Leószilárdite, the first Na,Mg-containing uranyl carbonate from the Markey Mine, San Juan County, Utah, USA

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© 2017 The Mineralogical Society. Leószilárdite (IMA2015-128), Na6Mg(UO2)2(CO3)6·6H2O, was found in the Markey Mine, Red Canyon, White Canyon District, San Juan County, Utah, USA, in areas with abundant andersonite, natrozippeite, gypsum, anhydrite, and probable hydromagnesite along with other secondary uranium minerals bayleyite, čejkaite, and johannite. The new mineral occurs as aggregates of pale yellow bladed crystals flattened on {001} and elongated along [010], individually reaching up to 0.2 mm in length. More commonly it occurs as pale yellow pearlescent masses to 2 mm consisting of very small plates. Leószilárdite fluoresces green under both LW and SW UV, and is translucent with a white streak, hardness of 2 (Mohs), and brittle tenacity with uneven fracture. The new mineral is readily soluble in room temperature H2O. Crystals have perfect cleavage along {001}, and exhibit the forms {110}, {001}, {100}, {101}, and {-101}. Optically, leószilárdite is biaxial (-), α = 1.504(1), β = 1.597(1), γ = 1.628(1) (white light); 2V (meas.) = 57(1)°, 2V (calc.) = 57.1°; dispersion r > v, slight. Pleochroism: X = colorless, Y and Z = light yellow; X < Y ≈ Z. The average of six wavelength dispersive spectroscopic (WDS) analyses provided Na2O 14.54, MgO 3.05, UO3 47.95, CO2 22.13, H2O 9.51, total 97.18 wt%. The empirical formula is Na5.60Mg0.90U2O28C6H12.60, based on 28 O apfu. Leószilárdite is monoclinic, C2/m, a = 11.6093(21), b = 6.7843(13), c = 15.1058(28) Å, β = 91.378(3)°, V = 1189.4(4) Å3 and Z = 2. The crystal structure (R1 = 0.0387 for 1394 reflections with Iobs > 4σI), consists of uranyl tricarbonate anion clusters [(UO2)(CO3)3]4- held together in part by irregular chains of NaO5(H2O) polyhedra sub parallel to [010]. Individual uranyl tricarbonate clusters are also linked together by three-octahedron units consisting of two Na-centered octahedra that share the opposite faces of a Mg-centered octahedron at the center (Na-Mg-Na), and have the composition Na2MgO12(H2O)4. The name of the new mineral honours the Hungarian-American physicist, inventor, and biologist Dr. Leó Szilárd (1898-1964).

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Mineralogical Magazine