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Meisserite (IMA2013-039), Na5(UO2)(SO4)3(SO3OH)(H2O), is a new uranyl sulfate mineral from the Blue Lizard mine, San Juan County, Utah (USA). It is named in honour of the prominent Swiss mineralogist Nicolas Meisser. The new mineral was found in a sandstone matrix and is associated with chalcanthite, copiapite, ferrinatrite, gypsum, johannite and another new Na-bearing uranyl sulfate, belakovskiite (IMA2013-075). Meisserite is a secondary mineral formed by the post-mining weathering of uraninite. The mineral is triclinic, P1, a = 5.32317(10), b = 11.5105(2), c = 13.5562(10) Å, α = 102.864(7)°, β = 97.414(7)°, γ = 91.461(6)°, V = 801.74(6) Å3, and Z = 2. Crystals are prisms elongated on [100], up to 0.3 mm long, exhibiting the forms {010} and {001}. Meisserite is pale green to yellowish green, translucent to transparent and has a very pale yellow streak. It is brittle, with fair cleavage on {100} and {001}, and uneven fracture. The Mohs hardness is estimated at 2. Meisserite is somewhat hygroscopic and easily soluble in water. The calculated density based on the empirical formula is 3.208 g/cm3. Meisserite exhibits bright yellow green fluorescence under both long- and shortwave UV radiation. The mineral is optically biaxial (-), with α = 1.514(1), β = 1.546(1), γ = 1.557(1) (measured in white light). The measured 2V is 60(2)° and the calculated 2V is 60°. Dispersion is r > v, perceptible, and the optical orientation is X≈a, Z≈c*. The mineral is pleochroic, with X (colourless) < Y (pale yellow) ≈ Z (pale greenish yellow). The empirical formula of meisserite (based on 19 O a.p.f.u.) is Na5.05(U0.94O2)(SO4)3[SO2.69(OH)1.31](H2O). The Raman spectrum is dominated by the symmetric stretching vibrations of UO22+, SO42- and also weaker O-H stretching vibrations. The eight strongest powder X-ray diffraction lines are [dobs in Å (hkl) Irel]: 13.15 (001) 81, 6.33 (012) 62, 5.64 (021,020) 52, 5.24 (100,012,101) 100, 4.67 (101) 68, 3.849 (12 1,102,022) 48, 3.614 (032,113) 41, and 3.293 (1 13,004) 43. The crystal structure of meisserite (R1 = 0.018 for 3306 reflections with Iobs > 3σI) is topologically unique among known structures of uranyl minerals and inorganic compounds. It contains uranyl pentagonal bipyramids linked by SO4 groups toform chains. Na+ cations bond to O atoms in the chains and to an SO3OH group and an H2O group between the chains, thereby forming a heteropolyhedral framework. [ABSTRACT FROM AUTHOR]