Bobcookite, NaAl(UO2)2(SO4)4·18H2O and wetherillite, Na2Mg(UO2)2(SO4)4·18H2O, two new uranyl sulfate minerals from the Blue Lizard mine, San Juan County, Utah, USA
AbstractThe new minerals bobcookite (IMA 2014-030), NaAl(UO2)2(SO4)4·18H2O and wetherillite (IMA 2014-044), Na2Mg(UO2)2(SO4)4·18H2O, were found in the Blue Lizard mine, San Juan County, Utah, USA, where they occur together as secondary alteration phases in association with boyleite, chalcanthite, dietrichite, gypsum, hexahydrite, johannite, pickeringite and rozenite.Bobcookite descriptive details: lime green to greenish-yellow massive veins and columnar crystals; transparent; vitreous lustre; bright greenish-white fluorescence; pale greenish yellow streak; hardness (Mohs) 2½; brittle; conchoidal fracture; no cleavage; moderately hygroscopic; easily soluble in cold H2O; densitycalc= 2.669 g cm–3. Optically, biaxial (–), α = 1.501(1), β = 1.523(1), γ = 1.536(1) (white light); 2Vmeas.= 78(1)°; 2Vcalc.= 74°; dispersionr<v, moderate. Pleochroism:Xcolourless,Yvery pale yellow-green,Zpale yellow-green;X<Y<Z. EDS analyses yielded the empirical formula Na0.97Al1.09(U1.02O2)2(S0.98O4)4(H2O)18. Bobcookite is triclinic,P1,a= 7.7912(2),b= 10.5491(3),c= 11.2451(8) Å , α = 68.961(5), β = 70.909(5), γ = 87.139(6)°,V= 812.79(8) Å3andZ= 1. The structure (R1= 1.65% for 3580Fo> 4σF) contains [(UO2)(SO4)2(H2O)] chains linked by NaO4(H2O)2octahedra to form layers. Hydrogen bonds to insular Al(H2O)6 octahedra and isolated H2O groups hold the structure together. The mineral is named for Dr Robert (Bob) B. Cook of Auburn University, Alabama, USA.Wetherillite descriptive details: pale greenish-yellow blades; transparent; vitreous lustre; white streak; hardness (Mohs) 2; brittle; two cleavages, {101} perfect and {010} fair; conchoidal or curved fracture; easily soluble in cold H2O; densitycalc= 2.626 g cm–3. Optically, biaxial (+), α = 1.498(1), β = 1.508(1), γ = 1.519(1) (white light); 2Vmeas.= 88(1)°, 2Vcalc.= 87.9°; dispersion isr<v, distinct; optical orientation:Z=b,X∧a= 54° in obtuse β; pleochroism:Xcolourless,Ypale yellow-green,Zpale yellow-green;X<Y≈Z. EDS analyses yielded the empirical formula Na1.98(Mg0.58Zn0.24Cu0.11Fe0.092+)Σ1.02(U1.04O2)2(S0.98O4)4(H2O)18. Wetherillite is monoclinic,P21/c,a= 20.367(1),b= 6.8329(1),c= 12.903(3) Å, β = 107.879(10)°,V= 1709.0(5) Å3andZ= 2. The structure (R1= 1.39% for 3625Fo> 4σF) contains [(UO2)(SO4)2(H2O)] sheets parallel to {100}. Edge-sharing chains of Na(H2O)5O polyhedra link adjacent uranyl sulfate sheets forming a weakly bonded three-layer sandwich. The sandwich layers are linked to one another by hydrogen bonds through insular Mg(H2O)6octahedra and isolated H2O groups. The mineral is named for John Wetherill (1866–1944) and George W. Wetherill (1925–2006).