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The crystal structure of a novel natural uranyl sulfate, Ca(UO₂)₆(SO₄)₂O₂(OH)₆·12H₂O (CaUS), has been determined using data collected under ambient conditions at the Swiss–Norwegian beamline BM01 of the European Synchrotron Research Facility (ESRF). The compound is monoclinic, P2₁/c, a = 11.931(2), b = 14.246(6), c = 20.873(4) Å, β = 102.768(15), V = 3460.1(18) ų, and R₁ = 0.172 for 3805 unique observed reflections. The crystal structure contains six symmetrically independent U⁶⁺ atoms forming (UO₇) pentagonal bipyramids that share O^…O edges to form hexamers oriented parallel to the (010) plane and extended along [1–20]. The hexamers are linked via (SO₄) groups to form [(UO₂)₆(SO₄)₂O₂(OH)₆(H₂O)₄]²⁻ chains running along the c-axis. The adjacent chains are arranged into sheets parallel to (010). The Ca²⁺ ions are coordinated by seven O atoms, and are located in between the sheets, providing their linkage into a three-dimensional structure. The crystal structure of CaUS is closely related to that of uranopilite, (UO₂)₆(SO₄)O₂(OH)₆·14H₂O, which is also based upon uranyl sulfate chains consisting of hexameric units formed by the polymerization of six (UO₇) pentagonal bipyramids. However, in uranopilite, each (SO₄) tetrahedron shares its four O atoms with (UO₇) bipyramids, whereas in CaUS, each sulfate group is linked to three uranyl ions only, and has one O atom (O16) linked to the Ca²⁺ cation. The chains are also different in the U:S ratio, which is equal to 6:1 for uranopilite and 3:1 for CaUS. The information-based structural complexity parameters for CaUS were calculated taking into account H atoms show that the crystal structure of this phase should be described as very complex, possessing 6.304 bits/atom and 1991.995 bits/cell. The high structural complexity of CaUS can be explained by the high topological complexity of the uranyl sulfate chain based upon uranyl hydroxo/oxo hexamers and the high hydration character of the phase. Full article