• journal_title：American Mineralogist
• Contributor：Jakub Plášil ; Karla Fejfarová ; Michal Dušek ; Radek Škoda ; Jan Rohlíček
• Publisher：Mineralogical Society of America
• Date：2013-04-01
• Format：text/html
• Language：en
• Identifier：10.2138/am.2013.4331
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：98
• issue：4
• firstpage：549
• section：Actinides in Geology, Energy, and the Environment

The crystal structure of čejkaite, Na₄(UO₂)(CO₃)₃, from the type locality, was determined for the first time by single-crystal X-ray diffraction. In contrast to the previously reported pseudohexagonal triclinic symmetry, the current data indicate čejkaite is monoclinic, triply twinned, and belongs to the space group Cc. Refined unit-cell parameters are a = 9.2919(8), b = 16.0991(11), c = 6.4436(3) Å, β = 91.404(5)°, and V = 963.62(12) ų. The monoclinic unit cell is also supported by the good fit to the powder diffraction data. The structure of čejkaite consists of uranyl tricarbonate clusters, forming sheets sub-parallel to (001) by sharing edges with (NaΦ) polyhedra. Sheets are interconnected through the uranyl O atoms and columns of (Na1Φ) polyhedra that share their trigonal faces. All Na atoms in the structure are in sixfold coordination. The structure refinement yielded R_obs = 0.0424 for 1687 observed reflections [I_obs > 3σ(I)] and 0.0538 for all 2016 unique reflections. Refinement and bond-valence analysis of the structure confirmed the previously proposed formula Na₄(UO₂)(CO₃)₃, Z = 4.