Abstract
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) Å3, 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.
Highlights
Most of the new uranium sulfates are the products of secondary low-temperature hydrothermal processes, which are often associated with the crystallization of very complex mineral species such as ewingite, Mg8 Ca8 (UO2 )24 (CO3 )30 O4 (OH)12 (H2 O)138, which is the most structurally complex mineral known today [6]
Each uranyl ion is coordinated by five O atoms that belong either to sulfate groups, hydroxyl ions, or H2 O molecules
CaUSrelated is to that closely related to 4)Othat of uranopilite, 4
Summary
It would not be an exaggeration to say that, within the last few years, uranium mineralogy and crystal chemistry has witnessed a true renaissance due to the discoveries of exceptional suites of new uranium minerals in Jáchymov, Czech Republic [1,2,3,4,5,6] and San Juan County, Utah, USA [7,8,9,10,11,12,13,14,15,16,17,18,19].The diversity of new natural uranyl sulfates is of particular interest [1,2,3,4,5,7,8,9,10,11,12,13,14,15,17,18,20,21], since most of them do not have direct synthetic analogues, and are new to the synthetic inorganic chemistry as well. Uranyl ions are interlinked via tetrahedral TO4 groups (T = S, Cr, Se, Mo) into finite clusters, chains, or layers, in which the interaction between adjacent uranyl groups is mediated by the hexavalent T6+ cations For this group of minerals and synthetic compounds, the U:T ratio is usually smaller than one, with the ratio of 1:2 = 0.5 probably being the most common. There exists a group of uranyl sulfate mineral structures with an U:S ratio larger than one (e.g., 2:1 for the minerals of the zippeite group [1,5,17,24,25,26,27,28]) or equal to one (e.g., in adolfpateraite, K(UO2 )(SO4 )(OH)(H2 O) [2], and johannite, Cu(UO2 ) (SO4 ) (OH)2 ·8H2 O [29]). Uranyl ions are linked via common O2−
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