Abstract

The new mineral ammoniozippeite (IMA2017-017), (NH4)2[(UO2)2(SO4)O2]·H2O, was found in both the Blue Lizard mine San Juan County, Utah, and the Burro mine, San Miguel County, Colorado, USA. At both mines, it occurs as a low-temperature, secondary phase. The mineral is yellow to yellowish orange with pale yellow streak and fluoresces dull green yellow under 405 nm laser light. Crystals are transparent and have vitreous luster. It is brittle, with Mohs hardness of about 2½, splintery fracture, and three cleavages: {010} and {001} perfect, {100} good. The calculated density for the ideal formula is 4.433 g/cm3. Crystals are acicular to bladed, elongate on [100], up to about 0.2 mm in length at the Blue Lizard, and up to 2 mm at the Burro. Ammoniozippeite is optically biaxial (+) with α = 1.678(2), β = 1.724(3), γ = 1.779(3) (white light); the measured 2V is 87.1(5)°; r < v dispersion is weak; the optical orientation is X = b, Y = c, Z = a; and pleochroism is X colorless, Y orange yellow, and Z yellow orange (XY < Z). Electron microprobe analyses (WDS mode) provided the empirical formulae [(NH4)1.97Na0.03]Σ2.00(U1.00O2)2(S1.01O4)O2·H2O and [(NH4)1.99K0.06Na0.04]Σ2.09(U1.01O2)2(S0.97O4)O2·H2O for crystals from the Burro and Blue Lizard mines, respectively. The five strongest X-ray powder diffraction lines for Burro mine material are [dobs Å(I)(hkl)]: 7.17(100)(020), 3.580(21)(040), 3.489(42)(203), 3.138(63)(223), and 1.6966(18)(229,426). Ammoniozippeite is orthorhombic, Ccmb, a 8.7944(3), b 14.3296(7), c 17.1718(12) Å, V 2164.0(2) Å3, and Z = 8. The structure of ammoniozippeite (R1 = 0.0396 for 932 reflections with Io > 2σI) contains edge-sharing zig-zag chains of pentagonal bipyramids that are linked by sharing corners with SO4 groups, yielding a [(UO2)2(SO4)O2]2– sheet based on the zippeite-type topology. The interlayer region contains two NH4+ groups and one H2O group pfu, statistically distributed over three sites.

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