Uranyl Selenite Research Articles

Complete Crystal Structures and Elastic Properties of the Uranyl Minerals Johannite, Pseudojohannite and Derriksite

Due to the high solubility of uranyl sulfate and selenite minerals, the investigation involving the determination of the crystal structures and physical properties of these minerals is essential in actinide environmental chemistry for the simulation of uranium migration from uraninite deposits and nuclear waste repositories. However, the determination of the complete crystal structures of the uranyl sulfate minerals johannite (Cu(UO2)2(SO4)2(OH)2 ·8H2O) and pseudojohannite (Cu3(UO2)4(SO4)2O4(OH)2 ·12H2O) and the uranyl selenite mineral derriksite (Cu4[((UO2)(SeO3)2(OH)6]) has not been feasible so far. In this work, the crystal structures of these minerals, including the positions of the hydrogen atoms, are determined using first principles solid-state methods based on periodic density functional theory using plane wave basis sets and pseudopotentials. The lattice parameters and associated geometrical variables as well as the corresponding X-ray diffraction patterns derived from the computed crystal structures are in excellent agreement with their experimental counterparts, derived from the corresponding experimental structures lacking the hydrogen atom positions. The complete crystal structure of derriksite is also determined by refinement from X-ray diffraction data, the resulting structure being consistent with the computed one. The knowledge of the positions of H atoms is of fundamental importance not only because they define the corresponding hydrogen bond networks holding together the atoms in the structures, but also because it allows for the efficient, inexpensive and safe determination of the physical properties using first principles methods. This feature is particularly important in the case of uranium-containing minerals due to their radiotoxicity, complicating the handling of the samples and experimental measurements. In this work, from the computed crystal structures, the elasticity tensors of these minerals are computed using the finite displacement method and a rich set of elastic properties including the bulk, Young’s and shear moduli, the Poisson’s ratio, ductility, anisotropy and hardness indices and bulk modulus derivatives with respect to pressure derivatives are determined.

The Role of Acidity in the Synthesis of Novel Uranyl Selenate and Selenite Compounds and Their Structures

Herein, the novel uranyl selenate and selenite compounds Rb2[(UO2)2(SeO4)3], Rb2[(UO2)3(SeO3)2O2], Rb2[UO2(SeO4)2(H2O)]·2H2O, and (UO2)2(HSeO3)2(H2SeO3)2Se2O5 have been synthesized using either slow evaporation or hydrothermal methods under acidic conditions and their structures were refined using single crystal X-ray diffraction. Rb2[(UO2)2(SeO4)3] synthesized hydrothermally adopts a layered 2D tetragonal structure in space group P42/ncm with a = 9.8312(4) Å, c = 15.4924(9) Å, and V = 1497.38(15) Å, where it consists of UO7 polyhedra coordinated via SeO4 units to create units UO2(SeO4)58− moieties which interlink to create layers in which Rb+ cations reside in the interspace. Rb2[(UO2)3(SeO3)2O2] synthesized hydrothermally adopts a layered 2D triclinic structure in space group P1¯ with a = 7.0116(6) Å, b = 7.0646(6) Å, c = 8.1793(7) Å, α = 103.318(7)°, β = 105.968(7)°, γ = 100.642(7)° and V = 365.48(6) Å3, where it consists of edge sharing UO7, UO8 and SeO3 polyhedra that form [(UO2)3(SeO3)2O2] layers in which Rb+ cations are found in the interlayer space. Rb2[UO2(SeO4)2(H2O)]·2H2O synthesized hydrothermally adopts a chain 1D orthorhombic structure in space group Pmn21 with a = 13.041(3) Å, b = 8.579(2) Å, c = 11.583(2) Å, and V = 1295.9(5) Å3, consisting of UO7 polyhedra that corner share with one H2O and four SeO42− ligands, creating infinite chains. (UO2)2(HSeO3)2(H2SeO3)2Se2O5 synthesized under slow evaporation conditions adopts a 0D orthorhombic structure in space group Cmc21 with a = 28.4752(12) Å, b = 6.3410(3) Å, c = 10.8575(6) Å, and V = 1960.45(16) Å3, consisting of discrete rings of [(UO2)2(HSeO3)2(H2SeO3)2Se2O5]2. (UO2)2(HSeO3)2(H2SeO3)2Se2O5 is apparently only the second example of a uranyl diselenite compound to be reported. A combination of single crystal X-ray diffraction and bond valance sums calculations are used to characterise all samples obtained in this investigation. The structures uncovered in this investigation are discussed together with the broader family of uranyl selenates and selenites, particularly in the context of the role acidity plays during synthesis in coercing specific structure, functional group, and topology formations.

Crystal Chemistry and Structural Complexity of Natural and Synthetic Uranyl Selenites

Comparison of the natural and synthetic phases allows an overview to be made and even an understanding of the crystal growth processes and mechanisms of the particular crystal structure formation. Thus, in this work, we review the crystal chemistry of the family of uranyl selenite compounds, paying special attention to the pathways of synthesis and topological analysis of the known crystal structures. Comparison of the isotypic natural and synthetic uranyl-bearing compounds suggests that uranyl selenite mineral formation requires heating, which most likely can be attributed to the radioactive decay. Structural complexity studies revealed that the majority of synthetic compounds have the topological symmetry of uranyl selenite building blocks equal to the structural symmetry, which means that the highest symmetry of uranyl complexes is preserved regardless of the interstitial filling of the structures. Whereas the real symmetry of U-Se complexes in the structures of minerals is lower than their topological symmetry, which means that interstitial cations and H2O molecules significantly affect the structural architecture of natural compounds. At the same time, structural complexity parameters for the whole structure are usually higher for the minerals than those for the synthetic compounds of a similar or close organization, which probably indicates the preferred existence of such natural-born architectures. In addition, the reexamination of the crystal structures of two uranyl selenite minerals guilleminite and demesmaekerite is reported. As a result of the single crystal X-ray diffraction analysis of demesmaekerite, Pb2Cu5[(UO2)2(SeO3)6(OH)6](H2O)2, the H atoms positions belonging to the interstitial H2O molecules were assigned. The refinement of the guilleminite crystal structure allowed the determination of an additional site arranged within the void of the interlayer space and occupied by an H2O molecule, which suggests the formula of guilleminite to be written as Ba[(UO2)3(SeO3)2O2](H2O)4 instead of Ba[(UO2)3(SeO3)2O2](H2O)3.

ChemInform Abstract: Synthesis, Structure, and Photoluminescence Properties of an Organically‐Templated Uranyl Selenite.

AbstractThe uranyl selenite (H2en) [(UO2)(SeO3)(HSeO3)] (NO3)·0.5H2O is synthesized by slow evaporation of an aqueous mixture of UO2(NO3)2, SeO2, and en (56% yield).

Synthesis, Structure, and Photoluminescence Properties of an Organically‐Templated Uranyl Selenite

AbstractThe organically‐templated uranyl selenite, (H2en)[(UO2)(SeO3)(HSeO3)](NO3)·0.5H2O (1) (en = 1,2‐ethylenediamine) was synthesized and characterized by elemental analyses, IR spectroscopy, TG, and single‐crystal X‐ray diffraction. Compound 1 crystallizes in the orthorhombic system, space group Pbca, with a = 13.170(3) Å, b = 11.055(2) Å, c = 18.009(4) Å, V = 2621.8(9) Å3, M = 1316.19, Z = 4, Dcal = 3.334 g·cm–3, μ(Mo‐Kα) = 17.998 mm–1, GOF = 1.059, R1 = 0.0263, wR2 = 0.0532 [I>2σ(I)]. The X‐ray diffraction analysis reveals that compound 1 has a three‐dimensional (3D) supramolecular structure. It contains negatively charged [UO2(HSeO3)(SeO3)]– inorganic anion layers and is balanced by [H2en]2+ cations and NO3– anions located in the interlayers. Furthermore, the photoluminescence properties of 1 were investigated.

Topologically and geometrically flexible structural units in seven new organically templated uranyl selenates and selenite–selenates

Single crystals of seven novel uranyl oxysalts of selenium with protonated methylamine molecules, [C2H8N]2[(UO2)(SeO4)2(H2O)] (I), [C2H8N]2[(UO2)2(SeO4)3(H2O)] (II), [C4H15N3][H3O]0.5[(UO2)2(SeO4)2.93(SeO3)0.07(H2O)](NO3)0.5 (III), [C2H8N]3[H5O2][(UO2)2(SeO4)3(H2O)2]2(H2O)5 (IV), [C2H8N]2[H3O][(UO2)3(SeO4)4(HSeO3)(H2O)](H2SeO3)0.2 (V), [C4H12N]3[H3O][(UO2)3(SeO4)5(H2O)] (VI), and [C2H8N]3(C2H7N)[(UO2)3(SeO4)4(HSeO3)(H2O)] (VII) have been prepared by isothermal evaporation from aqueous solutions. Their crystal structures have been solved by direct methods and their uranyl selenate and selenite–selenate units investigated using black-and-white graphs from the viewpoints of topology of interpolyhedral linkages and isomeric variations. The crystal structure of IV is based upon complex layers with unique topology, which has not been observed previously in uranyl selenates. Investigations of the statistics and local distribution of the U–Obr–Se bond angles demonstrates that shorter angles associate with undulations, whereas larger angles correspond to planar areas of the uranyl selenite layers.

ChemInform Abstract: Incorporation of Neptunium(VI) into a Uranyl Selenite.

AbstractNpVI is incorporated into compound (IV) with the highest level of Np uptake (12% substitution of NpVI for UVI) observed for an uranyl compound so far.

Incorporation of Neptunium(VI) into a Uranyl Selenite

The incorporation of neptunium(VI) into the layered uranyl selenite Cs[(UO(2))(HSeO(3))(SeO(3))] has yielded the highest level of neptunium uptake in a uranyl compound to date with an average of 12(±3)% substitution of Np(VI) for U(VI). Furthermore, this is the first case in nearly 2 decades of dedicated incorporation studies in which the oxidation state of neptunium has been determined spectroscopically in a doped uranyl compound and also the first time in which neptunium incorporation has resulted in a structural transformation.

CRYSTAL STRUCTURES OF SIX NEW URANYL SELENATE AND SELENITE COMPOUNDS AND THEIR RELATIONSHIP WITH URANYL MINERAL STRUCTURES

Three uranyl selenates, one mixed uranyl selenate-selenite, and two uranyl selenite compounds have been synthesized using mild hydrothermal techniques. The structure of each has been determined using single-crystal diffraction data collected with Mo K αX-ray radiation and an APEX CCD-based detector. The isotypic compounds ( 1 ) Cs 2 Ni 3 (H 2 O) 4 [(UO 2 ) 8 (SeO 4 ) 4 O 8 ] and ( 2 ) Cs 2 Co 3 (H 2 O) 4 [(UO 2 ) 8 (SeO 4 ) 4 O 8 ] are monoclinic, space group C 2/ c , with cell parameters of a 8.755(3), b 14.228(5), c 17.927(8) A, β103.739(3)° and a 8.744(2), b 14.380(4), c 17.901(5) A, β103.654(3)°, respectively. The structure of ( 3 ) Cs 2 (H 2 O) 4 [(UO 2 ) 4 (SeO 4 ) 2 O 2 (OH) 2 ] is triclinic, space group P 1 with cell parameters a 8.6877(4), b 8.7164(4), c 8.9589(8) A, α95.964(10), β99.155(10), and γ117.318(10)°. The mixed selenate-selenite phase ( 4 ) Cs 2 (H 2 O) 5 [(UO 2 ) 7 (SeO 4 ) 2 (SeO 3 ) 2 O 4 ]. H 2 O is monoclinic, space group P 2 1 /m, with cell parameters a 9.1381(3), b 15.0098(5), c 15.1732(5) A, β91.171(10)°. The compound ( 5 ) Li 2 (H 2 O) 6 [(UO 2 ) 3 (SeO 3 ) 2 O 2 ] is monoclinic, with cell parameters P 2 1 / c , a 7.5213(9), b 7.0071(9), c 17.3282(19) A, β98.834(2)°. ( 6 ) Cs 2 (H 2 O) 2 [(UO 2 ) 4 (SeO 3 ) 5 ] is monoclinic, P 2 1 / n , with cell parameters a 10.913(3), b 12.427(3), c 18.448(4) A, β90.393(3)°. Compounds 1 , 2 , and 3 contain sheets of uranyl pentagonal bipyramids and selenate tetrahedra with the zippeite-type topology. Compound 4 is a unique mixed selenate-selenite compound that contains sheets that are topologically intermediate between the zippeite-type and phosphuranylite-type topologies. Compound 5 contains a uranyl selenite sheet based on the phosphuranylite anion-topology, as found in the minerals marthozite and guilleminite. Compound 6 contains a novel sheet consisting of uranyl pentagonal and hexagonal bipyramids, as well as SeO 3 pyramids.

Raman spectroscopic study of the uranyl selenite mineral demesmaekerite Pb2Cu5(UO2)2(SeO3)6(OH)6·2H2O

AbstractThe Raman spectrum of the uranyl selenite mineral demesmaekerite was studied, complemented by the infrared spectrum and tentatively interpreted. The observed bands were attributed to the stretching and bending vibrations of (UO2)2+, (SeO3)2− and OH groupings. UO bond lengths in uranyl and OH···O hydrogen bond lengths were calculated from Raman and/or infrared spectra and compared with published data. Copyright © 2008 John Wiley & Sons, Ltd.

Near- and mid-infrared spectroscopy of the uranyl selenite mineral haynesite (UO 2) 3(SeO 3) 2(OH) 2·5H 2O

Near- and mid-infrared spectra of uranyl selenite mineral haynesite (UO 2) 3(SeO 3) 2(OH) 2·5H 2O, were studied and assigned. Observed bands were assigned to the stretching vibrations of uranyl and selenite units, stretching, bending and libration modes of water molecules and hydroxyl ions, and δ U OH bending vibrations. U O bond lengths in uranyl and hydrogen bond lengths O H…O were inferred from the spectra.

Raman spectroscopic study of the uranyl selenite mineral marthozite Cu[(UO2)3(SeO3)2O2]·8H2O

AbstractThe mineral marthozite, a uranyl selenite, has been characterised by Raman spectroscopy at 298 K. The bands at 812 and 797 cm−1 were assigned to the symmetric stretching modes of the (UO2)2+ and (SeO3)2− units, respectively. These values gave the calculated UO bond lengths in uranyl of 1.799 and/or 1.814 Å. Average UO bond length in uranyl is 1.795 Å, inferred from the X‐ray single crystal structure analysis of marthozite by Cooper and Hawthorne. The broad band at 869 cm−1 was assigned to the ν3 antisymmetric stretching mode of the (UO2)2+ (calculated UO bond length 1.808 Å). The band at 739 cm−1 was attributed to the ν3 antisymmetric stretching vibration of the (SeO3)2− units. The ν4 and the ν2 vibrational modes of the (SeO3)2− units were observed at 424 and 473 cm−1. Bands observed at 257, and 199 and 139 cm−1 were assigned to OUO bending vibrations and lattice vibrations, respectively. OH···O hydrogen bond lengths were inferred using Libowiztky's empirical relation. The infrared spectrum of marthozite was studied for complementation. Copyright © 2008 John Wiley & Sons, Ltd.

An Application of near Infrared and Mid-Infrared Spectroscopy to the Study of Uranyl Selenite Minerals: Derriksite, Demesmaekerite, Guilleminite and Haynesite

The near infrared (NIR) spectra of the natural uranyl selenite minerals that include derriksite, demesmaekerite, guilleminite and haynesite are examined as a potential indicator of uranium occurring geologic materials at the earth's surface. NIR analysis, complimented with mid-IR studies, was used to investigate the co-ordination of UO22+ and Cu2+ in the uranyl selenites. Bands obtained from the infrared spectra of selenites are interpreted in terms of the stretching vibrations of uranyl, selenite units and OH groups and bending modes. NIR spectra of the uranyl selenite minerals exhibit distinctive characteristics of uranyl ion (UO2)2+ absorptions over the range of 11,500–8000 cm−1. The high- range NIR spectrum of Cu-bearing uranyl derriksite is resolved into two bands, UO22+ 8070 cm−1 and Cu2+ 7175 cm−1. The effect of lead in demesmaekerite leads to distortion of the spectrum and the NIR bands are observed for uranyl ion at 11,305 cm−1 and 8475 cm−1 and for Cu2+ at 7430 cm−1. The δ U–OH bending vibrations are characterised by a strong absorption feature centred at 1015 cm−1 in haynesite. A significant shift for UOH bending vibrations and the absence of ν1 and ν3 vibrations of UO22+ at the expense of Cu2+ are reflected in the spectrum of derriksite. The complexity of bands with shifts to low wavenumbers could take place due to the additional cations of Pb and Cu in the structure of demesmaekerite. NIR spectroscopy has proven to be a most useful tool for the identification of and distinction between different uranyl selenite minerals.

Synthesis and crystal structure of a new uranyl selenite(IV)-selenate(VI), [C5H14N]4[(UO2)3(SeO4)4(HSeO3)(H2O)](H2SeO3)(HSeO4)

Crystals of a new uranyl selenite(IV)-selenate(VI), [C5H14N]4[(UO2)3(SeO4)4(HSeO3)(H2O)]·(H2SeO3)(HSeO4) were obtained by evaporation from aqueous solutions. The compound crystallizes in the triclinic system, space group $$P\bar 1$$ , a = 11.7068(9), b = 14.8165(12), c = 16.9766(15) A, α = 73.899(6)°, β = 76.221(7)°, γ = 89.361(6)°, V = 2743.0(4) A3, Z = 2. The crystal structure was solved by direct methods and refined to R 1 = 0.081 (wR 2 = 0.150) for 6966 reflections with |F hkl | ≥ 4σ(|F hkl |). The structure is based on [(UO2)3(SeO4)4(HSeO3)(H2O)]3− layers formed by joining uranyl pentagonal bipyramids, selenate tetrahedra, and selenite pyramids. The [HSe(VI)O4]− anions, [H2Se(IV)O3] molecules, and protonated methylbutylamine cations are arranged between the layers.

A Raman spectroscopic study of the uranyl selenite mineral haynesite

The mineral haynesite, a uranyl selenite, has been characterised by Raman spectroscopy at 298 and 77 K. Two bands at 811.5 and 800.2 cm-1 are assigned to the symmetric stretching modes of the (UO2)2+ and (SeO3)2- units respectively. These values give calculated U-O bond lengths of 1.799 and/or 1.801 A. The broad band at 861.8 cm-1 is assigned to the ν3 antisymmetric stretching mode of the (UO2)2+ (calculated U-O bond length 1.813 A). Additional bands are observed in the 77 K spectrum. In the spectroscopy of selenite compounds the position of the antisymmetric stretching vibration occurs at lower wavenumbers than the symmetric stretching mode and thus the band at 746.6 cm-1 is attributed to the ν3 antisymmetric stretching vibration of the (SeO3)2- units. The ν4 and the ν2 vibrational modes of the (SeO3)2- units are observed at 418.5 cm-1 and 472.1 cm-1. Bands observed at 278.3, 257.3 and 218.8 cm-1 are assigned to OUO bending vibrations.

Hydrothermal synthesis and crystal chemistry of the new strontium uranyl selenites, Sr[(UO2)3(SeO3)2O2]•4H2O and Sr[UO2(SeO3)2

The reaction of UO 3 with SeO 2 in the presence of SrCl 2 ⋅6H 2 O and Sr(OH) 2 ⋅8H 2 O in supercritical water at 425 °C for 3 d results in the formation of the new strontium uranyl selenites, Sr[(UO 2 ) 3 (SeO 3 ) 2 O 2 ]⋅4H 2 O (1) and Sr[UO 2 (SeO 3 ) 2 ] (2). The single crystal X-ray structures of type 1 and type 2 were solved by direct methods and refined by full-matrix least-squares methods. Crystallographic data (193 K): (1), monoclinic, space group C2/m, a = 17.014(2), b = 7.0637(7), and c = 7.1084(7) Å, β = 100.544(2)°, Z = 2, R(F) = 0.0361 for 79 parameters with 1132 reflections with I > 2σ(I), wR2 = 0.0998 for all data; (2), triclinic, space group P1̅ , a = 5.6722(4), b = 6.7627(5), and c = 11.2622(8) Å, α = 104.698(1)°, β = 93.708(1)°, γ = 109.489(1)°, Z = 2, R(F) = 0.0373 for 110 parameters with 1902 reflections with I > 2σ(I), wR 2 = 0.0856 for all data. The structure of type 1 contains two-dimensional 2 ∞ [(UO 2 ) 3 (SeO 3 ) 2 O 2 ] 2 - sheets with the same topology as those found in guilleminite, Ba[(UO 2 ) 3 (SeO 3 ) 2 O 2 ]⋅3H 2 O, and marthozite, Cu[(UO 2 ) 3 (SeO 3 ) 2 O 2 ](H 2 O) 8 . Sr 2+ cations and H 2 O groups occur between the layers. In contrast, the structure of type 2 contains one-dimensional 1 ∞ [UO 2 (SeO 3 ) 2 ] 2- ribbons with Sr 2+ cations residing between them. This compound is isostructural with its Ca 2+ -containing analog.

Larisaite, Na(H3O)(UO2)3(SeO3)2O2 4H2O, a new uranyl selenite mineral from Repete mine, San Juan County, Utah, U.S.A.

Larisaite, a new uranyl selenite with the idealized formula Na(H3O)(UO2)3(SeO3)2O2 · 4H2O, has been found in a sedimentary rock from Repete mine near Blanding, San Juan Co., Utah, U.S.A., in association with quartz, haynesite, andersonite, wolsendorfite, uranophane, gypsum, calcite and montmorillonite. The mineral is named in memory of Russian mineralogist and crystallographer Larisa Nikolaevna Belova (1923–1998) who made a significant contribution to the knowledge on the uranium minerals. Larisaite forms coarse lamellar crystals up to 1 mm and radial aggregates up to 2 mm. It is transparent or translucent, yellow, lustre vitreous, streak yellow. Fluorescence under the UV light is green (wavelengths of excitation 250 nm). Larisaite is sectile, with Mohs' hardness 1, perfect cleavage on (010) and uneven fracture across the cleavage direction. Calculated density is 4.50 g/cm3 from the crystal structure refinement and 4.46 g/cm3 from the empirical formula. Optically biaxial (−), α 1.597(2), β 1.770(5), γ 1.775(5); −2V = 20°. Dispersion is strong, r α (light greenish-yellow). IR spectrum is given. Average values for 3 point microprobe analyses (wt.%, ranges are given in brackets) are: Na2O 2.04 (1.82–2.32), K2O 0.69 (0.62–0.76), CaO 0.23 (0.17–0.30), UO3 72.19 (71.77–72.64), SeO2 18.12 (17.83–18.48); H2O content determined by Penfield method is 7.64; total 100.91 wt.%; contents of Mg, Sr, Ba, Pb, Zn, Mn, Ni, Co, Cu, Fe, Al, Si, S, As, Cl, F are lower than detection limits i. e. 2)3.09(SeO3)2O24.1H20.1-(Kp/Kc) = 0.013 (“superior”). The crystal structure has been determined (R=0.067). Larisaite is monoclinic, space group P 11 m ; a = 6.9806(9), b = 7.646(1), c = 17.249(2) A, γ = 90.039(4)°, V = 920.64 A3, Z = 2. The strongest lines in the powder diffraction pattern [ d , A ( I , %) ( hkl )] are: 8.63 (43) (002), 7.67 (100) (010), 3.85 (40) (−113, 020, 113), 3.107(77) (211), 2.874 (53) (006, −115). By the U: Se ratio, the values of unit cell parameters and the structure type, larisaite is related to haynesite, guilleminite and piretite. In common with guilleminite, uranium polyhedra and SeO3 triangles form the sheets, however the distribution of interlayer cations and H2O molecules is different. Holotype specimen is deposited in the Geoscientific Collections of Freiberg University of Mining and Technology, Faculty of Geosciences, Geotechnics and Mining, Freiberg, Germany (the inventory number 80251).

Do Secondary and Tertiary Ammonium Cations Act as Structure‐Directing Agents in the Formation of Layered Uranyl Selenites?

AbstractFor Abstract see ChemInform Abstract in Full Text.

Do secondary and tertiary ammonium cations act as structure-directing agents in the formation of layered uranyl selenites?

Two new layered uranyl selenites, [C(4)H(12)N(2)](0.5)[UO(2)(HSeO(3))(SeO(3))] (1) and [C(6)H(14)N(2)](0.5)[UO(2)(HSeO(3))(SeO(3))].0.5H(2)O.0.5CH(3)CO(2)H (2), have been isolated from mild hydrothermal reactions. The preparation of 1 was achieved by reacting UO(2)(C(2)H(3)O(2))(2).2H(2)O with H(2)SeO(4) in the presence of piperazine at 130 degrees C for 2 d. Crystals of 2 were synthesized by reacting UO(2)(C(2)H(3)O(2))(2).2H(2)O, H(2)SeO(4), and 1,4-diazabicyclo[2.2.2]octane at 150 degrees C for 2 d. The structure of 1 consists of UO(2)(2+) cations that are bound by bridging HSeO(3)(-) anions and chelating/bridging SeO(3)(2)(-) anions to yield UO(7) pentagonal bipyramids. The joining of the uranyl moieties by the hydrogen selenite and selenite anions creates two-dimensional 2(infinity) [UO(2)(HSeO(3))(SeO(3))](-) layers that extend in the bc-plane. The stereochemically active lone pair of electrons on the HSeO(3)(-) and SeO(3)(2)(-) anions align along the a-axis making each layer polar. The 2(infinity)[UO(2)(HSeO(3))(SeO(3))](-) layers and piperazinium cations stack in a AA'BAA'B sequence where two layers stack on one another without intervening piperazinium cations. While each 2(infinity)[UO(2)(HSeO(3))(SeO(3))](-) layer is polar, in the AA' stacking, the polarity of the second sheet is reversed with respect to the first, yielding an overall structure that is centrosymmetric. The structure of 2 is constructed from uranyl cations that are bound by three bridging SeO(3)(2)(-) and two bridging HSeO(3)(-) anions to create UO(7) pentagonal bipyramids. The linking of the uranyl cations by the HSeO(3)(-) and SeO(3)(2-) anions creates 2(infinity)[UO(2)(HSeO(3))(SeO(3))](-) layers that extend in the ac-plane. In 1 and 2, the organic ammonium cations form hydrogen bonds with the anionic uranyl selenite layers. Crystallographic data: 1, monoclinic, space group P2(1)/c, a = 10.9378(5) A, b = 8.6903(4) A, c = 9.9913(5) A, beta = 90.3040(8) degrees, Z = 4; 2, orthorhombic, space group Pnma, a = 13.0858(8) A, b = 17.555(1) A, c = 10.5984(7) A, Z = 8.

A new uranyl carbonate sheet in the crystal structure of fontanite, Ca[(UO2)3(CO3)2O2](H2O)6

The structure of fontanite, Ca[(UO 2 ) 3 (CO 3 ) 2 O 2 ](H 2 O) 6 , is monoclinic, space group P2 1 /n, a = 6.968(3), b = 17.276(7), c = 15.377(6) Å, β = 90.064(6)°, V = 1851(1) Å 3 , Z = 4. The structure was solved by direct methods and refined on the basis of F 2 for all unique reflections using leastsquares techniques to an agreement index (R1) of 9.9%. The structure contains two symmetrically distinct uranyl pentagonal bipyramids, one uranyl hexagonal bipyramid, and two CO 3 triangles. The uranyl polyhedra form chains by sharing equatorial edges, and CO 3 groups occur on either side of the chains, where they share equatorial edges of the uranyl hexagonal bipyramids. The CO 3 groups share their third ligand with a uranyl pentagonal bipyramid of an adjacent chain, resulting in uranyl carbonate sheets of composition [(UO 2 ) 3 (CO 3 ) 2 O 2 ] 2- . The single symmetrically unique Ca 2+ cation is located between the sheets, and is coordinated by two O atoms of uranyl ions of adjacent sheets, and six H 2 O groups. The uranyl carbonate sheet in fontanite is novel, but is based upon the phosphuranylite anion topology that is the basis of uranyl phosphate, uranyl selenite, and uranyl sulfate sheets in a variety of minerals.