Synchrotron Diffraction Study of the Crystal Structure of Ca(UO2)6(SO4)2O2(OH)612H2O, a Natural Phase Related to Uranopilite

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] 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.

Despite intense field investigations conducted from 1988 to 2015, to date, only one specimen of the mineral is known, which was collected only four years after the end of the mining works, and deposited in Musée Cantonal de Géologie of Lausanne (Switzerland) under research collection number: XRD-NM-0005. The data on this mineral presently known do not allow its full description as a new mineral species, until more material is found.

Chemical Composition
Semi-quantitative chemical analyses were carried out by means of scanning electron microscope CamScan MV2300 coupled with an energy-dispersive spectrometer Inca x-sight Oxford Instruments (Institut des sciences de la Terre, Faculté des géosciences et de l'environnement, University of Lausanne, Lausanne, Switzerland). Five analyses of 100-s each were conducted using a large focused beam scanned over a surface of about 100 µ m 2 , an accelerating voltage of 20 kV, a sample current of 40 µ A, and a vacuum of 1.5 10 −5 Pa. The following X-ray lines and analytical standards were used: U Mα-UO2, Ca Kα-CaSO4, and S Kα-CaSO4. Despite various analytical problems due to intense dehydratation under electron beam, only Ca, U, S, Ca, and O were detected with the average ratio Ca:U:S~1:5.6:2.2.

Synchrotron Single-Crystal X-Ray Diffraction Study
Synchrotron diffraction experiment was performed under ambient conditions at the Swiss-Norwegian beamline BM01 of the European Synchrotron Research Facility (ESRF) with an imaging plate area detector (Mar345; 2300  2300 pixels) that had a crystal-to-detector distance of 160 mm. A yellow needle-like crystal of CaUS was mounted on a tapered glass fiber and centered using a high-magnification CCD (charge-coupled device) camera. Diffraction data were measured using monochromatized radiation (λ = 0.80000 Å) in an oscillation mode by rotating the crystal in  by 2° two minutes per frame; 100 frames were measured. The intensities were integrated and merged with the program CrysAlis. Lorentz and polarization corrections were applied; absorption effects were corrected using SADABS (Rint = 0.144). The structure was solved using the SHELXS program [36].
Despite intense field investigations conducted from 1988 to 2015, to date, only one specimen of the mineral is known, which was collected only four years after the end of the mining works, and deposited in Musée Cantonal de Géologie of Lausanne (Switzerland) under research collection number: XRD-NM-0005. The data on this mineral presently known do not allow its full description as a new mineral species, until more material is found.

Chemical Composition
Semi-quantitative chemical analyses were carried out by means of scanning electron microscope CamScan MV2300 coupled with an energy-dispersive spectrometer Inca x-sight Oxford Instruments (Institut des sciences de la Terre, Faculté des géosciences et de l'environnement, University of Lausanne, Lausanne, Switzerland). Five analyses of 100-s each were conducted using a large focused beam scanned over a surface of about 100 µm 2 , an accelerating voltage of 20 kV, a sample current of 40 µA, and a vacuum of 1.5 10 −5 Pa. The following X-ray lines and analytical standards were used: U Mα-UO 2 , Ca Kα-CaSO 4 , and S Kα-CaSO 4 . Despite various analytical problems due to intense dehydratation under electron beam, only Ca, U, S, Ca, and O were detected with the average ratio Ca:U:S~1:5.6:2.2.

Synchrotron Single-Crystal X-Ray Diffraction Study
Synchrotron diffraction experiment was performed under ambient conditions at the Swiss-Norwegian beamline BM01 of the European Synchrotron Research Facility (ESRF) with an imaging plate area detector (Mar345; 2300 × 2300 pixels) that had a crystal-to-detector distance of 160 mm. A yellow needle-like crystal of CaUS was mounted on a tapered glass fiber and centered using a high-magnification CCD (charge-coupled device) camera. Diffraction data were measured using monochromatized radiation (λ = 0.80000 Å) in an oscillation mode by rotating the crystal in ϕ by 2 • two minutes per frame; 100 frames were measured. The intensities were integrated and merged with the program CrysAlis. Lorentz and polarization corrections were applied; absorption effects were corrected using SADABS (R int = 0.144). The structure was solved using the SHELXS program [36]. The agreement factor for the final model was R 1 = 0.173 for 3805 unique observed reflections with |F o | ≥ 4σ F . The crystallographic information and refinement parameters are given in Table 1. The poor quality of the diffraction data (the highest resolution = 1.09 Å) did not allow the refinement of positions of oxygen atoms in an anisotropic approximation. In addition, soft restraints were imposed upon the uranyl U-O and some S-O distances in order to keep the bond lengths of the uranyl ions within the crystal chemically realistic values. No positions of H atoms could be determined. Atom coordinates, isotropic parameters, and bond-valence sums (calculated using bond-valence parameters for the U-O bonds taken from Burns et al. [37] and for other bonds from Gagné and Hawthorne [38]) are given in Table 2. Table 3 provides the anisotropic displacement parameters for the U, Ca, and S atoms. Selected bond lengths are given in Table 4. Supplementary crystallographic data have been deposited in the Inorganic Crystal Structure Database (CSD 1873912), and can be obtained from Fachinformationszentrum Karlsruhe via https://www.ccdc.cam.ac.uk/structures/.
The crystal structure of CaUS contains three different types of H 2 O groups. The H 2 O29-H 2 O32 groups (corresponding to the O W 29-O W 32) are bonded to U 6+ cations, and therefore are the parts of the basic structural unit, i.e., the uranyl sulfate chains. The H 2 O33-H 2 O36 groups are bonded to Ca 2+ cations, and are located in between the chains, whereas the H 2 O37-H 2 O40 groups are held in the structure by the system of hydrogen bonds. Since the positions of the H atoms are unknown, no attempt was made to outline the possible hydrogen bonding system, since each potential donor/acceptor O atom has too many adjacent O atoms at the distances appropriate for the formation of a hydrogen bond.
The results of the bond valence analysis are given in Table 2. The bond valence sums (BVSs) for U atoms are in the range of 6.02-6.36 v.u., which is acceptable, taking into account that the U=O bond lengths within the uranyl ions were constrained. The BVSs for the S1, S2, and Ca sites are 5.85 v.u., 6.09 v.u., and 1.77 v.u., respectively. The BVSs for the O atoms assigned to hydroxyl ions range from 0.88 v.u. to 1.27 v.u., those for the H 2 O molecules vary from 0.00 (for the groups not bonded to U or Ca) to 0.49 v.u. The BVSs for the O atoms are in the range of 1.82-2.20 v.u., except for the O20 site, for which the BVS is equal to 1.43 v.u. However, the O20 atom is bonded to the S 6+ cation only, and has two adjacent H 2 O groups and one OH group at distances ranging from 2.74 Å to 2.97 Å. Therefore, it is most probable that this atom is involved in three moderate hydrogen bonds, which saturate its bond valence requirements.
According to the crystal structure refinement, the crystal chemical formula of CaUS can be written as Ca[(UO 2 ) 6 (SO 4 ) 2 O 2 (OH) 6  The results of the bond valence analysis are given in Table 2. The bond valence sums (BVSs) for U atoms are in the range of 6.02-6.36 v.u., which is acceptable, taking into account that the U=O bond lengths within the uranyl ions were constrained. The BVSs for the S1, S2, and Ca sites are 5.85 v.u., 6.09 v.u., and 1.77 v.u., respectively. The BVSs for the O atoms assigned to hydroxyl ions range from 0.88 v.u. to 1.27 v.u., those for the H2O molecules vary from 0.00 (for the groups not bonded to U or Ca) to 0.49 v.u. The BVSs for the O atoms are in the range of 1.82-2.20 v.u., except for the O20 site, for which the BVS is equal to 1.43 v.u. However, the O20 atom is bonded to the S 6+ cation only, and has two adjacent H2O groups and one OH group at distances ranging from 2.74 Å to 2.97 Å . Therefore, it is most probable that this atom is involved in three moderate hydrogen bonds, which saturate its bond valence requirements.
According to the crystal structure refinement, the crystal chemical formula of CaUS can be written as Ca[(UO2)6(SO4)2O2(OH)6(H2O)4]·(H2O)8 (with the chemical formula of the uranyl sulfate chain given in square brackets).
Calculations have been performed in several steps. At first, the structural complexity of the uranyl sulfate chain has been analyzed, taking into account its real rod symmetry group (RG, Figure 6a). Secondly, the topological complexity of the chain (according to the maximal RG) has been calculated ( Figure 6b). Then, complexity parameters for the uranyl sulfate substructure (i.e., two chains per unit cell) and for the whole structure have been calculated using ToposPro package [42], and are given in Table 5 for comparison. It should be taken into account that to process the complexity measures, the
Calculations have been performed in several steps. At first, the structural complexity of the uranyl sulfate chain has been analyzed, taking into account its real rod symmetry group (RG, Figure 6a). Secondly, the topological complexity of the chain (according to the maximal RG) has been calculated ( Figure 6b). Then, complexity parameters for the uranyl sulfate substructure (i.e., two chains per unit cell) and for the whole structure have been calculated using ToposPro package [42], and are given in The information-based complexity parameters [40,41] for CaUS are given in Table 5.
Calculations have been performed in several steps. At first, the structural complexity of the uranyl sulfate chain has been analyzed, taking into account its real rod symmetry group (RG, Figure 6a). Secondly, the topological complexity of the chain (according to the maximal RG) has been calculated ( Figure 6b). Then, complexity parameters for the uranyl sulfate substructure (i.e., two chains per unit cell) and for the whole structure have been calculated using ToposPro package [42], and are given in Table 5 for comparison. It should be taken into account that to process the complexity measures, the positions of all of the H atoms have been assigned manually in calculated positions considering the distribution of the H-bonding system. Complexity calculations show that the crystal structure of CaUS should be described as very complex, possessing 6.304 bits/atom and 1991.995 bits/cell. For comparison, the crystal structure of uranopilite should be considered as complex (6.304 bits/atom and 995.997 bits/cell), while the most frequent values of structural complexity of the natural uranyl sulfates (including H-atoms) are between 500-600 bits/cell [43]. 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. Both features are typical for low-temperature mineral phases that form in the oxidation zones of mineral deposits [37,[44][45][46].  [43]. 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. Both features are typical for low-temperature mineral phases that form in the oxidation zones of mineral deposits [37,[44][45][46].  Rod group is an affine subperiodic three-dimensional group type with one-dimensional translation that are classified analogously to the space groups. Subindex "a" in a rod group symbol indicates the (−cba) setting with the translation along the a direction. The symmetry operations for (a) are: 1 /2 + x, y, −z; and for (b) are: 1 /2 − x, −y, z; 1 /2 + x, y, −z; and −x, −y, −z.