Abstract
Paddlewheelite, MgCa5Cu2[(UO2)(CO3)3]4·33H2O, is a new uranyl carbonate mineral found underground in the Svornost mine, Jáchymov District, Bohemia, Czech Republic, where it occurs as a secondary oxidation product of uraninite. The conditions leading to its crystallization are complex, likely requiring concomitant dissolution of uraninite, calcite, dolomite, chalcopyrite, and andersonite. Paddlewheelite is named after its distinctive structure, which consists of paddle-wheel clusters of uranyl tricarbonate units bound by square pyramidal copper “axles” and a cubic calcium cation “gearbox.” Paddle wheels share edges with calcium polyhedra to form open sheets that are held together solely by hydrogen bonding interactions. The new mineral is monoclinic, Pc, a = 22.052(4), b = 17.118(3), c = 19.354(3) Å, β = 90.474(2)°, V = 7306(2) Å3 and Z = 4. Paddlewheelite is the second-most structurally complex uranyl carbonate mineral known after ewingite and its structure may provide insights into the insufficiently described mineral voglite, as well as Cu–U–CO3 equilibrium in general.
Highlights
Recent uranyl carbonate mineral discoveries have uncovered remarkably complex topological arrangements that formed under restrictive conditions of Ca–U–Cu2 (UO2 )3 (CO3) equilibria [1,2,3]
These new minerals lend their complexity to hydrated calcium polyhedra found in their structures, which can share numerous geometrical elements with uranyl tricarbonate units (UTC), (UO2 )(CO3 )3 4−, to form multidimensional structures
It is well understood that Ca–UTC complexes are some of the most stable aqueous uranyl species in groundwater [6,7], and that uranyl carbonate minerals are pervasive alteration products found in oxidizing areas of abandoned uranium mines and their tailings [8], and may be found on Minerals 2018, 8, 511; doi:10.3390/min8110511
Summary
Recent uranyl carbonate mineral discoveries have uncovered remarkably complex topological arrangements that formed under restrictive conditions of Ca–U–CO3 equilibria [1,2,3]. We describe the crystal-chemical and spectroscopic properties of the new Ca–Mg–Cu uranyl carbonate mineral paddlewheelite, whose structure contains several first known instances for uranyl minerals, including isolated square pyramidal copper polyhedra and calcium cations in cubic coordination. These two unique cation polyhedra bind to UTC units, forming an extraordinary paddlewheel motif arranged into an open-sheet topology that may provide several clues to understanding the structure and formation of the poorly described mineral voglite, Ca2 Cu(UO2 )(CO3 )4 ·6H2 O [9].
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