Four crystal structures of the uranyl carbonate mineral andersonite were refined from single–crystal X–ray diffraction data. One sample is a natural andersonite from the Monte Cristo mine, Utah, USA. Structure refinement at 297 K yielded the composition Na2Ca[(UO2)(CO3)3]·5.28H2O, space group \(R{\overline{3}}m\) , a = 17.9012(6) Å, c = 23.7978(12) Å, Z = 18, and R1 = 0.0139 for 2283 Fo > 4σ(Fo). Three synthetic andersonite samples refined at T = 297, 200, and 100 K in space group \(R{\overline{3}}m\) (R1 = 0.0122–0.0144) have similar compositions with 5.27–5.29 structure–based H2O per formula unit. Unlike the natural sample, they exhibit an additional electron-density peak (X) with ρₚₑₐₖ=1.8–2.8 e⁻/ų, attributed to an as–yet unidentified lattice defect. The crystal structure of andersonite consists of a framework with composition Na2Ca[(UO2)(CO3)3]·5H2O, constructed from uranyl tricarbonate groups and CaO6(H2O) polyhedra organized into {Ca(H2O)[(UO2)(CO3)3]}2− layers linked by NaO4(H2O)2 and NaO2(H2O)4 octahedra. The framework contains a distinctive closed cavity bounded by twelve Na–bonded H2O groups forming a hexagonal prism and six carbonate oxygen atoms. Each cavity accommodates two symmetry–equivalent interstitial H2O molecules, corresponding to ⅓ H2O per formula unit at full occupancy. The result is a unique water-in-water capsule. Site splitting affects approximately ¹⁄₆ of the twelve H2O groups surrounding the interstitial H2O in all structures and is attributed to hydrogen–bond donation to the cavity water molecules. After providing deeper insight into the complexity of the andersonite structure and its close relationships to línekite and markeyite-type minerals, a comparison with existing crystal structures of andersonite is presented. It is shown that the idealized chemical formula of andersonite should be written as Na2Ca[(UO2)(CO3)3]·5⅓H2O and that there is no space to accommodate additional H2O.
