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    Topic review

    Uranyl Carbonate Minerals

    Submitted by: Vladislav Gurzhiy

    Definition

    Uranyl carbonates are one of the largest groups of secondary uranium(VI)-bearing natural phases being represented by 40 minerals approved by the International Mineralogical Association, overtaken only by uranyl phosphates and uranyl sulfates. Uranyl carbonate phases form during the direct alteration of primary U ores on contact with groundwaters enriched by CO2, thus playing an important role in the release of U to the environment. The presence of uranyl carbonate phases has also been detected on the surface of “lavas” that were formed during the Chernobyl accident.

    1. Introduction of Uranyl Carbonate Minerals

    Uranyl carbonate phases play a very important role in all processes related to the nuclear fuel cycle. under the influence of groundwaters enriched with CO2, which can be derived from the dissolution of host carbonate rocks or from the atmosphere [1][2][3][4][5]. In dissolved form, uranyl carbonates can play an important role in U release to the environment. And of course, it should not be forgotten that uranyl-carbonate mineralization has been described among the alteration products of the “lavas” that were formed during the accident at the fourth reactor of the Chernobyl nuclear power plant in 1986 [6][7].

    There are 40 uranyl carbonate mineral species approved by the International Mineralogical Association as of 1 November 2020, thus making this group one of the most representative among secondary uranium minerals, coming third only after phosphates and sulfates [8][9]. Despite a fairly large number of known compounds, the structural diversity is not as great as one might expect. It is also of interest that about a quarter of approved minerals still have their crystal structures undetermined. The amount of synthetic structurally characterized uranyl carbonates is inferior to natural phases but can give an idea of the crystallization conditions present in the environment.

    Current work is devoted to reviewing the topological diversity and growth conditions of natural uranyl carbonates and their synthetic analogs. Information-based complexity measures have been performed to determine contributions of various substructural building blocks and particular topological types into the complexity of the whole structure, which is related to the stability of a crystalline compound.

    2. Synthetic Uranyl Carbonates

    First of all, it should be noted that the number of synthetic phases for which the structures were determined is significantly inferior to the structurally characterized natural uranyl carbonates in the ratio of 19:32. Whereas for other groups of U(VI)-bearing compounds this proportion is usually opposite [8][10][11]. The first structurally characterized synthetic uranyl carbonate, to our knowledge, was one of the simplest phases 4a, sodium-bearing Na4(UO2)(CO3)3 [12]. It is of interest that the first crystal structure of the natural uranyl carbonate, rutherfordine (41), was reported the year before [13]. The papers of K. Mereiter from the TU Wien (Austria) should be certainly noted in the first row among the works devoted to the synthesis and structural studies of synthetic uranyl carbonates. Then, the studies carried out by the A.M. Fedoseev and I.A. Charushnikova from the Frumkin Institute of Physical chemistry and Electrochemistry RAS (Russian Federation) and by V.N. Serezhkin from the Samara State University (Russian Federation) should be mentioned. A substantial portion of the synthetic uranyl carbonate compounds was synthesized and studied by P. C. Burns and his colleagues from the University of Notre Dame (USA), who significantly contribute to the studies of uranyl carbonate minerals as well.
    All synthetic experiments can be roughly divided into two groups. Moreover, the majority of inorganic uranyl carbonates were synthesized by evaporation at room temperature and only a few of them were obtained from hydrothermal conditions. Uranyl nitrate hexahydrate was usually used as the source of U. But in some experiments, more specific reagents were used: UO2 powder (for 1 and 2), UO2(CO3) (for 7a), Ag4[UO2(CO3)3] (for 8a), α-UO2MoO4(H2O)2 (for 10), and Na4UO2(CO3)3 (for 21). Potassium, sodium, cesium, or thallium carbonates were used as the source of CO3 ions within the synthetic experiments. Compounds 36 and 10 can be considered as exceptions due to the usage of guanidine carbonate and carbamide in the respective syntheses. Several protocols of synthetic experiments deserve special attention. Thus, compound 1 [14] was formed as the result of the dissolution of UO2 powder in the solution of K2CO3 and H2O2 at room temperature. Later it was filtered through a 0.45 µm polyamide syringe filter, and an additional 1.5 mL of 35 wt% H2O2 was added. Afterward it was transferred to a borosilicate scintillation vial and layered with methanol. The compound 2 [15] was obtained similarly, except for the scintillation vial step. The crystals of 4a [16] were obtained by hydrothermal synthesis at 135 °C in a sealed silica glass tube at about 20 MPa. The compound 7a [17] was synthesized by evaporation at room temperature, but before being left to evaporate, the dissolution of the precipitate was achieved via heating the solution over a steam bath. The crystals of 7b [18] were obtained from the solution that was stirred for five days. The compound 11 [19] was synthesized by slow addition of uranyl nitrate solution to the solution of Tl2(CO3)2 at the temperature of c.a. 57 °C. The solvent was removed in a vacuum desiccator for two months from the resulting yellow-green solution, and crystals were then taken from the precipitate. To obtain the crystals of 36 [20], the initial solution was stirred vigorously for several days, afterwards, it was centrifuged and the supernatant was removed via pipet. The crystals were obtained from the precipitate, which was slowly cooled to 5 °C under a CO2 atmosphere.
    Nine of uranyl carbonate minerals have synthetic analog, which was also obtained mostly by evaporation at room temperature, except for the crystals of 7 [21], which were formed during the two months of evaporation at vacuum-desiccator. The compound 4a [22] was synthesized by hydrothermal reaction at 220 °C. The crystals of 41 [23] were obtained by purging the solution of UO3 with 70 kPa CO2 at the glove box for 24 h.
    Very special attention can be paid to compound 4a [22], which may be the same sodium uranyl carbonate that was found among alteration products of the “lavas” resulting from the nuclear accident of the Chernobyl nuclear power plant [6][7].

    3. Topological Analysis

    The majority of uranyl carbonate crystal structures are based on finite clusters, which are represented by only two topological types (Table 1). The topological variety of layered uranyl carbonate complexes is significantly larger; however, the amount of compounds, which structures are based on the 2D units, is much lower. There are only ten uranyl carbonate compounds known with a layered structure, and all of them are natural phases.
    The crystal structures of two synthetic K-bearing uranyl carbonates 1 [14] and 2 [15] differing only in the hydration state and one uranyl carbonate, templated by guanidinium molecules 3 [24], are based on the finite clusters of the cc0-1:2-9 topological type (Figure 1a,b). In terms of polyhedral representation, these clusters can be described as a hexagonal bipyramid that shares two of its equatorial edges with CO3 groups spaced by one non-shared equatorial edge. There is a peroxide molecule arranged in the equatorial plane spaced from a carbonate group by another non-shared edge. Such topological type was also described in the structures of two uranyl nitrate compounds, pure inorganic [25] and organically templated [26], in which the peroxide group was replaced by two H2O molecules. It should be noted that this topological type has an isomer, if two equatorial CO3 groups are trans-arranged, being spaced by two non-shared edges, in contrast to the cis-arrangement in the structures of 13. The trans-isomer is one of the most common types of uranyl nitrate finite clusters [27], while it has not been observed in the structures of uranyl carbonates at all.
    Figure 1. Finite clusters in the crystal structures of natural and synthetic uranyl carbonates and their graphical representations (see Table 1 and text for details). Legend: see Figure 1; peroxide molecule is indicated by red bond (a); see Section 3.3 for details.
    Uranyl tricarbonate cluster (UTC), which is shown in Figure 1c, is the most common structural unit among the natural and synthetic uranyl carbonate phases. There are 39 compounds known (Table 1), whose structures are based on these finite clusters, and is in sum 2.5 times more than the amount of all other structurally characterized uranyl carbonates (15). The topology of UTC belongs to the cc0-1:3-2 type (Figure 1d). This topology can be obtained from the previous cis-cc0-1:2-9 type by the replacement of the peroxide molecule by the third CO3 group, resulting in the formation of a triangular cluster with the uranyl hexagonal bipyramid arranged in its core and ideal -6m2 point group symmetry.
    Table 1. Crystallographic characteristics of natural and synthetic uranyl carbonates.
    No. Chemical Formula Mineral Name Sp.Gr. a, Å/
    α, °
    b, Å/
    β, °
    c, Å/
    γ, °
    Ref.
    Finite Clusters
    cc0-1:2-9
    1 K4[(UO2(CO3)2(O2)](H2O)   P21/n 6.9670(14)/90 9.2158(18)/91.511(3) 18.052(4)/90 [14]
    2 K4[(UO2(CO3)2(O2)](H2O)2.5   P21/n 6.9077(14)/90 9.2332(18)/91.310(4) 21.809(4)/90 [15]
    3 (CN3H6)4[UO2(CO3)2(O2)]·H2O   Pca21 15.883(1)/90 8.788(2)/90 16.155(1)/90 [24]
    cc0-1:3-2
    4 Na4(UO2)(CO3)3 Čejkaite Cc 9.2919(8)/90 16.0991(11)/91.404(5) 6.4436(3)/90 [28]
    4a Na4(UO2)(CO3)3   P-3c 9.3417/90 9.3417/90 12.824/120 [22][16][12]
    4b Na4(UO2)(CO3)3 Cejkaite
    old model
    P-1 9.291(2)/90.73(2) 9.292(2)/90.82(2) 12.895(2)/120.00(1) [29]
    5 K3Na(UO2)(CO3)3   P-62c 9.29(2)/90 9.29(2)/90 8.26(2)/120 [30]
    6 K3Na(UO2)(CO3)3(H2O) Grimselite P-62c 9.2507(1)/90 9.2507(1)/90 8.1788(1)/120 [23][31][32][33]
    6a Rb6Na2((UO2)(CO3)3)2(H2O) Rb analogue of
    Grimselite
    P-62c 9.4316(7)/90 9.4316(7)/90 8.3595(8)/120 [34]
    7 K4(UO2)(CO3)3 Agricolaite C2/c 10.2380(2)/90 9.1930(2)/95.108(2) 12.2110(3)/90 [35]
    7a K4UO2(CO3)3   C2/c 10.247(3)/90 9.202(2)/95.11(2) 12.226(3)/90 [21]
    7b K4(UO2)(CO3)3   C2/c 10.240(7)/90 9.198(4)/95.12(4) 12.222(12)/90 [17]
    58 Rb4(UO2)(CO3)3   C2/c 10.778(5)/90 9.381(2)/94.42(3) 12.509(3)/90 [18]
    8 Cs4UO2(CO3)3(H2O)6   P21/n 11.1764(4)/90 9.5703(4)/ 96.451(2) 18.5756(7)/90 [36]
    8a Cs4(UO2(CO3)3)(H2O)6   P21/n 18.723(3)/90 9.647(2)/96.84(1) 11.297(2)/90 [37]
    9 Cs4(UO2(CO3)3)   C2/c 11.5131(9)/90 9.6037(8)/93.767(2) 12.9177(10)/90 [38]
    10 (NH4)4(UO2(CO3)3)   C2/c 10.679(4)/90 9.373(2)/96.43(2) 12.850(3)/90 [39]
    11 Tl4((UO2)(CO3)3)   C2/c 10.684(2)/90 9.309(2)/94.95(2) 12.726(3)/90 [19]
    12 Mg2(UO2)(CO3)3(H2O)18 Bayleyite P21/a 26.560(3)/90 15.256(2)/92.90(1) 6.505(1)/90 [40][41]
    13 CaMg(UO2)(CO3)3(H2O)12 Swartzite P21/m 11.080(2)/90 14.634(2)/99.43(1) 6.439(1)/90 [40][42][43]
    14 Ca2(UO2)(CO3)3(H2O)11 Liebigite Bba2 16.699(3)/90 17.557(3)/90 13.697(3)/90 [40][44][45][46]
    15 Ca9(UO2)4(CO3)13·28H2O Markeyite Pmmn 17.9688(13) 18.4705(6) 10.1136(4) [47]
    16 Ca8(UO2)4(CO3)12·21H2O Pseudomarkeyite P21/m 17.531(3) 18.555(3) 9.130(3)/103.95(3) [48]
    17 Sr2UO2(CO3)3)(H2O)8   P21/c 11.379(2)/90 11.446(2)/93.40 (1) 25.653(4)/90 [49]
    18 Na6Mg(UO2)2(CO3)6·6H2O Leószilárdite C2/m 11.6093(21)/90 6.7843(13)/91.378(3) 15.1058(28)/90 [50]
    19 Na2Ca(UO2)(CO3)3(H2O)5.3 Andersonite R-3m 17.8448(4)/90 17.8448(4)/90 23.6688(6)/120 [23][40][44][51][52][53][54][55][56][57][58][59][60][61]
    20 Na2Ca8(UO2)4(CO3)13·27H2O Natromarkeyite Pmmn 17.8820(13) 18.3030(4) 10.2249(3) [48]
    21 Ca3Na1.5(H3O)0.5(UO2(C O3)3)2(H2O)8   Pnnm 18.150(3)/90 16.866(6)/90 18.436(3)/90 [62]
    22 K2Ca(UO2)(CO3)3·6H2O Braunerite P21/c 17.6725(12)/90 11.6065(5)/101.780(8) 29.673(3)/90 [63]
    23 K2Ca3[(UO2)(CO3)3]2·8(H2O) Linekite Pnnm 17.0069(5)/90 18.0273(5)/90 18.3374(5)/90 [64]
    23a K2Ca3((UO2(CO3)3)2(H2O)6   Pnnm 17.015(2)/90 18.048(2)/90 18.394(2)/90 [65]
    24 SrMg(UO2)(CO3)3(H2O)12 Swartzite-(Sr) P21/m 11.216(2)/90 14.739(2)/99.48(1) 6.484(1)/90 [40][42]
    25 Na0.79Sr1.40 Mg0.17
    (UO2(C O3)3)(H2O)4.66
      Pa-3 20.290(3)/90 20.290(3)/90 20.290(3)/90 [66]
    26 MgCa5Cu2(UO2)4(CO3)12
    (H2O)33
    Paddlewheelite Pc 22.052(4)/90 17.118(3)/90.474 (2) 19.354(3)/90 [67]
    27 Na8[(UO2)(CO3)3](SO4)2·3H2O Ježekite P-62m 9.0664(11)/90 9.0664(11)/90 6.9110(6)/120 [68]
    28 NaCa3(UO2)(CO3)3 (SO4)F(H2O)10 Schröckingerite P-1 9.634(1)/91.41(1) 9.635(1)/92.33(1) 14.391(2)/120.26(1) [40][69][70][71][72]
    29 MgCa4F2[UO2(CO3)3]2
    (H2O)17.29
    Albrechtschraufite P-1 13.569(2)/115.82(1) 13.419(2)/107.61(1) 11.622(2)/92.84(1) [73][74]
    30 Ca5(UO2(CO3)3)2(NO3)2
    (H2O)10
      P21/n 6.5729(9)/90 16.517(2)/90.494(3) 15.195(2)/90 [75]
    31 Ca6(UO2(CO3)3)2Cl4(H2O)19   P4/mbm 16.744(2)/90 16.744(2)/90 8.136(1)/90 [75]
    32 Ca12(UO2(CO3)3)4Cl8(H2O)47   Fd-3 27.489(3)/90 27.489(3)/90 27.489(3)/90 [75]
    33 Nd2Ca[(UO2)(CO3)3](CO3)2
    (H2O)10.5
    Shabaite-(Nd) P-1 8.3835(5)/90.058(3) 9.2766(12)/89.945(4) 31.7519(3)/90.331(4) [76][77]
    34 [C(NH2)3]4[UO2(CO3)3]   R3 12.3278(1)/90 12.3278(1)/90 11.4457(2)/120 [78]
    35 (C4H12N)4[UO2(CO3)3]·8H2O   P21/n 10.5377(18)/90 12.358(2)/99.343(4) 28.533(5)/90 [79]
    cc0-1:2-10
    36 [C(NH2)3]6[(UO2)3(CO3)6]
    (H2O)6.5
      P-1 6.941(2)/95.63(2) 14.488(2)/98.47(2) 22.374 (2)/101.88(2) [20]
    Nanoclusters
    37 Mg8Ca8(UO2)24(CO3)30O4
    (OH)12(H2O)138
    Ewingite I41/acd 35.142(2)/90 35.142(2)/90 47.974(3)/90 [80]
    Layers
    544234 (β-U3O8)
    38 CaU(UO2)2(CO3)O4(OH)
    (H2O)7
    Wyartite P212121 11.2706(8)/90 7.1055(5)/90 20.807(1)/90 [81][82][83]
    38a Ca(U(UO2)2(CO3)0.7O4(OH)1.6) (H2O)1.63 Wyartite
    dehydrated
    Pmcn 11.2610(6)/90 7.0870(4)/90 16.8359(10)/90 [84]
    61524232 (phosphuranylite)
    39 Ca(UO2)3(CO3)2O2(H2O)6 Fontanite P21/n 6.968(3)/90 17.276(7)/90.064(6) 15.377(6)/90 [85][86]
    61524236 (roubaultite)
    40 Cu2(UO2)3(CO3)2O2(OH)2
    (H2O)4
    Roubaultite P-1 7.767(3)/92.16(4) 6.924(3)/90.89(4) 7.850(3)/93.48(4) [87][88]
    6132-I (rutherfordine)
    41 (UO2)(CO3) Rutherfordine Imm2 4.840(1)/90 9.273(2)/90 4.298(1)/90 [23][89][13][90][91]
    42 Ca(H2O)3[(UO2)3(CO3)3.6O0.2] Sharpite Cmcm 4.9032(4) 15.6489(11) 22.0414(18) [92]
    42a Ca(UO2)6(CO3)5(OH)4·6H2O Sharpite Orth 21.99(2) 15.63(2) 4.487(4) [93][94]
    6132-II (widenmannite)
    43 Pb2[(UO2)(CO3)2] Widenmannite Pmmn 4.9350(7)/90 9.550(4)/90 8.871(1)/90 [31][95][96][97]
    Layers of Miscellaneous Topology
    44 Y2(UO2)4(CO3)3O4·14H2O Kamotoite-(Y) P21/n 12.3525(5) 12.9432 (5)/99.857(3) 19.4409(7) [98][99]
    45 [(Y4.22Nd3.78)(H2O)25(UO2)16O8
    (OH)8(CO3)16](H2O)14
    Bijvoetite-(Y) B21 21.234(3)/90 12.958(2)/90.00(2) 44.911(7)/90 [100][101]
    46 Ca(UO2)(CO3)2·5H2O Meyrowitzite P21/n 12.376(3) 16.0867(14)/107.679(13) 20.1340(17) [102]
    Minerals with Undefined Structures
    47 Cu2(Ce,Nd,La)2(UO2)(CO3)5
    (OH)2·1.5H2O
    Astrocyanite-(Ce) Hex 14.96(2)/90 14.96(2)/90 26.86(4)/120 [103]
    48 (UO2)(CO3)·H2O Blatonite Hex or Trig 15.79(1)/90 15.79(1)/90 23.93(3)/120 [104]
    49 (UO2)(CO3)·nH2O Joliotite Orth 8.16 10.35 6.32 [31]
    50 CaGd2(UO2)24(CO3)8Si4O28·
    60H2O
    Lepersonnite-(Gd) Pnnm or Pnn2 16.23(3)/90 38.74(9)/90 11.73(3)/90 [100]
    51 Ca(UO2)(CO3)2·3H2O Metazellerite Pbn21 or Pbnm 9.718(5) 18.226(9) 4.965(4) [105]
    52 (UO2)2(CO3)(OH)2·4H2O Oswaldpeetersite P21/c 4.1425(6) 14.098(3)/103.62(1) 18.374(5) [106]
    53 Ca3Mg3(UO2)2(CO3)6(OH)4·
    18H2O
    Rabbitite Mon 32.6(1) 23.8(1)/~90 9.45(5) [107]
    54 Ca(UO2)3(CO3)(OH)6·3H2O Urancalcarite Pbnm or Pbn21, 15.42(3) 16.08(4) 6.970(6) [108]
    55 Ca2Cu(UO2)(CO3)4·6H2O Voglite P21 or P21/m 25.97 24.50/104.0 10.70 [109][110]
    56 Ca(UO2)(CO3)2·5H2O Zellerite Pmn21 or Pmnm 11.220(15) 19.252(16) 4.933(16) [105][111]
    57 CaZn11(UO2)(CO3)3(OH)20·
    4H2O
    Znucalite Orth 10.72(1) 25.16(1) 6.325(4) [112]
    57a CaZn12(UO2)(CO3)3(OH)22·
    4H2O
    Znucalite Tricl 12.692(4)/89.08(2) 25.096(6)/91.79(2) 11.685(3)/90.37(3) [113]
    Compound 36 [20], to our knowledge, is the only compound whose structure is based on the triuranyl hexacarbonate finite cluster (Figure 1e). The cluster is built by three vertex-sharing in a cyclic manner uranyl hexagonal bipyramids. Each cavity at the exterior side of such a cycle is occupied by a CO3 group to form a large triangle, each side of which is built by alternating two bipyramids and three carbonate groups. The topology of the uranyl carbonate cluster (Figure 1f) in the structure of 36 belongs to the cc0-1:2-10 type. The architecture of this cluster can be also described as trebling of the UTC cluster with keeping triangular motif and ideal -6m2 point group symmetry.
    Probably the most remarkable structure not only among the uranyl carbonate compounds but among all known minerals, was described in ewingite (37) [80]. Ewingite is a calcium-magnesium oxo-hydroxy-hydrate uranyl carbonate natural phase, whose structure is built by 24 uranyl pentagonal and hexagonal bipyramids interlinked with each other and CO3 groups, to form nanoclusters 2.3 nm in diameter (Figure 1g). Three fundamental building units can be distinguished within the uranyl carbonate cluster in ewingite. These are 4 trimers of edge-sharing pentagonal bipyramids, 6 cis-isomer of the uranyl bicarbonate unit (cc0-1:2-9), and 6 UTC complex (Figure 1h–j). Moreover, linkage of all building units occurs only through the carbonate groups. Mg and Ca ions as well as H2O molecules are arranged both inside and in between the U-bearing nanoclusters.
    The crystal structures of wyartite (38) [81][82][83][84] and its dehydrated form are based on the similar layered complexes that belong to the so-called β-U3O8-sheet anion topology (Figure 3a,b). Topology has the 544234 ring symbol and consists of infinite chains of edge-sharing pentagons that are linked with the neighbor chains through the common vertices and separated by the chains of squares and triangles. The crystal structures of 38 and 38a are remarkable for being the only U(V)-bearing natural phases. Pentagons are occupied by the U6+ ions, squares correspond to the irregular U5+O7 polyhedra, whereas triangles are vacant. Carbonate groups share edges with the U5+-centered polyhedra and are arranged towards the interlayer space.
    Figure 2. 2D complexes in the crystal structures of natural and synthetic uranyl carbonates and their anion topologies or graphical representation (see Table 1 and text for details).
    The crystal structure of fontanite (39) [85][86] is based on the layered uranyl carbonate complexes, which correspond to the, so-called, phosphuranylite anion topology [114] (Figure 2c,d) with the 61524232 ring symbol. The topology consists of two types of alternating infinite chains. The first type of chains is formed by edge-sharing dimers of pentagons that are interlinked by edge-sharing hexagons. The second type of chain is formed by alternating edge-sharing triangles and squares. All hexagons and pentagons are occupied by the uranyl ions, all triangles are occupied by carbonate groups, while all squares are vacant. It should be noted that phosphuranylite anion topology is very common among the U-bearing natural and synthetic phases and is represented by a wide variety of isomers, which differ in the occupancy of polygons. Thus, hexagons may be vacant, and triangles may be occupied by tetrahedral, trigonal pyramidal, or planar trigonal oxyanions (e.g., [8][10]).
    Another anion topology that consists of hexagons, pentagons, squares and triangles with the 61524236 ring symbol (Figure 2e,f) was described in the structure of roubaultite (40) [87][88]. Roubaultite anion topology contains the same infinite chains of edge-sharing pentagon dimers linked by edge-sharing hexagons that were observed in the phosphuranylite topology. But in the structure of 40, these chains are separated by infinite chains of edge-sharing squares decorated with trimers of edge-sharing triangles on the sides. All hexagons and pentagons are also occupied by the uranyl ions, as I was realized in the phosphuranylite topology. Squares are occupied by Cu-centered slightly distorted octahedra. The middle triangle from each trimer is occupied by the carbonate group, leaving the other two triangles vacant.
    The simplest uranyl carbonate, at least according to the chemical composition, rutherfordine (UO2)(CO3) [23][89][13][90][91], has a layered structure. The anion topology is also rather simple; it consists of parallel chains of edge-sharing hexagons separated by hourglass dimers of edge-sharing triangles (Figure 3g,h). All hexagons are occupied by the uranyl ions, and one triangle from each dimer is occupied by the CO3 group, keeping the second triangle vacant. The crystal structure of sharpite (42) is also based on the layered complexes that belong to the same rutherfordine anion topology. However, the polyhedral representation appeared to be much more complex. Thus, the layer can be described as being formed by the 1D modules of rutherfordine topology. Each module represents a triple band of edge-sharing uranyl hexagonal bipyramids, a part of the triangular spaces between which are occupied by carbonate groups. These modules are arranged in the structure at approximate right angles to each other and are linked by the Ca-centered polyhedra, which are arranged on the crests of sawtooth waves (Figure 3a,b). Despite the curvature of such zigzag layers (in contrast to flat layered structure in rutherfordine) and the arrangement of Ca ions in the centers of square antiprisms, projection of such corrugated layers onto the (010) plane corresponds to the rutherfordine topology, with the equatorial arrangement of Ca polyhedra ligands having hexagon shape.
    Figure 3. The crystal structures of sharpite (a,b) and meyrowitzite (c) with simplified topological representation (d). Legend: see Figure 1; Ca polyhedra = light blue.
    The crystal structure of widenmannite (43) [31][95][96][97] is based on layered complexes, the topology of which consists of hexagons and triangles with the same 6132 ring symbol as was found in the structure of rutherfordine. However, the arrangement of polygons in both structures is different. Widenmannite anion topology is built by the hexagons linked by vertex-sharing to other six hexagons, while all of its six edges are shared with triangles, thus forming trihexagonal tiling, which was used by Johannes Kepler in his book [115] and is also known under the kagome pattern name. In the ideal structure of the widenmannite (Figure 2i,j) each second row of hexagonal bipyramids should be vacant, but in the real structure the disorder with partial occupancy of the U sites takes place, which results in the occupation of all hexagons by the uranyl ions and half of the triangles oriented in the same direction are occupied by carbonate groups, keeping another half vacant.
    The crystal structures of two REE-bearing minerals kamotoite-(Y) (44) [98][99] and bijvoetite-(Y) (45) [100][101] are based on highly remarkable and very rare layered complexes that have not been observed in any other natural or synthetic compound. The topology of the 2D complex is based on infinite chains of alternating dimers of edge-sharing uranyl hexagonal and pentagonal bipyramids (Figure 2k,l). Dimers of pentagonal bipyramids are arranged along the chain’s extension, while dimers of hexagonal bipyramids are arranged perpendicularly. Each hexagonal bipyramid shares two of its oblique equatorial edges, not taking part in the linkage between U polyhedra, with CO3 groups. These chains are linked into the 2D structure via irregular Y3+- or Nd3+-centered coordination polyhedra through the 6th non-shared equatorial edge from one chain and two O atoms of two carbonate groups from the neighbor chain. It should be noted that the resulting U- and REE-bearing layers are electroneutral, so the 3D structure formation is provided by the H-bonding system, which involves H2O molecules from the coordination sphere of REE atoms and from the interlayer space.
    The final to date topological type, which has been described in the structures of natural and synthetic uranyl carbonates, is observed in the structure of calcium uranyl carbonate mineral meyrowitzite (46) [102]. The structure of the layered complex is composed of UTC clusters sharing apical vertices of CO3 groups with uranyl pentagonal bipyramids (Figure 3c). This structural type, unlike the rest of the topologies described herein, is the least dense in terms of the interconnection of U coordination polyhedra, and its topology will become clearer if graphical representation is used for illustration (Figure 3d). Thus, if uranyl pentagonal bipyramid is represented by grey nods, U hexagonal bipyramids by white nods, and the line between nods appearing if pentagonal bipyramid shares an equatorial O atom with the CO3 group from the UTC cluster, the resulting graph of the complex will correspond to one of the most common cc1-1:2-4 topological type among the U(VI) bearing structures in general (e.g., [8][10]). Since the concept of graphical representation is violated, this description is more appropriate to use not as a direct interpretation of the topology, but as an approximate model.

    4. Structural and Topological Complexity

    Structural complexity measures have been implemented in several stages and the results of calculations are listed in Figure 4 and Figure 5 and Table 2. At the first stage, the topological complexity (Tl), according to the maximal point (for finite clusters) or layer symmetry group has been determined, as these are the basic structure building units. At the second stage, the structural complexity (Sl) of the U-bearing complexes has been calculated taking into account its real symmetry. The next informational contribution comes from the stacking (LS) of finite clusters and layered complexes (if more than one complex is within the unit cell). The fourth contribution to the total structural complexity is derived by the interstitial structure (IS). The last portion of information comes from the interstitial H bonding system (H). It should be noted that the H atoms related to the U-bearing clusters and layers were considered as a part of those complexes, but not within the contribution of the H-bonding system. Complexity parameters for the whole structure have been determined using ToposPro software [116].
    Figure 4. Ladder diagrams showing contributions to structural complexity (bits per unit cell) (a) and normalized contributions (in %) (b) for the structures based on uranyl carbonate finite clusters. Legend: TI = topological information; SI = structural information; LS = layer stacking; IS = interstitial structure; HB = hydrogen bonding. 
    Figure 5. Ladder diagrams showing contributions to structural complexity (bits per unit cell) (a) and normalized contributions (in %) (b) for the structures based on uranyl carbonate layers. Legend: see Figure 4.

     

    The entry is from 10.3390/cryst11060704

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