via hydrogen bonding. The Raman spectrum shows features consistent with OH and/or H_2O in multiple structural environments. The region between the slabs may host excess Al in place of some As.