Tapiaite, Ca5Al2(AsO4)(4)(OH)(4)center dot 12H(2)O, a new mineral from the Jote mine, Tierra Amarilla, Chile

摘要

Tapiaite (IMA2014-024), Ca5Al2(AsO4)(4)(OH)(4)center dot 12H(2)O, 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, joteite, 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 colourless blades, flattened on {10 (1) over bar} and elongated and striated along [010], up to similar to 0.5 mm long, and exhibiting the forms {10 (1) over bar}, {101} and {111}. The blades are commonly intergrown in subparallel bundles and less commonly in sprays. The mineral is transparent and has a white streak and vitreous lustre. The Mohs hardness is estimated to be between 2 and 3, the tenacity is brittle, and the fracture is splintery. It has two perfect cleavages on {101} and {10 (1) over bar}. The calculated density based on the empirical formula is 2.681 g cm(-3). It is optically biaxial (+) with alpha = 1.579(1), beta = 1.588(1), gamma = 1.610(1) (white light), 2V(meas) = 66(2)degrees and 2V(calc) = 66 degrees. The mineral exhibits no dispersion. The optical orientation is X approximate to [10 (1) over bar]; Y = b, Z approximate to [101]. The electron-microprobe analyses (average of five) provided: Na2O 0.09, CaO 24.96, CuO 0.73, Al2O3 10.08, Fe2O3 0.19, As2O5 40.98, Sb2O5 0.09, H2O 23.46 (structure), total 100.58 wt.%. In terms of the structure, the empirical formula (based on 32 O a.p.f.u.) is (Ca4.83Cu0.102+Na0.03)(Sigma 4.96)(Al2.14Fe0.033+)(Sigma 2.17) [(As3.875+Sb0.015+)(Sigma 3.88)O-16][(OH)(3.76)(H2O)(0.24)](Sigma 4)(H2O)(10)center dot 2H(2)O. The mineral is easily soluble in RT dilute HCl. Tapiaite is monoclinic, P2(1), with unit-cell parameters a = 16.016(1), b = 5.7781(3), c = 16.341(1) angstrom, beta = 116.704(8)degrees, V = 1350.9(2) angstrom(3) and Z = 2. The eight strongest lines in the powder X-ray diffraction pattern are [d(obs) A(I)(hkl)]: 13.91(100)((1) over bar 01), 7.23(17)(200,002), 5.39(22)(110,011), 4.64(33)((1) over bar 12,(2) over bar 11,(3) over bar 03), 3.952(42)((1) over bar 13,(3) over bar 11,(2) over bar 13), 3.290(35)((2) over bar 14,(4) over bar 12,(1) over bar 14,(4) over bar 11), 2.823(39)(303,(3) over bar 15) and 2.753(15)((3) over bar 13,(1) over bar 15,121,(5) over bar 11). The structure of tapiaite (R-1 = 5.37% for 1733 F-o > 4 sigma F) contains Al(AsO4)(OH)(2) chains of octahedra and tetrahedra that are topologically identical to the chain in the structure of linarite. CaO8 polyhedra condense to the chains, forming columns, which are decorated with additional peripheral AsO4 tetrahedra. The CaO8 polyhedra in adjacent columns link to one another by corner-sharing to form thick layers parallel to {10 (1) over bar} and the peripheral AsO4 tetrahedra link to CaO6 octahedra in the interlayer region, resulting in a framework structure.