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

Leoszilardite (IMA2015-128), Na (sub 6) Mg(UO (sub 2) ) (sub 2) (CO (sub 3) ) (sub 6) .6H (sub 2) O, 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, cejkaite 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 long. More commonly it occurs as pale yellow pearlescent masses to 2 mm consisting of very small plates. Leoszilardite fluoresces green under both longwave and shortwave ultraviolet light, 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 H (sub 2) O. Crystals have perfect cleavage along {001}, and exhibit the forms {110}, {001}, {100}, {101} and {101}. Optically, leoszilardite is biaxial (-), alpha = 1.504(1), beta = 1.597(1), gamma = 1.628(1) (white light); 2V (meas.) = 57(1) degrees , 2V (calc.) = 57.1 degrees ; dispersion r > v, slight. Pleochroism: X = colourless, Y and Z = light yellow; X < Y approximately Z. The average of six wavelength dispersive spectroscopic analyses provided Na (sub 2) O 14.54, MgO 3.05, UO (sub 3) 47.95, CO (sub 2) 22.13, H (sub 2) O 9.51, total 97.18 wt.%. The empirical formula is Na (sub 5.60) Mg (sub 0.90) U (sub 2) O (sub 28) C (sub 6) H (sub 12.60) , based on 28 O apfu. Leoszilardite is monoclinic, C2/m, a = 11.6093(21), b = 6.7843(13), c = 15.1058(28) A, beta = 91.378(3) degrees , V = 1189.4(4) A (super 3) and Z = 2. The crystal structure (R (sub 1) = 0.0387 for 1394 reflections with I (sub obs) > 4sigma I), consists of uranyl tricarbonate anion clusters [(UO (sub 2) )(CO (sub 3) ) (sub 3) ] (super 4-) held together in part by irregular chains of NaO (sub 5) (H (sub 2) O) polyhedra sub parallel to [010]. Individual uranyl tricarbonate clusters are also linked together by three-octahedron units consisting of two Na-centred octahedra that share the opposite faces of a Mg-centred octahedron at the centre (Na-Mg-Na), and have the composition Na (sub 2) MgO (sub 12) (H (sub 2) O) (sub 4) The name of the new mineral honours the Hungarian-American physicist, inventor and biologist Dr. Leo Szilard (1898-1964).