1839

The Canadian Mineralogist Vol. 43, pp. 1839-1894 (2005)

U6+ MINERALS AND INORGANIC COMPOUNDS: INSIGHTS INTO AN EXPANDED STRUCTURAL HIERARCHY OF CRYSTAL STRUCTURES

PETER C. BURNS§ Department of Civil Engineering and Geological Sciences, 156 Fitzpatrick Hall, University of Notre Dame, Notre Dame, Indiana 46556, U.S.A., and Chemistry Division, Argonne National Laboratory, 9700 South Cass, Argonne, Illinois 60439, U.S.A.

ABSTRACT

The crystal structures of uranyl minerals and inorganic uranyl compounds are important for understanding the genesis of U deposits, the interaction of U mine and mill tailings with the environment, transport of actinides in soils and the vadose zone, the performance of geological repositories for nuclear waste, and for the development of advanced materials with novel applications. Over the past decade, the number of inorganic uranyl compounds (including minerals) with known structures has more than doubled, and reconsideration of the structural hierarchy of uranyl compounds is warranted. Here, 368 inorganic crystal structures that contain essential U6+ are considered (of which 89 are minerals). They are arranged on the basis of the topological details of their structural units, which are formed by the polymerization of polyhedra containing higher-valence cations. Overarching structural categories correspond to those based upon isolated polyhedra (8), fi nite clusters (43), chains (57), sheets (204), and frameworks (56) of polyhedra. Within these categories, structures are organized and compared upon the basis of either their graphical representations, or in the case of sheets involving sharing of edges of polyhedra, upon the topological arrangement of anions within the sheets.

Keywords: uranium, actinide, crystal structure, structural hierarchy, nuclear waste.

SOMMAIRE

Les structures cristallines des minéraux et des composés inorganiques uranylés revêtent une grande importance pour bien comprendre la genèse des gisements d’uranium, l’interaction de haldes et des déchêts de mines avec l’environnement, le transfert des actinides dans les sols et la zone vadose, et l’effi cacité des sites d’enfouissement des déchêts nucléaires, et pour développer de matériaux nouveaux ayant des applications de haute technologie. Au cours de la dernière décennie, le nombre de composés inorganiques contenant le groupe uranyle (y inclus les minéraux) dont la structure est connue a plus que doublé, et une reconsi- dération de la hiérarchie structurale des composés uranylés est appropriée. Ici, 368 structures de cristaux inorganiques contenant le U6+ comme composant essentiel (dont 89 sont des minéraux) sont passées en revue. Elles sont regroupées sur la base de détails topologiques de leurs unités structurales, formées par la polymérisation de polyèdres contenant des cations de valence élevée. Les catégories structurales générales correspondent à celles fondées sur les polyèdres isolés (8), regroupements fi nis (43), chaînes (57), feuillets (204), et trames (56) de polyèdres. A l’intérieur de ces catégories, les structures sont organisées et comparées en fonction de soit leurs représentations graphiques, ou bien dans le cas de feuillets impliquant le partage d’arêtes de polyèdres, en fonction de l’agencement topologique des anions dans ces feuillets.

(Traduit par la Rédaction)

Mots-clés: uranium, actinide, structure cristalline, hiérarchie structurale, déchêts nucléaires.

§ E-mail address: [email protected] 1840 THE CANADIAN MINERALOGIST

INTRODUCTION primary minerals in some occurrences. Owing to their structural and chemical complexities, and the experi- Crystals containing U6+ have been the focus of mental diffi culties associated with their characteriza- extensive research over several decades. The fi rst crystal tion, understanding of uranyl minerals as a whole lags structures of inorganic U6+ (uranyl) compounds appeared behind the current level of understanding of most major in the literature several decades ago (e.g., Evans 1963), mineral groups. The structures are fully characterized and by 1996, about 180 structures were known (Burns for less than 50% of described uranyl minerals, and et al. 1996). The introduction of CCD-based detectors many aspects of their chemical composition, stability of X-rays to mineralogy (Burns 1998a) greatly extended and occurrence still require study. the reach of single-crystal X-ray diffraction into the Burns et al. (1996) presented a structural hierarchy study of complex U6+ compounds. Today, at least 368 of 180 inorganic synthetic and natural uranyl phases. structures of inorganic uranyl compounds are known, Burns (1999a) expanded the hierarchy in the case of which 89 correspond to minerals. These materials of minerals only. It is based upon the polymeriza- exhibit fascinating and complex atomic arrangements, tion of those polyhedra that contain higher-valence owing to the myriad of possible types of connections cations. Low-valence cations, including H atoms and between uranyl polyhedra and other uranyl polyhedra, H bonds, were ignored for the purpose of erecting the as well as polyhedra containing other higher-valence hierarchy. cations. Current research concerning uranyl compounds Here I provide a revised and greatly expanded hier- is driven by the search for novel solids with important archy of 368 structures of inorganic uranyl compounds materials properties (e.g., Locock & Burns 2002a, (including 89 minerals). The volume of known struc- Krivovichev et al. 2002a, Krivovichev & Burns 2002a, tures mandates a concise description of structures. Almond et al. 2002a, Burns et al. 2005, Krivovichev et Illustrations of all of the structural units are provided, al. 2005a,b), and their importance in the environment and formulae and crystallographic parameters are given (e.g., Li & Burns 2001a, Burns et al. 2000, 2004a, in the tables for each of the 368 structures. I hope that Hughes Kubatko et al. 2003). Knowledge of the struc- the reader will gain an appreciation of the complex tures of uranyl minerals is important for understanding beauty of this chemical class of compounds, and that the genesis of U deposits (Frondel 1958), and uranyl the structural hierarchy will provide a basis for further minerals exhibit substantial structural and chemical studies in actinide crystal chemistry. diversity, refl ecting geochemical conditions dominant I am delighted that this contribution appears in the during their formation. 50th Anniversary edition of The Canadian Mineralogist, Uranyl minerals and compounds are of consider- not only because it is one of the premier mineralogical able environmental importance. They are signifi cant journals in the world, but also because of the tremendous for understanding water–rock interactions in U-rich impact that the Mineralogical Association of Canada has rocks (Frondel 1958). Uranyl minerals exert an impact had on my continuing education and career. on the mobility of actinides in contaminated soils (e.g., Buck et al. 1996, Roh et al. 2000) and in vadose-zone COORDINATION POLYHEDRA sediments polluted with actinides, such as the Hanford and Savanna River sites in the U.S.A. (e.g., Yamakawa Almost all U6+ in crystal structures is present as 2+ & Traina 2001). Precipitation of uranyl phosphate an approximately linear UO2 uranyl ion. The formal minerals in the vadose zone of contaminated sites by the valence of the uranyl ion is 2+, so it must be coor- addition of phosphate has been proposed as a means to dinated by anions in a stable crystal structure. The mitigate U6+ plumes in groundwater (Fuller et al. 2002). uranyl ion is typically coordinated by four, fi ve or six Uranyl minerals can be bio-precipitated (Macaskie et ligands, arranged at the equatorial vertices of square, al. 2000), and potentially provide sources of phosphate pentagonal and hexagonal bipyramids, respectively for biological consumption, as well as redox-active U (Fig. 1). The bipyramids are capped by the O atoms of to serve as an electron acceptor needed for metabolism the uranyl ions. of bacteria. Uranyl minerals are important products of Burns et al. (1997a) provided U6+–O bond-length alteration of nuclear waste forms under simulated condi- and uranyl ion bond-angle data for ~100 crystal struc- tions of geological repositories, such as those expected tures. Only well-refi ned structures analyzed by single- in the proposed repository at Yucca Mountain, Nevada crystal X-ray diffraction, with fi nal R indices less than (e.g., Finch et al. 1999a, Finn et al. 1996, Wronkiewicz 7%, were included. The bond and angle distributions et al. 1996). for uranyl polyhedra have been revised to incorporate About 200 uranyl minerals have been described data for structures reported since 1997, using the same from nature. They usually occur in the oxidized zones criteria adopted by Burns et al. (1997a). A total of 222 of U deposits, where it is common for several uranyl structures have been included. The U6+– (: O, OH, [6] 6+ [7] 6+ mineral species to coexist, commonly in intimate inter- H2O) bond-length distributions for U , U and growths. In many cases, uranyl minerals are secondary [8]U6+ are provided in Figure 2. and result from the alteration of UO2+x, but they are HIERARCHY OF U6+ MINERALS AND INORGANIC COMPOUNDS 1841

Consider fi rst the case of [7]U6+ (U6+ coordinated by In some cases, the solid state and solution chemistry seven atoms, including the uranyl ion); the distribution of actinyl ions involves signifi cant cation–cation inter- of U6+–O bond lengths in 270 polyhedra containing actions in which an O atom of an actinyl ion is also an [7]U6+ in 143 structures is presented in Figure 2b. The equatorial ligand of a neighboring polyhedron (Krot & distribution is bimodal, as each polyhedron contains a Grigoriev 2004). The cation–cation designation arises 6+ uranyl ion. The average U –OUr (Ur: uranyl) bond- from the formal valences of the actinyl ions, both of length is 1.793 Å, with a standard deviation of 0.035 which are cations. Such interactions are relatively 6+ 5+ Å, whereas for the U –Oeq bonds, the average is 2.368 common for Np , but are rather unusual in the case Å with a standard deviation of 0.100 Å. The distribu- of U6+ (Sullens et al. 2004), presumably owing to the 6+ 6+ tions of both the U –OUr and U –Oeq bonds are rather fact that the bonding requirements of the O atoms of regular. the uranyl ions are nearly met by the bonds within the The bond-length distribution for [8]U6+ is shown in uranyl ion alone. To date, there are fi ve inorganic uranyl Figure 2a. Although the sample size is smaller than that compounds that exhibit cation–cation interactions of [7]U6+, it is still apparent that all [8]U6+ polyhedra in (1.4% of known structures), and each of these structures well-refi ned structures contain a uranyl ion, and the is composed of a framework of polyhedra. 6+ average U –OUr bond length for 35 polyhedra in 32 structures is 1.783 Å, with a standard deviation of 0.030 STRUCTURAL HIERARCHY OF URANYL PHASES 6+ Å. The distribution for the U –Oeq bonds is rather irreg- ular, with an average of 2.460 Å and a standard devia- Structural hierarchies are a means to organize a [8] tion of 0.107 Å. There are few U–Oeq bonds less than wealth of complex and strikingly different structures ~2.4 Å, causing the asymmetric distribution of bond into a cohesive framework. They provide consider- lengths apparent in Figure 2a. This feature presumably able insight into the crystal chemistry of the focus is related to the geometrical limitations of packing six compounds, as the hierarchical arrangement is usually coplanar O atoms around a single uranyl ion. based upon structural connectivity. Structural hierar- The bond-length distribution for [6]U6+ is shown chies permit recognition of structural trends within in Figure 2c. As discussed in detail by Burns et al. large groups of compounds that are generally obscured (1997a), not every [6]U6+ polyhedron in well-refi ned by the complexity of the class of compounds. They can structures has an associated uranyl ion. Several reported be the basis for understanding the linkages between structures contain [6]U6+ in octahedral or distorted mineral occurrences and underlying mineral structures, octahedral coordination, thus there are three modes in especially in low-temperature geochemical environ- the bond-length distribution for [6]U6+ that correspond ments that present complex assemblages of hydrous 6+ 6+ to U –OUr and U –Oeq bond lengths of uranyl square minerals. However, consideration of minerals alone bipyramids, and the more regular bond-lengths of octa- in the development of a structural hierarchy is akin to hedral coordination (centered at ~2.1 Å). Considering viewing a ballet with tunnel vision, as minerals repre- only the 54 uranyl square bipyramids in 47 well-refi ned sent only the subset of a chemical class that is stable 6+ structures, the average U –OUr bond-length is 1.816 over geological times under geochemical conditions. As Å, with a corresponding standard deviation of 0.050 such, the structural hierarchy presented herein includes 6+ 6+ Å, and the U –Oeq bond-length is 2.264 Å, standard all inorganic compounds that contain U as a necessary deviation 0.064 Å. structural constituent. Using the bond-valence parameters provided by Following the approach of Burns et al. (1996) and Burns et al. (1997a), the typical bond-valences of the Burns (1999a), structures of uranyl compounds are 6+ 6+ U –OUr and U –Oeq bonds of each coordination poly- arranged here on the basis of their structural units [6] 6+ [6] 6+ hedron are: U –OUr = 1.59 valence units (vu), U – consisting of those cation-centered polyhedra of higher [7] 6+ [7] 6+ Oeq = 0.71 vu, U –OUr = 1.64 vu, U –Oeq = 0.53 bond-valence. Structures naturally fall into fi ve catego- [8] 6+ [8] 6+ vu, U –OUr = 1.67 vu, U –Oeq = 0.44 vu (Fig. 3). ries corresponding to isolated polyhedra, fi nite clusters These values are consistent with extensive linkage of of polyhedra, chains of polyhedra, sheets of polyhedra, uranyl polyhedra through equatorial ligands, either with and frameworks of polyhedra. In each case the struc- other uranyl polyhedra or other polyhedra containing tural unit is illustrated, and pertinent crystallographic higher-valence cations. The bond-valences incident information is provided in the Tables. 6+ upon the OUr atoms, due to the bond to U alone, In most cases, assignment of a structure to one of range from 1.59 to 1.67 vu. Thus, the apical (uranyl) the fi ve classes is straightforward. In a few structures, vertices of the uranyl bipyramids are seldom shared sheets of polyhedra are linked together by sharing with polyhedra containing higher-valence cations, as anions, forming a framework, or the sheets are linked this would overbond the O position. The two-dimen- through uranyl ions located in interlayer positions, sional polymerization of uranyl polyhedra mandated by which also results in a framework. Such structures the distribution of bond strengths within the polyhedra usually contain sheets that are similar or identical to the favors the formation of sheets of polyhedra. sheets that occur isolated in other structures; as such, 1842 THE CANADIAN MINERALOGIST

2+ FIG. 1. Uranyl coordination polyhedra in uranyl compounds. Shown are (a) the (UO2) uranyl ion, (b) uranyl square bipyra- mid, (c) uranyl pentagonal bipyramid, (d) uranyl hexagonal bipyramid, (e) uranyl square bipyramid, (f) uranyl pentagonal bipyramid, (g) uranyl hexagonal bipyramid.

they typically will be grouped with those structures ion present. The structure of Cs2[(UO2)Cl4] contains despite their three-dimensional connectivity. UrCl4 square bipyramids, with Cl atoms at the equato- In many cases, the chemical formulae used in the rial positions. The structure of [(UO2)Cl2(H2O)3] has text and tables have been re-arranged from those given a Ur(H2O,Cl)5 polyhedron, with two equatorial Cl by the original authors to emphasize structural features atoms. The structures of [(UO2)(H2O)5](ClO4)2 and and to facilitate comparisons. In Tables that list clusters [(UO2)(H2O)5](ClO4)2•2H2O each contain isolated and chains, structures based upon similar structural Ur(H2O)5 pentagonal bipyramids, as well as isolated units are grouped, and double lines are used to separate perchlorate tetrahedra. those that have different graphical representations. In Table 4 (see below), structures based upon sheets with STRUCTURES CONTAINING FINITE CLUSTERS identical graphical representations are grouped, and OF POLYHEDRA those involving different graphs are separated by double lines. In Tables 5 through 8 (see below), all of which There are 43 compounds based upon fi nite clusters list structures that contain sheets of polyhedra, double of polyhedra of higher bond-valence, which represents lines separate structures with different anion-topologies, a significant growth from the 22 structures in this and dotted lines separate structures with distinct sheets, class reviewed by Burns et al. (1996). Of the 21 new but the same underlying anion-topology. structures reported since 1996, only two correspond to minerals, and only seven of the total in this class are STRUCTURES CONTAINING ISOLATED POLYHEDRA minerals. The structures are arranged in Table 2 on the basis of their graphs; those with identical graphs are Eight structures contain isolated U6+ polyhedra grouped together, regardless of the chemical identities (Table 1). In these structures, there are no direct link- of the constituents of the clusters. ages between the U6+ polyhedra; linkages all take Graphical representations of crystal structures are a place through low-valence cations. In the structures of powerful approach to reducing structural complexity for Ca3[UO6], Sr3[UO6], K2Li4[UO6] and Li6[UO6], the the establishment of underlying relationships amongst U6+ cation is octahedrally coordinated, with no uranyl groups of structures. In the case of uranyl compounds, HIERARCHY OF U6+ MINERALS AND INORGANIC COMPOUNDS 1843

FIG. 2. Bond-length distributions in uranium-bearing polyhedra in well-refi ned structures. (a) Uranyl hexagonal bipyramids, (b) uranyl pentagonal bipyramids, (c) uranyl square bipyramids and other six-coordinated U6+ polyhedra.

FIG. 3. Uranyl polyhedra and the corresponding average bond-valence (in valence units, vu) incident upon each vertex owing to the U6+–O bond within the polyhedron. 1844 THE CANADIAN MINERALOGIST

this approach works best where most of the connec- tricarbonate clusters are linked only through bonds tions between polyhedra containing higher-valence to low-valence cations, including H. A topologically cations occur through the sharing of vertices. The graph identical cluster containing nitrate triangles instead of is obtained from the structure by representing each carbonate is found in the structures of Rb[(UO2)(NO3)3] distinct type of polyhedron with a colored circle, and the and K[(UO2)(NO3)3]. number of vertices shared between adjacent polyhedra A uranyl square bipyramid shares each equato- are shown by connectors between the colored circles. rial vertex with a different tetrahedron in the cluster Throughout this paper, black circles are used to represent with composition [(UO2)(XO4)4] shown in Figure 4b. uranyl polyhedra, and white circles represent tetrahedra This cluster was recently found in three compounds or triangles containing higher-valence cations. Consider (Table 2), with the X cation being Mo6+ in two struc- for example the clusters shown in Figures 4c and 4i. The tures and Cr6+ in the third case. A more complex uranyl graph corresponding to the cluster in Figure 4c has two molybdate cluster composed of two uranyl pentagonal black circles and eight white circles, which corresponds bipyramids and eight MoO4 tetrahedra is known from to the number of uranyl polyhedra and tetrahedra in the Na6[(UO2)(MoO4)4] and Na3Tl3[(UO2)(MoO4)4] (Fig. corresponding cluster. It is also immediately apparent 4c, Table 2). Each equatorial ligand of each uranyl that there are two types of white circles, those that are pentagonal bipyramid is shared with a tetrahedron, and connected to two black circles, and those that are linked two tetrahedra provide a bridge between the uranyl to only one black circle. These white circles correspond pentagonal bipyramids. to the two- and one-connected tetrahedra of the cluster, Remarkable clusters of uranyl pentagonal bipyra- respectively. The graph also indicates that all linkages mids and sulfate tetrahedra occur in Na6[(UO2) between the polyhedra in the cluster arise by the sharing (SO4)4](H2O)2, Na10[(UO2)(SO4)4](SO4)2(H2O)3, of single vertices only (the linkages are indicated by and KNa5[(UO2)(SO4)4](H2O) (Fig. 4d). One sulfate single connectors in the graph). In contrast, the graph tetrahedron shares an edge with the bipyramid, a corresponding to the cluster shown in Figure 4i has connectivity that had not been previously observed in three types of white circles, those that are connected to a hydrous uranyl sulfate. Three additional sulfate tetra- a single black circle by a single connector, those that hedra are linked to the other three equatorial vertices of are connected to two black circles by single connectors, the bipyramid. The sharing of edges between polyhedra and those that are connected to a single black circle containing U6+ and S6+ cations presumably entails by two connectors. The presence of two connectors in signifi cant cation–cation repulsion. However, crystals the cases of two of the white circles indicates that an of these compounds several millimeters in maximum edge is shared between the uranyl polyhedron and the dimension were grown at 70°C in uranyl sulfate solu- corresponding tetrahedron. tions, which implies cluster stability, even in solutions Uranyl carbonate minerals that crystallize from (e.g., Burns & Hayden 2002). alkaline solutions contain the uranyl tricarbonate Six uranyl nitrates contain isolated clusters with 4– cluster, [(UO2)(CO3)3] (Fig. 4a). A uranyl hexagonal uranyl polyhedra and nitrate triangles. The cluster in bipyramid shares three edges with carbonate triangles, Figure 4e has a uranyl hexagonal bipyramid that shares and all equatorial anions of the bipyramids constitute two of its trans equatorial edges with nitrate triangles, ligands of the carbonate groups. This cluster is known and the remaining two vertices of the bipyramid are from seven mineral structures as well as 10 synthetic shared with two different nitrate triangles. This cluster compounds (Table 2). In each case, independent uranyl is the basis of the structure of Rb2[(UO2)(NO3)4], HIERARCHY OF U6+ MINERALS AND INORGANIC COMPOUNDS 1845 1846 THE CANADIAN MINERALOGIST

FIG. 4. Clusters (and their graphical representa- tions) of uranyl polyhedra and other polyhedra containing higher-valence cations that occur in the structures of uranyl compounds. The corre- sponding compounds are listed in Table 2.

which is the only structure known in which nitrate a uranyl bipyramid. This phase was grown at elevated triangles are linked to uranyl polyhedra by sharing temperature, and its stability under ambient conditions only a single vertex; in all other cases, edges are is uncertain. shared between uranyl and nitrate polyhedra. Two Four clusters contain only uranyl bipyramids that clusters that are graphical isomers contain a single share equatorial edges (Figs. 4j, k, l, m). Two are uranyl hexagonal bipyramid that shares two equatorial simple dimers of either square bipyramids or pentagonal edges with nitrate triangles (Figs. 4f, g). The former bipyramids (Figs. 4l, m). One is a trimer of uranyl is found in the structure of [(UO2)(NO3)2](H2O)6, pentagonal bipyramids (Fig. 4k), and one contains four [(UO2)(NO3)2](H2O)2 and [(UO2)(NO3)2](H2O)3, the uranyl pentagonal bipyramids that share edges (Fig. 4j). latter is found in [(UO2)(NO3)2(H2O)2](H2O). In a more The latter two clusters each contain oxygen atoms that complex cluster found in [(UO2)2(OH)2(NO3)2](H2O)4, are common to three uranyl polyhedra. a uranyl hexagonal bipyramid shares an equatorial edge The uranyl borate cluster shown in Figure 4n was with a uranyl pentagonal bipyramid, as well as two synthesized by aging a boron-rich solution at room equatorial edges with nitrate triangles (Fig. 4h). In all temperature (Behm 1985). The cluster contains 16 but Rb2[(UO2)(NO3)4], the uranyl nitrate clusters are borate triangles and tetrahedra that surround a uranyl linked only though H bonds. hexagonal bipyramid; four of the borate polyhedra The structure of the anhydrous compound K4[(UO2) share edges with the uranyl bipyramid, and four addi- (SO4)3] possesses a remarkable cluster (Fig. 4i), in tional borate polyhedra share a single vertex with the which two uranyl pentagonal bipyramids are bridged by bipyramid. sharing vertices with two sulfate tetrahedra, and each The cluster shown in Figure 4o contains a uranyl bipyramid also shares an additional vertex with another pentagonal bipyramid that shares two vertices with sulfate tetrahedron, and an edge with a fourth sulfate perchlorate tetrahedra. The remaining equatorial tetrahedron. This is the only example of an anhydrous vertices of the bipyramid are H2O groups, and the phase in which a sulfate tetrahedron shares an edge with clusters are linked through H bonding only. HIERARCHY OF U6+ MINERALS AND INORGANIC COMPOUNDS 1847

STRUCTURES CONTAINING INFINITE CHAINS The chains of uranyl pentagonal bipyramids and OF POLYHEDRA tetrahedra shown in Figures 5j, 5k, 5l and 5m, which are known from eight compounds (Table 3), are graphical Burns et al. (1996) listed 19 uranyl compounds isomers. Each bipyramid shares equatorial vertices with with structures based upon infi nite chains of polyhedra three tetrahedra, whereas the remaining two equatorial of higher bond-valence, of which fi ve correspond to vertices correspond to H2O groups. Each tetrahedron is minerals. Currently, 57 such structures are known, linked to three bipyramids, and the chains differ in the of which 10 are minerals (Table 3). The remarkable details of the orientation of the tetrahedra. expansion of this structural class is largely attributable Two chain types have been discovered that contain to research focused on understanding linkages between uranyl pentagonal bipyramids, chromate tetrahedra, and uranyl polyhedra and tetrahedra containing Mo6+, S6+ iodate pyramids (Fig. 5n, four structures, Fig. 5o, two or Cr6+, as well as various polyhedra containing cations structures). In these unusually complex chains, pairs of with stereoactive lone-electron pairs such as Se4+ and uranyl pentagonal bipyramids are bridged by sharing I5+, which occur in 16 of the new structures. Thirty- vertices with two chromate tetrahedra, and the resulting nine structures containing uranyl polyhedra and any of tetramers of polyhedra are connected into chains by the lone-pair stereoactive cations Te4+, Se4+ and I5+ are sharing vertices with iodate pyramids. In both chains, known; remarkably, 41% of these contain infi nite chains, the tetrahedra are two-connected, and two vertices of whereas only 16% of all inorganic uranyl compounds each are terminal. In chain 5o, each bipyramid is linked with known structures are based upon chains. Presum- to two tetrahedra and two pyramids. In the case of chain ably, the irregular polyhedra surrounding lone-pair 5n, iodate pyramids also decorate the chains by sharing stereoactive cations, and the existence of the lone pair a single vertex with the uranyl pentagonal bipyramids, of electrons rather than an additional ligand, somewhat resulting in bipyramids that are fi ve-connected within favor polymerization in one dimension only. the chains. Chains of polyhedra in uranyl compounds are The chains shown in Figures 5p (one structure) and arranged in Figure 5 on the basis of their graphs, 5q (two structures) combine uranyl pentagonal bipyra- with chains dominated by the sharing of only vertices mids and molybdate tetrahedra. Pairs of bipyramids between polyhedra provided fi rst, followed by chains share a vertex, resulting in dimers, and two tetrahedra that also involve sharing of polyhedron edges. Crystal- bridge the dimers on two sides by sharing vertices with lographic parameters of corresponding structures are each bipyramid. The resulting tetramers are connected given in Table 3. into chains by sharing vertices with four different The simple chain of uranyl square bipyramids tetrahedra. Thus, each bipyramid is five-connected that share vertices shown in Figure 5a occurs in within the chain, whereas the tetrahedra are each Na4[(UO2)O3], Ca2[(UO2)O3], Sr2[(UO2)O3] and two-connected. Li4[(UO2)O3]. Each of the structures is anhydrous, and Two types of chain contain only uranyl polyhedra linkages between the chains involve lower-valence that share edges. The structures of moctezumite and cations only. [(UO2)Cl2H2O] have simple chains of pentagonal bipyr- The chains illustrated in Figures 5b through 5f are amids that share edges (Fig. 5r), whereas studtite has graphical isomers. The chains contain either uranyl chains of edge-sharing hexagonal bipyramids (Fig. 5s). square bipyramids or pentagonal bipyramids, and each Studtite is one of only two known peroxide minerals, is four-connected within the chain, as illustrated in the and it is the only one with a known structure (Burns & corresponding graph. Bipyramids are linked through Hughes 2003). Each hexagonal bipyramid contains two either tetrahedra or pyramids, each of which is two- peroxide groups that constitute edges of the bipyramid; connected within the chain. Chain 5b contains either both of the O atoms of the peroxide group are bonded arsenate, molybdate, or phosphate tetrahedra. Chains 5c, to the central U6+ cation. The remaining two vertices of 4+ 4+ 5d, and 5f have lone-pair stereoactive Se or Te , as each bipyramid are H2O groups. Polyhedra are linked refl ected by the one-sided pyramidal coordination poly- by the sharing of peroxide groups, for which the corre- hedra about these cations. The structures corresponding sponding edges of the bipyramids are ~1.45 Å. Recent to each chain are listed in Table 3. studies have shown that this unusual mineral probably The chains of uranyl pentagonal bipyramids and grows because of the buildup of peroxide that forms by tetrahedra (or SeO3 pyramids) shown in Figures 5g, 5h alpha-radiolysis of water in nature (Hughes Kubatko and 5i correspond to an identical graph, and are known et al. 2003). from seven compounds (Table 3). Uranyl pentagonal The chains shown in Figures 5t and 5u are based bipyramids are bridged through two tetrahedra by the upon the same graph. That in Figure 5t is from the struc- sharing of vertices, such that four equatorial vertices of tures of parsonsite and hallimondite, and is composed of the bipyramids are also tetrahedral vertices of bridging uranyl pentagonal bipyramids and phosphate or arsenate (two-connected) tetrahedra. The fi fth equatorial vertex tetrahedra. Bipyramids share an edge, forming a dimer, of the bipyramid is shared with a non-bridging tetrahe- and dimers are linked into chains by sharing edges and dron that decorates the side of the chain. vertices with tetrahedra. Additional tetrahedra decorate 1848 THE CANADIAN MINERALOGIST HIERARCHY OF U6+ MINERALS AND INORGANIC COMPOUNDS 1849

the edges of the chains by sharing vertices with the pentagonal bipyramids that are bridged by the square polyhedra. The structures of Ca[(UO2)(SeO3)2] and bipyramids. Additional rigidity of the chain is provided Sr[(UO2)(SeO3)2] contain chains (Fig. 5u) in which by iodate pyramids that bridge adjacent dimers of selenite pyramids assume the role of tetrahedra in the pentagonal bipyramids, and the chain is decorated by parsonsite chain. iodate pyramids that share only a single vertex with The chain shown in Figure 5v is known only from the bipyramids. Sr[(UO2)(SeO3)2](H2O)2, and contains units similar The complex chain shown in Figure 5y is known to those in Ca[(UO2)(SeO3)2] and Sr[(UO2)(SeO3)2]. from three uranyl iodates (Table 3). The bulk of the These chains have the same dimers of edge-sharing chain is composed of uranyl pentagonal bipyramids uranyl pentagonal bipyramids, with two selenite pyra- that share equatorial edges, resulting in a chain that is mids that share edges with the bipyramids, but the two bipyramids wide. Chain rigidity is enhanced by linkages of these units into chains differ. iodate pyramids that bridge between bipyramids, and The backbone of the chain found in UO2(IO3)2 (Fig. additional iodate pyramids decorate the chain by sharing 5w) is composed of uranyl hexagonal bipyramids that single vertices with the bipyramids. share edges, and is thus related to the chains found in The chain found in uranopilite (Fig. 5z) is the most studtite (Fig. 5s). In UO2(IO3)2, the chains also contain complex chain found to date in a uranyl mineral. It iodate pyramids that are attached to either side by is composed of uranyl pentagonal bipyramids that sharing edges with the bipyramids. are linked by sharing equatorial edges to form six- The chain shown in Figure 5x occurs in three uranyl membered clusters. Within these clusters, two O atoms iodates, and is unusual in that it is the only chain that are bonded to three uranyl ions, and all but four of the contains uranyl ions in two different coordinations. equatorial anions that terminate the cluster are H2O The zig-zag chain is formed by the sharing of edges groups. Identical clusters of six bipyramids are linked between uranyl pentagonal bipyramids and square by sharing vertices with sulfate tetrahedra, such that bipyramids, arranged such that there are dimers of each tetrahedron is four-connected. Chains are linked 1850 THE CANADIAN MINERALOGIST HIERARCHY OF U6+ MINERALS AND INORGANIC COMPOUNDS 1851

FIG. 5. Chains (and their graphical representations) of uranyl polyhedra and other polyhedra containing higher-valence cations that occur in the structures of uranyl compounds. The corresponding compounds are listed in Table 3. into the three-dimensional structure by H bonds, both packing of anions, with smaller cations populating directly between adjacent chains, and to interstitial H2O interstitial positions to produce the observed sheets. groups. Hydrogen bonds also extend between clusters As shown by Miller et al. (1996) and Burns (1999a), of bipyramids within the same chain. the sheet anion-topologies can be assessed as stacking sequences of a small number of distinct chains of poly- STRUCTURES WITH INFINITE SHEETS OF POLYHEDRA gons, which provides a useful means of comparison of the topologies as well as a short-hand notation. This Structures containing infi nite sheets of polyhedra approach has demonstrated that some of the extraordi- of higher bond-valence are the most abundant class in narily complex sheets recently found in minerals such uranyl minerals and synthetic inorganic compounds, as vandendriesscheite and wölsendorfi te are composed accounting for 204 of the 368 structures examined of modules of simpler sheets found in other minerals or herein. The dominance of sheets of polyhedra arises synthetic compounds (Burns 1997, 1999b). from the uneven distribution of bond strengths within uranyl polyhedra. The bond-valence requirements of Sheets dominated by the sharing of vertices the OUr atoms are nearly satisfi ed by the bond to the U6+ cation alone, thus these O atoms usually do not The sheet anion-topology approach works well for form bonds to other higher-valence cations. In contrast, most sheets that are dominated by the sharing of edges the equatorial ligands of the uranyl bipyramids must between polyhedra of higher bond-valence. However, participate in additional significant bonding for a sheets connected only by the sharing of vertices stable confi guration to result. In some cases, equatorial between polyhedra correspond to a myriad of complex ligands correspond to H2O groups that are not bonded anion-topologies that defy usefulness. This was not a to additional high-valence cations. However, it is more substantial setback for Burns et al. (1996) or Burns common for uranyl polyhedra to link to other uranyl (1999a), but the recent discovery of many such sheets polyhedra or other polyhedra of higher bond-valence in synthetic compounds, especially those containing through the equatorial ligands, which favors the forma- hexavalent cations in tetrahedral coordination (such as tion of sheets. Mo6+, Cr6+, S6+), requires a new approach to their hier- Burns et al. (1996) and Burns (1999a) grouped archical organization. To further understanding of the sheets on the basis of the topological arrangement of relationships within sheets dominated by vertex sharing, anions within the sheet. In this approach, all anions that Krivovichev & Burns (2003j) developed a graphical are bonded to at least two cations within the sheet are approach for the analysis of such sheets. This approach considered, and those separated by less than ~3.5 Å are has considerable merit, as it is generally possible to fi nd connected by lines. Removal of all atoms from further parent graphs that may be used to derive the graphs of consideration results in the sheet anion-topology, which, a diverse range of sheets by simple deletion of vertices when projected onto a plane, is a two-dimensional tiling and connectors. of space that represents the topological arrangement of Krivovichev (2004) has greatly expanded the treat- anions within the sheet from which it was derived. The ment of Krivovichev & Burns (2003j) to include a utility of this approach is that sheets of polyhedra that graphical approach to structures containing zero-, one-, bear little immediate resemblance commonly have the and two-dimensional units with corner-sharing between same underlying sheet anion-topology. The sheet anion- coordination polyhedra. He developed a method of topology is a representation of an approximate closest graphical representation of these structures, and intro- 1852 THE CANADIAN MINERALOGIST HIERARCHY OF U6+ MINERALS AND INORGANIC COMPOUNDS 1853

duced short-hand notations for the required graphs. parent graph because a series of graphs may be derived His study included structures in which two adjacent from it, which correspond to known structures, by polyhedra share no more than one common vertex, and simply deleting connectors or vertices, together with where there is no linkage between polyhedra of the same all of their associated connectors in the case of deleted type. These criteria match many of the sheets containing vertices. In the case of the {3.6.3.6} graph, Krivovichev uranyl polyhedra that are dominated by vertex sharing (2004) found 34 derivative graphs that correspond to (Table 4), and Krivovichev (2004) arranged these struc- known structures, with M:T ratios of 1:2, 2:3, 4:3, 13:18, tures on the basis of their graphs, together with coverage 3:8, 5:8, 3:5, and 1:1 (where M represents non-tetrahe- of non-uranium structures. dral polyhedra such as uranyl pentagonal bipyramids, Krivovichev (2004) defi ned the connectedness, s, and T corresponds to tetrahedra). Krivovichev (2004) of a polyhedron as the number of adjacent polyhedra provided a designation for the graphs of the general with which it shares common corners. Consider the form L M/T c, where L designates that the graph is a graph shown in Figure 6a. It contains black circles and sheet, M/T gives the ratio of non-tetrahedral polyhedra white circles. Each black circle is connected to six white to tetrahedra, and c is an alphabetic designator used to circles, and white circles are connected to three black distinguish various graphs with the same M/T ratio. circles. The elementary unit of the graph is a rhomb that Several sheets considered by Krivovichev (2004) 4+ 5+ contains two black and two white circles. The values of and herein have either Se O3 or I O3 polyhedra. In s at the nodes of the rhomb written in cyclic order are these cases, the cations contain stereoactive lone pairs {3.6.3.6}, which may be used to designate the graph. of electrons that occur in the position of the fourth Krivovichev (2004) denoted the {3.6.3.6} graph as a vertex of a distorted tetrahedron. For the purposes of 1854 THE CANADIAN MINERALOGIST

FIG. 6. Sheets (and their graphical representations) of uranyl polyhedra and other polyhedra containing higher-valence cations that are dominated by the sharing of vertices. The corresponding compounds are listed in Table 4. HIERARCHY OF U6+ MINERALS AND INORGANIC COMPOUNDS 1855

4+ 5+ the current discussion, Se O3 and I O3 polyhedra are upon graph L1/2g (Fig. 6m). In these structures, the treated as tetrahedra. uranyl dimolybdate sheets are connected through uranyl Twenty-nine uranyl compounds contain sheets pentagonal bipyramids located in the interlayer, resulting with corresponding graphs that are derived from the in frameworks. They are included here as sheets because {3.6.3.6} parent graph (Table 4). Polyhedral repre- the fundamental portion of the structure corresponds to sentations of each distinct sheet, together with their a graph whose parent is {3.6.3.6}. Within the sheet, each corresponding graphs, are shown in Figure 6. bipyramid is linked to fi ve tetrahedra, and tetrahedra Two sheets are known that correspond to the are either connected to two or three bipyramids. The graph L1/2b (Fig. 6b). The sheet in the structure of connectivity of the tetrahedra traversing around the [UO2(IO3)2(H2O)] contains uranyl pentagonal bipyra- bipyramids is 2–3–3–2–3, which is identical to graph 5+ mids and I O3 pyramids. The sheet in Cs2[(UO2) L1/2d. The distinction between sheets based upon (SeO4)2(H2O)](H2O) is composed of uranyl pentagonal graphs L1/2g and L1/2d lies in the specifi c bipyramids bipyramids and selenate tetrahedra. In each case the to which each tetrahedron bridges. bipyramids are only connected to four tetrahedra, and The uranyl chromate sheet in the structure of the fi fth equatorial vertex of the bipyramid is occu- K2[(UO2)2(CrO4)3(H2O)2](H2O)4 (Fig. 6p) is the only pied by H2O. Each tetrahedron is connected to two example of a uranyl compound with a sheet based upon bipyramids. the graph designated L2/3e (Fig. 6o). This is a relatively The structure of (NH4)2[(UO2)(SO4)2(H2O)](H2O) open sheet in which each bipyramid is linked to four contains the sheet shown in Figure 6f, which is based tetrahedra, with the fi fth vertex of the bipyramid corre- upon the L1/2c graph shown in Figure 6e. The sponding to H2O. Of the four tetrahedra that are linked bipyramids share equatorial vertices with four sulfate to any bipyramid, two bridge between two bipyramids tetrahedra, and H2O is present at the fi fth equatorial and two are connected to three bipyramids. vertex of the bipyramid. As in L1/2b, each tetrahedron Five structures contain sheets that are based upon is connected to two bipyramids. Sheets based upon the graph designated L3/5b (Fig. 6q). Each contains graphs L1/2b and L1/2c differ in the orientation of the a uranyl chromate sheet with a bipyramid-to-tetrahe- bipyramids and the unshared equatorial vertices of the dron ratio of 3:5 (Fig. 6r). All bipyramids share their bipyramids. Sheets based on L1/2b have all unshared fi ve equatorial vertices with fi ve tetrahedra, and all vertices of the bipyramids extending in the same direc- fi ve of the tetrahedra are connected to three different tion, but these vertices alternate in opposite directions in bipyramids. the case of sheets based upon L1/2c (Figs. 6d, f). The structures of -Cs2[(UO2)2(MoO4)3] and Twenty-three structures contain sheets with a M: Cs2[(UO2)2(SO4)3] contain identical sheets (Fig. 6t), T ratio of 1:2; they contain twice as many tetrahedra and are the only known uranyl compounds that are as uranyl pentagonal bipyramids. Sixteen correspond based upon graphs derived from a parent graph with to the graph designated L1/2d by Krivovichev (2004) the connectivity {5.3.5.3}{5.3.5.3} (Fig. 6s) (Krivovi- (Table 4). Of these, eight have molybdate tetrahedra chev 2004). Each bipyramid in this sheet is linked to (Fig. 6h) and seven contain selenite pyramids or fi ve tetrahedra, of which three of the tetrahedra are selenate tetrahedra (Fig. 6i), and all but three crystallize connected to three bipyramids and two are linked to in space group P21/c or P21/n. Each uranyl pentagonal four bipyramids. bipyramid in the uranyl molybdate and uranyl selenite The {4.4.4.4} graph shown in Figure 6u corresponds or selenate sheet shares vertices with five distinct to the autunite anion-topology of Burns et al. (1996). tetrahedra, but tetrahedra are linked either to two or The autunite-type sheet shown in Figure 6v is formed three bipyramids. The connectivity of the tetrahedra by uranyl square bipyramids and tetrahedra that are traversing around the bipyramids is 2–3–3–2–3 in each connected by sharing vertices, such that each bipyramid case. All known structures containing this sheet have is connected to four tetrahedra, and each tetrahedron is monovalent cations in the interlayer regions, where H2O linked to four bipyramids. Structures of the autunite also occurs in some cases. and meta-autunite groups contain this sheet, with either The structures of Na2[(UO2)(MoO4)2] and Ag2[(UO2) phosphate or arsenate tetrahedra. The sheet is based (SeO3)2] contain sheets based upon the L1/2e graph upon the autunite anion-topology that is composed of shown in Figure 6j. Examination of the sheets shown in squares that share vertices and edges (Burns et al. 1996). Figures 6k and 6l shows that each bipyramid is linked This is a large family of structures, with 17 refi ned thus to fi ve tetrahedra, and that tetrahedra are connected far, of which 10 correspond to minerals. The structures to either two or three bipyramids. The connectivity vary considerably in the occupation and connectivity of the tetrahedra traversing around the bipyramids is of their interlayer constituents (e.g., Locock & Burns 2–2–3–3–3 in both cases, which differs from L1/2d, for 2003a, d). The minerals of this group are commonly which the connectivity is 2–3–3–2–3. associated with uranium deposits containing phosphate, The isostructural compounds Mg[(UO2)3(MoO4)4] and have a signifi cant impact upon uranium mobility in (H2O)8 and Zn[(UO2)3(MoO4)4](H2O)8 contain the such situations due to their low solubilities (Murakami uranyl molybdate sheet shown in Figure 6n that is based et al. 1997). Current studies are addressing the possi- 1856 THE CANADIAN MINERALOGIST bility that enhanced formation of autunite-group The structure of (NH4)[(UO2)F(SeO4)](H2O) is minerals in the subsurface may reduce uranium mobility based upon the uranyl selenate sheet shown in Figure 7f. in the vadose zones of contaminated sites. The sheet contains uranyl pentagonal bipyramids The sheet of vertex-sharing uranyl square bipyramids that are linked into chains by the sharing of a single shown in Figure 6w is also based upon the {4.4.4.4} vertex between adjacent bipyramids. According to graph presented in Figure 6u. This sheet occurs in Blatov et al. (1989), the vertices shared between the seven structures (Table 4). In fi ve cases, the structures bipyramids correspond to F, which is uncommon in are anhydrous, and sheets are connected by bonds to uranyl compounds. The three equatorial vertices of interlayer cations including Li, Ba, Sr, Na and Pb. In the bipyramids that are not shared within the chain are -UO2(OH)2 and -UO2(OH)2, all of the equatorial linked to selenate tetrahedra. Each selenate tetrahedron vertices of the uranyl square bipyramids are occupied by is connected to three bipyramids of two chains, resulting hydroxyl groups, and linkages between adjacent sheets in the uranyl selenate sheet. It is interesting to note are assured through H bonds only. that this is the only known sheet that involves uranyl The structure of K4[(UO2)(PO4)2] contains the pentagonal bipyramids and tetrahedra in which the unusual sheet of uranyl square bipyramids and phos- bipyramids are linked by sharing single vertices. phate tetrahedra shown in Figure 6y. This sheet can be The structure of Co(H2O)6[(UO2)5(SO4)8(H2O)] obtained from the autunite sheet (Fig. 6v) by deleting (H2O)5 contains the complex sheet of uranyl pentagonal every second bipyramid. The sheet contains twice as bipyramids and sulfate tetrahedra shown in Figure 7g. many tetrahedra as bipyramids, each bipyramid is linked The structure has two types of bipyramids; those that through its equatorial vertices to four tetrahedra, but the share all fi ve, and those that share only four of their tetrahedra are only connected to two bipyramids. equatorial vertices with sulfate tetrahedra. In the latter Miscellaneous sheets based upon uranyl pentagonal case, the fi fth equatorial vertex of the bipyramid corre- 4+ bipyramids and tetrahedra (or Se O3 pyramids) domi- sponds to H2O. There are three types of tetrahedra nated by single linkages between polyhedra are shown in the sheet, which share two, three, or four of their in Figure 7. The structures of Pb[(UO2)(SeO3)2] and vertices with bipyramids. The inclusion of H2O in an Ba[(UO2)(SeO3)2] contain sheets with identical compo- equatorial position of some of the bipyramids, as well sitions, but different connections between polyhedra as the connection of some sulfate tetrahedra to only two (Figs. 7a, b). In both sheets, each bipyramid is linked bipyramids, results in small pores within the sheets of to four different SeO3 pyramids, three by the sharing polyhedra. of a single vertex, and one by sharing an edge. Half of the SeO3 groups are linked to two bipyramids only Sheet anion-topologies containing by sharing single vertices (designated A), whereas the triangles and pentagons other half are connected to one bipyramid by sharing a vertex, and another by sharing an edge (designated Those structures based upon sheet anion-topologies B). The distinction between the two sheets lies in the containing only triangles and pentagons are presented arrangement of these two types of SeO3 groups. In the in Table 5 and Figure 8. Only fi ve such topologies have sheet shown in Figure 7a, a circuit about any bipyramid been found in structures, and in all but one case, all involves the sequence ABAB, whereas the circuits in the of the pentagons of the anion topologies are occupied sheet in Figure 7b involve the sequence AABB. with uranyl ions, giving uranyl pentagonal bipyramids, The structure of K[(UO2)(CrO4)(OH)](H2O)1.5 whereas the triangles remain vacant. Thus, the sheet contains dimers of edge-sharing uranyl pentagonal anion-topologies in Figure 8 represent five distinct bipyramids, and all of the equatorial vertices of the types of sheets composed only of uranyl pentagonal bipyramids that are not shared within the dimer are bipyramids. linked to chromate tetrahedra (Fig. 7c). Each tetrahedron Protasite anion-topology: The protasite anion- is connected to three different dimers of bipyramids. topology (Fig. 8a) is the basis for two distinct types The structures of Cs[(UO2)(PO3)3] (Fig. 7d) and of sheet. Population of each pentagon of the anion [(UO2)H(PO3)3] (Fig. 7e) are closely related in that topology with a uranyl ion results in the well-known each contains sheets of uranyl pentagonal bipyramids -U3O8-type sheet of edge-sharing uranyl pentagonal and phosphate tetrahedra, with groups of tetrahedra bipyramids (Fig. 8b). This sheet is the basis of seven linked by vertex sharing. In Cs[(UO2)(PO3)3], three known uranyl oxide hydrate minerals, as well as four phosphate tetrahedra are linked, and non-bridging O synthetic phases. The sheet topology is compatible with atoms are shared with four different bipyramids. All a variety of interlayer constituents, including Pb, Na, K, of the equatorial vertices of the bipyramids are shared Ca, Sr, Ba and Cs. The net charge of the sheet is vari- with tetrahedra. In [(UO2)H(PO3)3], phosphate tetra- able, as there are four distinct distributions of hydroxyl hedra are linked into dimers and trimers, and only four ions known for this anion topology (Fig. 9). of the equatorial vertices of the bipyramids are shared Three uranyl sulfates are known that contain a sheet with tetrahedra. based upon the protasite anion-topology (Fig. 8c). In HIERARCHY OF U6+ MINERALS AND INORGANIC COMPOUNDS 1857

FIG. 7. Miscellaneous sheets of uranyl polyhedra and other polyhedra containing higher-valence cations that are dominated by the sharing of vertices. The corresponding compounds are listed in Table 4.

FIG. 8. Sheets (and their anion-topologies) of uranyl poly- hedra and other polyhedra containing higher-valence cations that are based upon anion topologies containing triangles and pentagons. The corresponding compounds are listed in Table 5. 1858 THE CANADIAN MINERALOGIST

2+ this sheet, uranyl pentagonal bipyramids are connected In fourmarierite, Pb cations and H2O groups occupy by sharing vertices with sulfate tetrahedra, such that interlayer positions, but the interlayers of schoepite each bipyramid and each tetrahedron is four-connected and metaschoepite contain only H2O. In a study of the within the sheet. crystal chemistry of fourmarierite, Li & Burns (2000a) Na[(UO2)4O2(OH)5](H2O)2 anion-topology: The found evidence for signifi cant variability of both the structure of Na[(UO2)4O2(OH)5](H2O)2, which was amount of Pb in the interlayer, and the OH content obtained by mild hydrothermal synthesis, represents of the sheets. Li & Burns (2000a) also synthesized a the first occurrence of its underlying sheet anion- crystal intermediate in composition between schoepite topology (Fig. 8d). The composition of this sheet and fourmarierite, suggesting that a solid-solution series is distinct from any known sheet with the protasite involving a coupled substitution is possible between anion-topology, despite the fact that it is composed these minerals. Li & Burns (2000a) proposed the only of uranyl pentagonal bipyramids. The sheet in general formula Pb(1–x)[(UO2)4O(3–2x)(OH)(4+2x)](H2O)4 Na[(UO2)4O2(OH)5](H2O)2 is closely related to that of for fourmarierite. Ca[(UO2)4O3(OH)4](H2O)2, as outlined below. Vandendriesscheite anion-topology: The vanden- Fourmarierite anion-topology: The fourmarierite driesscheite anion-topology is the most complex anion-topology (Fig. 8f) is the basis of topologically topology known that contains only pentagons and identical sheets found in fourmarierite, schoepite, and triangles (Figs. 8h, i). The sheet is truly extraordinary metaschoepite. The sheets in each mineral are arrange- in that it contains a 41 Å primitive repeat-distance, a ments of uranyl pentagonal bipyramids; in fourmari- refl ection of the complex arrangement of pentagons and erite, the sheets have a net negative charge, whereas triangles in its corresponding anion-topology. Notably, those in schoepite and metaschoepite are electroneutral. there are several simpler topologies that are also based HIERARCHY OF U6+ MINERALS AND INORGANIC COMPOUNDS 1859

FIG. 9. Observed distributions of OH groups within the protasite anion-topology. The locations of OH groups are indicated by open circles. Vertices in the anion topology that are not marked by circles correspond to O atoms. upon pentagons and triangles, and factors that result triangles of the topology are vacant, and the Cs cations in the stabilization of the complex vandendriesscheite are located between the sheets of polyhedra. sheet are unclear. However, the sheet is related to a The distinction between the various sheets simpler sheet that contains only uranyl pentagonal contain ing only uranyl pentagonal bipyramids is most bipyramids. Using an approach involving stacking apparent in the distribution of the triangles within sequences of chains of polygons, Burns (1997) showed the sheet anion-topology, which remain vacant in the that the vandendriesscheite anion-topology is built populated sheets. In the case of the protasite anion- of modules of the much simpler protasite (-U3O8) topology and derivative sheets, each triangle is isolated anion-topology. from the others and points in the same direction. In Cs2[(UO2)2O3] anion-topology: The structures of Na[(UO2)4O2(OH)5](H2O)2, the triangles occur in pairs the alpha and beta polymorphs of Cs2[(UO2)2O3] each that share a vertex, resembling bowties, and there are possess sheets of uranyl pentagonal bipyramids (Fig. 8k) two distinct orientations of bowties. The fourmarierite that are based upon the sheet anion-topology shown in anion-topology also contains triangle bowties, but differs Figure 8j. Each of the pentagons in the anion topology from Na[(UO2)4O2(OH)5](H2O)2 in that each is oriented share three of their edges with adjacent pentagons, and in the same direction. The vandendriesscheite anion- all of the pentagons are populated by uranyl ions in the topology contains both isolated triangles and triangle observed sheets. The sheet contains a zig-zag chain of bowties; all bowties are aligned, whereas the isolated edge-sharing pentagonal bipyramids that is two poly- triangles occur in two orientations. The Cs2[(UO2)2O3] hedra wide (vertical in Fig. 8k), topologically identical anion-topology contains pairs of triangles that share an to that in the zippeite sheet (see below, Fig. 11v). Unlike edge, giving a fl attened rhomb. in the zippeite sheet, these chains are linked by sharing The uranyl oxide hydrates based upon sheets of of edges between bipyramids of adjacent chains. The uranyl pentagonal bipyramids are the fi rst minerals 1860 THE CANADIAN MINERALOGIST to form where uraninite, UO2+x, is altered in a moist, to occur because of different orientations of the silicate oxidizing environment (Finch & Ewing 1992). Studies tetrahedra. This may readily be seen by examining the of natural analogues, as well as laboratory-scale simu- sheets given in Figures 10c–e, with emphasis on the lations, have also demonstrated that several of these orientation of the apical, non-bridging, O or OH groups minerals form where spent nuclear fuel is altered in an of the silicate tetrahedra. The structure of oursinite, oxidizing environment in the presence of water, similar which was only recently reported, represents a new to conditions expected in the proposed geological repos- graphical isomer. With the exception of kasolite, all of itory for nuclear waste at Yucca Mountain, Nevada (e.g., the uranophane-group minerals with known structures Pearcy et al. 1994, Finn et al. 1996, Finch et al. 1999a). contain acid silicate groups, where the hydroxyl group Recently, Burns et al. (2004a) showed that Np5+ is is non-bridging within the sheet. incorporated into synthetic powders of the Na analogue Studies of natural analogues (Pearcy et al. 1994), of compreignacite, which supports the hypothesis that as well as laboratory simulations (Wronkiewicz et al. formation of such minerals in a geological repository 1992, 1996, Finch et al. 1999a) have shown that altera- may impact the future mobilities of key radionuclides. tion of spent nuclear fuel in the proposed geological repository at Yucca Mountain, Nevada, will likely result Sheet anion-topologies containing triangles, in the formation of uranophane and boltwoodite. Burns squares and pentagons et al. (1997b) argued that these minerals may incorpo- rate various radionuclides released from the nuclear Seventy-one uranyl compounds are known that waste, and as such, they may have an impact upon rates contains sheets that are based upon anion topologies of radionuclide release from the proposed repository. containing triangles, squares and pentagons. In all cases Using crystals of natural boltwoodite, Burns (1999c) at least some of the pentagons are populated by uranyl ion-exchanged Cs into interlayer positions of the ions; thus all sheets in this class contain uranyl pentag- structure by treating crystals with solutions containing onal bipyramids. The triangles of the anion topologies CsCl at elevated temperatures. Notably, ion exchange are occupied by BO3 groups in two structures, and by was accomplished for single crystals that remained the face of a tetrahedron in many structures. The square intact even following exchange, and ion exchange was sites of the anion topologies provide coordination envi- demonstrated by structure solution for a treated crystal. ronments for cations in square pyramidal or octahedral This was the fi rst demonstration of ion exchange in a coordination. single crystal of a uranyl mineral, and demonstrated that Uranophane anion-topology: The uranophane radioactive Cs can be accommodated in the interlayer anion-topology is elegant in its simplicity (Fig. 10a). It region of boltwoodite. Recently, Burns et al. (2004a) contains chains of edge-sharing pentagons that are sepa- showed that Np5+ is incorporated into powders of rated by chains of edge-sharing triangles and squares. uranophane synthesized at pH ~5 and 100°C, conditions This anion topology is the basis for a chemically diverse similar to those expected in the proposed repository group of compounds that includes the important group at Yucca Mountain where water may fi rst contact the of the uranophane minerals. In all but two of the repre- nuclear waste. sentative structures, the pentagons are all populated by The structure of ulrichite was recently revisited by uranyl ions, giving pentagonal bipyramids. In the case Kolitsch & Giester (2001); this work confi rmed many of ulrichite, half of the pentagons are populated by Ca, of the earlier-reported aspects of the structure, but and half of the pentagons contain Na in Na5.5(UO2)3 provides a new space-group assignment and signifi - (H0.5PO4)(PO4)3, but the Na is not considered to be part cantly improved precision of the structural parameters. of the structural unit for the purposes of illustration in Ulrichite contains a sheet that is based upon the urano- Figure 10h. There are 17 structures known that contain phane anion-topology, with half of the pentagons popu- sheets based upon the uranophane anion-topology, of lated by uranyl ions, and the other half populated by which 11 are minerals; they are grouped in Table 10 Ca8 polyhedra (Fig. 10f). The triangles of the topology (see below) according to the specifi c population of the are populated by phosphate tetrahedra. topology. The structure of umohoite was once considered In the compounds Na[(UO2)(BO3)] and Li[(UO2) to contain sheets of uranyl hexagonal bipyramids (BO3)], the triangles of the uranophane anion-topology (Makarov & Anikina 1963). A redetermination of are populated by BO3 groups (Fig. 10b). These anhy- the structure revealed a new uranyl molybdate sheet drous compounds are readily synthesized at elevated that is based upon the uranophane anion-topology temperatures, and contain topologically identical (Krivovichev & Burns 2000a). Each pentagon in the sheets. anion topology is populated by a uranyl ion, giving The uranophane-group minerals are uranyl silicates uranyl pentagonal bipyramids, and the squares of the that have sheets based upon the uranophane anion- topology are equatorial vertices of distorted Mo5 topology. In each case, the triangles of the topology polyhedra (Fig. 10g), and the non-sheet ligand is H2O. correspond to the faces of silicate tetrahedra, but three The interlayer of the structure contains only H2O, with graphical isomers of the uranyl silicate sheet are known the sheets connected by H bonds. HIERARCHY OF U6+ MINERALS AND INORGANIC COMPOUNDS 1861

FIG. 10. Sheets of uranyl polyhedra and other polyhedra containing higher-valence cations (and their anion topologies) that are based upon the uranophane anion-topology and that contain triangles, squares, and pentagons. The corresponding compounds are listed in Table 6.

The structures of K2[(UO2)2O3], [H2(UO2)3O4], A substantial family of synthetic uranyl phosphate [(UO2)(CuO4)], schmitterite, and K[(UO2)(NbO4)] framework structures has recently been discovered all contain sheets based upon the uranophane anion- that contains sheets based upon the uranophane anion- topology, in which all pentagons are occupied by uranyl topology, with the triangles populated by the faces of ions, and the squares of the topology are occupied by phosphate tetrahedra. Uranyl polyhedra in the inter- various cations (Figs. 10i–m). In K2[(UO2)2O3] (Fig. 10i) layer regions provide direct connections between the and [H2(UO2)3O4] (Fig. 10j), uranyl ions populate the sheets, and this family of structures is included with squares of the anion topology, resulting in uranyl square the framework structures discussed below (see below, bipyramids, with all squares occupied in K2[(UO2)2O3] Figs. 15m–o). and only half of the squares occupied in [H2(UO2)3O4]. Francevillite anion-topology: The francevillite Schmitterite is unusual in that it is the only uranyl anion-topology (Fig. 11a) consists of pairs of edge- tellurite with a structure based upon the uranophane sharing pentagons and edge-sharing squares as well anion-topology (Fig. 10l). Each square contains Te4+ as triangles. Twelve structures are known in which in a highly distorted pyramidal geometry owing to the sheets are based upon this anion topology, of which stereoactive lone-electron pair on the cation. four are minerals (Table 6). In all of the sheets, each 1862 THE CANADIAN MINERALOGIST HIERARCHY OF U6+ MINERALS AND INORGANIC COMPOUNDS 1863

of the pentagons of the anion topology is populated by that contains NbO5 square pyramids. Recently, fi ve uranyl ions, giving uranyl pentagonal bipyramids. The structures have been reported with the francevillite-type francevillite group of minerals, as well as synthetic sheet in which the squares are populated by CrO5 square uranyl vanadates, involves population of the squares of pyramids (Locock et al. 2004). These structures are the anion topology by VO5 square pyramids, resulting remarkable in that they contain pentavalent chromium, in dimers of edge-sharing pyramids. Within each dimer, which is uncommon in crystal structures. In each case, the apical vertices of the two adjacent square pyramids low-valence cations in the interlayers of the structures, point in opposite directions. One structure is known 1864 THE CANADIAN MINERALOGIST which also contain H2O groups in some cases, provide group shares an edge with one U polyhedron such that it linkages between the sheets. extends into the interlayer. The sheets are linked through -U3O8 anion-topology: The -U3O8 anion- bonds to Ca in the interlayer, as well as by a network of topology (Fig. 11c) is the basis of the structure of H bonds. The structures of ianthinite and spriggite each -U3O8, which contains U in multiple valence states, have sheets of uranyl pentagonal and square bipyramids and three minerals. This topology is similar to the that are topologically identical to those in -U3O8; in uranophane anion-topology in that both contain iden- spriggite, all of the U is hexavalent, whereas the U in tical chains of edge-sharing pentagons, as well as chains the structure of ianthinite has multiple valence states. of edge-sharing triangles and squares. The distinction On the basis of an imprecise refi nement of the structure, is that pentagons of adjacent chains share vertices only Burns et al. (1997c) reported that ianthinite probably 6+ 4+ in the -U3O8 anion-topology, which also contains contains both U and U , but the subsequent discovery more triangles. In the case of the sheet in -U3O8, all of wyartite suggests that ianthinite also may contain of the pentagons and squares of the anion topology are pentavalent U. populated by U. Wyartite is the only mineral known to Iriginite anion-topology: Iriginite, as well as two contain pentavalent uranium, although several synthetic synthetic uranyl molybdates, contain sheets based upon compounds contain uranium in this valence state the iriginite anion-topology, shown in Figure 11e. The (Burns & Finch 1999). The sheet of uranyl polyhedra sheets are identical in these three structures, and contain in wyartite is closely related to that in -U3O8, but one uranyl pentagonal bipyramids and distorted Mo6 of the U polyhedra contains pentavalent U, and the CO3 octahedra that form dimers by sharing an edge. Uranyl

FIG. 11. Sheets of uranyl polyhedra and other polyhedra containing higher-valence cations that are based upon anion topologies and that contain triangles, squares, and pentagons (other than the uranophane anion-topology). The corresponding compounds are listed in Table 6. HIERARCHY OF U6+ MINERALS AND INORGANIC COMPOUNDS 1865 1866 THE CANADIAN MINERALOGIST pentagonal bipyramids are linked to four different Mo6 larity to compreignacite, this phase has not been found octahedra, two by sharing edges and two by sharing as a mineral. The sheet within its structure consists of only vertices. In the case of [Ca(UO2)(Mo2O7)2], double-width chains of bipyramids that share equatorial pentagons of the anion topology are populated by Ca edges; adjacent chains are connected by the sharing of as well as U. vertices and edges between bipyramids. The chains Sayrite anion-topology: Two structures are known are similar to those of the curite-type sheet (Fig. 11l), that contain sheets of uranyl square and pentagonal except that they contain fewer square bipyramids, bipyramids (Fig. 11h) that are based upon the sayrite although these polyhedra are similarly distorted in the anion-topology (Fig. 11g). The squares and pentagons two structures. of the anion topology are each populated by uranyl ions; Ca[(UO2)4O3(OH)4](H2O)2 anion-topology: Despite thus the resulting sheet is composed of edge- and vertex- its chemical similarity with becquerelite, the structure sharing uranyl square and pentagonal bipyramids. The of Ca[(UO2)4O3(OH)4](H2O)2 is based upon a novel sheet is anhydrous in K2[(UO2)5O8](UO2)2, and the sheet of uranyl pentagonal bipyramids and distorted interlayer of the structure contains uranyl ions. In the square bipyramids (Fig. 11p). The sheet contains two case of the rare mineral sayrite, the sheet contains H distinct double-width chains of bipyramids, one that present as OH groups, and adjacent sheets are linked contains only pentagonal bipyramids, and one that 2+ by bonds to Pb cations and H2O groups located in contains equal proportions of square and pentagonal the interlayer. bipyramids. The chains are linked into sheets only by [Pb3(UO2)11O14] anion-topology: The isostructural the sharing of vertices between bipyramids. The sheet compounds [Pb3(UO2)11O14] and [Sr3(UO2)11O14] is similar to the curite-type sheet and the sheet found in contain complex sheets of uranyl square and pentag- K5[(UO2)10O8(OH)9](H2O) in the dominance of double- onal bipyramids and either Pb7 or Sr7 pentagonal width chains and the presence of highly distorted square bipyramids (Fig. 11j). In this case, inclusion of Pb and bipyramids. Ca[(UO2)4O3(OH)4](H2O)2 has not been Sr in the structural unit is warranted, as they assume found in nature, but it is readily synthesized under a similar structural roles as U. Adjacent sheets are linked variety of conditions, and it commonly occurs with directly through the Pb7 or Sr7 polyhedra. These becquerelite in products of synthesis (Glatz et al. 2002). structures are remarkable in the presence of Pb2+ and Recently, Kubatko et al. (2005) determined the heats Sr2+ within the sheets of uranyl polyhedra, which is of formation of several uranyl oxide hydrates using not known to occur in any of the structures of uranyl drop-solution calorimetry. Those results demonstrated oxide hydrates. that Ca[(UO2)4O3(OH)4](H2O)2 is unstable in terms of Curite anion-topology: The curite anion-topology enthalpy relative to the oxides; thus it appears to form (Fig. 11k), which contains pentagons, distorted squares, metastably in synthesis reactions. and triangles, is the basis of sheets of uranyl pentagonal The sheet in Ca[(UO2)4O3(OH)4](H2O)2 is closely and square bipyramids in curite and its synthetic Sr- related to the sheet of uranyl pentagonal bipyramids found substituted analogue. Although the square bipyramid in in the structure of Na[(UO2)4O2(OH)5](H2O)2 (Fig. 8e). this sheet is strongly distorted from an ideal geometry, The U(3) square bipyramid in Ca[(UO2)4O3(OH)4](H2O)2 similarly distorted square bipyramids also occur in the has only four equatorial ligands; if this were a pentagonal structures of K5[(UO2)10O8(OH)9](H2O) (Fig. 11n), bipyramid, the sheet would be topologically identical Ca[(UO2)4O3(OH)4](H2O)2 (Fig. 11p), and Pb2(H2O)[( to that in Na[(UO2)4O2(OH)5](H2O)2. Displacement of UO2)10UO12(OH)6(H2O)6] (see below, Fig. 15g). Li & a single atom by ~0.6 Å is the topological difference Burns (2000b) studied the structures of several crystals between these two sheets. of curite, including a synthetic crystal, and found that the Cs4[(UO2)5O7] anion-topology: The Cs4[(UO2)5O7)] Pb sites are partially occupied in each crystal studied. anion-topology (Fig. 11q) is the basis for the sheet (Fig. The general formula Pb3+x(H2O)2 [(UO2)4O4+x(OH)3–x]2 11r) found only in Cs4[(UO2)5O7]. The sheet contains was proposed, and Pb variability in the interlayer is both uranyl square and pentagonal bipyramids that are accompanied by changes of the hydroxyl content of connected by sharing equatorial edges and vertices. the sheet of uranyl polyhedra, which provides a charge- Adjacent sheets are linked through bonds to the Cs balance mechanism. Synthesis and structure determina- cations in the interlayer of the structure. tion of the Sr-substituted analogue of curite (Burns & Vandenbrandeite anion-topology: The vandenbran- Hill 2000a) demonstrate that the unusual sheet of uranyl deite anion-topology (Fig. 11s) is the basis for one sheet polyhedra found in curite is not a result of the distorted (Fig. 11t) that contains dimers of edge-sharing uranyl coordination environment about the interlayer Pb2+ pentagonal bipyramids that are linked by dimers of cation caused by a stereoactive lone-electron pair. edge-sharing Cu5 square pyramids. The OUr anions K5[(UO2)10O8(OH)9](H2O) anion-topology: Burns of the pentagonal bipyramids are apical ligands of the & Hill (2000b) reported the structure of a new K uranyl Cu5 square pyramids of adjacent sheets. oxide hydrate that contains complex sheets of uranyl Zippeite anion-topology: The zippeite anion- pentagonal bipyramids and highly distorted uranyl topology and corresponding sheet are shown in Figures square bipyramids (Fig. 11n). Despite its chemical simi- 11u and 11v. The sheet contains zig-zag chains of edge- HIERARCHY OF U6+ MINERALS AND INORGANIC COMPOUNDS 1867 sharing uranyl pentagonal bipyramids, and the chains six uranyl pentagonal bipyramids that are linked into a are connected by the sharing of vertices between the cluster by the sharing of equatorial edges. The resulting bipyramids and sulfate tetrahedra, such that each sulfate clusters are linked into a sheet both by sharing vertices tetrahedron is linked to four different bipyramids. The directly between clusters, and by sharing polyhedron formulae and crystallographic parameters for zippeite- edges with MoO5 groups. group compounds are listed in Table 6. Vochten et al. [Mg(UO2)(B2O5)] anion-topology: The [Mg(UO2) (1995) reported the fi rst structure of a zippeite-structure (B2O5)] anion-topology, and the corresponding hetero- material for the synthetic analogue of zippeite, the polyhedral sheet, are shown in Figures 11ac and 11ad, K-bearing mineral. Burns et al. (2003) presented a respectively. The sheet contains uranyl pentagonal revised structure for zippeite, as well as structures for bipyramids that share two equatorial edges with BO3 four additional synthetic analogues of zippeite-group groups that are part of B2O5 dimers of BO3 groups. minerals and three synthetic phases of the zippeite Each bipyramid also shares two equatorial vertices group. Brugger et al. (2003) reported the structure of with other BO3 groups, and MgO6 octahedra share an the new zippeite-group mineral marecottite. equatorial edge of a bipyramid, as well as vertices with Burns et al. (2003) showed that although the two other bipyramids. uranyl sulfate sheets are topologically identical in each Wölsendorfite anion-topology: The structure of zippeite-group structure studied, the sheets in some wölsendorfi te contains an extremely complex sheet of are anhydrous, whereas in others they may contain uranyl square and pentagonal bipyramids (Fig. 11af), hydroxyl groups located at equatorial vertices of the and has a primitive repeat-distance of 56 Å (Burns uranyl pentagonal bipyramids. The interlayer confi gura- 1999b). Wölsendorfi te contains the most topologically tions of zippeite-group phases are diverse, and include complex sheet known in any uranyl compound. By either monovalent or divalent cations as well as H2O considering the sheet anion-topology of wölsendorfi te groups. Burns et al. (2003) concluded that the distribu- (Fig. 11ae) as a stacking sequence of simpler chains, tion of interlayer constituents can be rationalized on Burns (1999b) demonstrated that the topology is the basis of the bonding requirements of the uranyl composed of modules of the simpler protasite (-U3O8) sulfate sheets. and -U3O8 anion topologies. Sheets are linked through K2[(UO2)2(WO5)O] anion-topology: Three topo- a complex interlayer that contains Pb and Ba cations, logically identical sheets (Fig. 11x) have recently been as well as H2O groups. found that are based upon the anion-topology shown in Cs4[(UO2)3O(MoO4)2(MoO5)] anion-topology: The Figure 11w, which contains clusters of eight pentagons uranyl molybdate sheet that occurs in the structure of linked by edge-sharing (Table 6). Each of these clusters Cs4[(UO2)3O(MoO4)2(MoO5)], and its corresponding is connected to four identical clusters by sharing one anion-topology, are shown in Figures 11ag and 11ah. vertex with each, which results in both triangles and The uranyl molybdate sheet contains uranyl pentagonal squares in the anion topology. All pentagons of the bipyramids, molybdate tetrahedra, and MoO5 poly- anion topology are populated by uranyl ions in each of hedra. Three bipyramids share edges and a common the three structures, giving uranyl pentagonal bipyra- O atom, resulting in trimers that are linked into chains mids, and three out of four of the squares are the bases by the sharing of polyhedron vertices. The MoO5 poly- of square pyramids containing W in two structures, and hedron occupies a pocket in the chain of bipyramids, Mo in the other. One square pyramid shares three of its where it shares two of its edges with bipyramids. The edges with uranyl bipyramids, whereas the other two chains of bipyramids and MoO5 polyhedra are linked share only one edge with a bipyramid, but two of their into sheets by sharing vertices with tetrahedra, and each vertices are shared with other bipyramids. Interlayer tetrahedron is linked to two bipyramids, one from the cations provide linkage between the sheets. chain on either side. K6[(UO2)5(VO4)2O5] anion-topology: Topologically Ag10[(UO2)8O8(Mo5O20)] anion-topology: The and chemically identical uranyl vanadate sheets (Fig. structures of Ag10[(UO2)8O8(Mo5O20)] and Na10[(UO2)8 11z) based upon the same anion-topology (Fig. 11y) O8(Mo5O20)] contain the sheet of uranyl pentagonal have recently been found in four anhydrous compounds bipyramids and MoO6 or WO6 octahedra shown in (Table 6). The sheet contains three-polyhedron-wide Figure 11aj, with the corresponding sheet anion- chains of uranyl square and pentagonal bipyramids, topology shown in Figure 11ai. The sheet consists identical to those found in the -U3O8-type sheet of complex chains of edge-sharing uranyl pentagonal (Fig. 11d), that are connected into sheets by sharing bipyramids that are in turn linked through sharing equa- edges and vertices with VO4 tetrahedra. Linkages torial edges and vertices of the bipyramids with dimers between sheets are provided by monovalent cations of edge-sharing MoO6 or WO6 octahedra. Identical located in the interlayer regions of the structures. sheets are linked into complex double sheets through K8[(UO2)8(MoO5)3O6] anion-topology: The uranyl a MoO4 tetrahedron that shares two equatorial vertices molybdate sheet contained in K8[(UO2)8(MoO5)3O6] is with MoO6 or WO6 octahedra in the sheet above as well shown in Figure 11ab, with its corresponding anion- as the sheet below. The unusual linkage of molybdate topology in Figure 11aa. The sheet contains groups of 1868 THE CANADIAN MINERALOGIST and tungstate polyhedra in this structure type results in are populated by uranyl ions, resulting in chains of pentanuclear Mo5O20 and W5O20 clusters. edge-sharing pentagonal and hexagonal bipyramids. K2(UO2)2(MoO4)O2 anion-topology: The structure Population of the triangles of the anion topology by of K2[(UO2)2(MoO4)O2] contains the sheet of uranyl phosphate tetrahedra, such that a face of the tetrahedron pentagonal bipyramids and molybdate tetrahedra shown corresponds to the triangle of the topology, results in the in Figure 11al, which is based upon the sheet anion- uranyl phosphate sheets of the phosphuranylite group. topology given in Figure 11ak. The sheet contains Four graphical isomers of this sheet are known; they chains of edge-sharing bipyramids two polyhedra wide differ in the orientations of apical (non-sheet) ligands that are cross-linked into a sheet by sharing equatorial of the phosphate tetrahedra (Figs. 12b–e). For example, vertices with molybdate tetrahedra. consider the orientations of the tetrahedra attached to a single chain of uranyl polyhedra in the phosphuranylite Sheet anion-topologies containing triangles, sheet (Fig. 12b). Tetrahedra attached to the chain on squares, pentagons and hexagons either side immediately opposite each other are in the same orientation. Along the side of a chain of uranyl Twenty uranyl phases, including 17 minerals, polyhedra, the tetrahedra alternate up and down. In contain sheets based upon anion topologies with the case of the vanmeersscheite sheet (Fig. 12c), all triangles, squares, pentagons and hexagons (Table 7). tetrahedra that share edges with a given chain of uranyl These include the large phosphuranylite group of uranyl polyhedra are in the same orientation. phosphate minerals, for which several new structures The interlayers of the structures of minerals of the have been described recently. phosphuranylite group are complex, and some contain Phosphuranylite anion-topology: The phosphura- more than one type of cation, as well as H2O groups. nylite anion-topology, shown in Figure 12a, contains The structures of phosphuranylite and vanmeersscheite pentagons that share edges, resulting in dimers, which are unusual in that both contain uranyl ions in all are in turn linked into chains by sharing edges with three common coordinations: square, pentagonal and hexagons. These chains are linked through chains of hexagonal bipyramids. The square bipyramids are edge-sharing triangles and squares, arranged such that located between the sheets in each structure, with the the triangles and squares alternate along the chain uranyl ions extending roughly parallel to the sheets. In length. Several populations of this anion topology are phosphuranylite, all four of the equatorial ligands of known, but none involves occupancy of the square the square bipyramid are apical (non-sheet) vertices sites. In most cases, both the pentagons and hexagons of the phosphate tetrahedra of sheets on either side. In

FIG. 12. Sheets of uranyl polyhedra and other polyhedra con- taining higher-valence cations that are based upon anion topologies and that contain triangles, squares, pentagons and hexagons. The corresponding compounds are listed in Table 7. HIERARCHY OF U6+ MINERALS AND INORGANIC COMPOUNDS 1869

contrast, the uranyl square bipyramid in the vanmeers- and the triangles of the anion topology are populated by scheite interlayer shares vertices with only two phos- carbonate triangles, such that the CO3 group shares an phate tetrahedra, one from each adjacent sheet. Linkage edge with a hexagonal bipyramid and one vertex with of the uranyl phosphate sheets through interlayer uranyl a pentagonal bipyramid of an adjacent chain. Calcium polyhedra results in open three-dimensional structures cations and H2O groups located in the interlayer provide in phosphuranylite and vanmeersscheite. linkages between the sheets in fontanite. The structure of hügelite contains sheets that are Four uranyl selenites, of which three are minerals, topologically identical to those in vanmeersscheite and contain sheets based upon the phosphuranylite anion- dumontite, but in this case, the triangles of the anion topology. Each sheet has chains of uranyl pentagonal 4+ topology are populated by arsenate tetrahedra. and hexagonal bipyramids linked by bonds to Se O3 Recent studies of uranyl carbonates have revealed pyramids with a base of three O atoms and Se4+ at the more structural complexity within this chemical class (Li fourth apex of the pyramid (Fig. 12g). The strongly et al. 2000, Hughes Kubatko & Burns 2003). Although distorted coordination environment about the Se4+ many uranyl carbonates that form under somewhat cation is due to the presence of a stereoactive lone-pair alkaline conditions contain the isolated uranyl tricar- of electrons on the cation that is directed away from bonate cluster, minerals formed under neutral to acidic the coordinating anions. In each structure, low-valence conditions tend to adopt sheet structures. Fontanite is cations and H2O groups provide linkages between the the only uranyl carbonate with a sheet based upon the sheets. phosphuranylite anion-topology (Fig. 12f). This sheet Johannite and Sr[(UO2)2(CrO4)2(OH)2](H2O)8 contains uranyl pentagonal and hexagonal bipyramids, contain sheets based upon the phosphuranylite anion- 1870 THE CANADIAN MINERALOGIST topology (Fig. 12h). However, in this case the hexagons of octahedra through the sharing of non-chain vertices of the topology are not occupied; the sheet contains of the CO3 triangles, as well as equatorial vertices of dimers of edge-sharing uranyl pentagonal bipyramids uranyl pentagonal bipyramids. Linkages between the that are linked only by sharing vertices with sulfate or sheets in roulbaultite occur by H bonds to interlayer chromate tetrahedra. Edges of the tetrahedra correspond H2O groups only. to the hexagons of the anion topology, thus occupancy of the hexagons by uranyl ions would result in short Miscellaneous anion-topologies with hexagons U6+–S6+ or U6+–Cr6+ interatomic distances. Roulbaultite anion-topology: The structure of roul- Twenty-one compounds, of which only one is a baultite contains the complex sheet shown in Figure 12j, mineral, are grouped as miscellaneous sheets with anion which is based upon the anion topology shown in Figure topologies containing hexagons (Table 8, Fig. 13). Only 12i. This structure has notable similarities to that of six anion topologies are needed to account for all of the fontanite (Fig. 12f). Both contain the same chain of 21 structures. uranyl pentagonal and hexagonal bipyramids, and CO3 -UO3 anion-topology: The simplest anion-topology triangles. In fontanite, these chains are linked directly involves a two-dimensional tiling of space using only through the sharing of vertices between the CO3 group hexagons (Fig. 13a), and corresponds to the sheets of one chain with uranyl pentagonal bipyramids of of uranyl hexagonal bipyramids (Fig. 13b) that are an adjacent chain. In contrast, the roulbaultite sheet known from 10 structures. The structure of -UO3 has also contains chains formed by the sharing of edges sheets constructed from hexagonal bipyramids, but in between Cu6 octahedra, and uranyl carbonate chains the reported structure, no uranyl ions are present, and are separated by chains of octahedra on either side. adjacent sheets are linked through the sharing of apical The uranyl carbonate chains are linked to the chains ligands between adjacent sheets. In the structure of - HIERARCHY OF U6+ MINERALS AND INORGANIC COMPOUNDS 1871

[(UO2)(OH)2], sheets of uranyl hexagonal bipyramids hexagonal bipyramids and SbO4 square pyramids (Fig. are connected through H bonds that extend through the 13l), with the sheet anion-topology shown in Figure interlayer region. Eight additional structures are based 13k. The sheet contains chains of edge-sharing bipyra- upon sheets of hexagonal bipyramids, with linkages mids similar to those in rutherfordine (Fig. 13d) and provided by low-valence cations in the interlayers [(UO2)(B2O3)O] (Fig. 13h). Unlike the case of ruther- (Table 8). fordine, these chains are not directly linked, but rather Rutherfordine anion-topology: The rutherfordine are connected through zigzag chains of edge-sharing anion-topology is composed of hexagons and triangles SbO4 square pyramids. Note that the SbO4 polyhedra (Fig. 13c). It contains chains of edge-sharing hexagons are distorted because of the presence of a stereoactive that are identical to those of the -UO3 anion-topology lone-electron pair, and are geometrically similar to the 4+ (Fig. 13a), but these chains are now connected in the Te O4 polyhedra in the structure of schmitterite (Fig. second dimension only by the sharing of vertices rather 10l). than edges, and adjacent chains are separated by trian- gles. The sheet in the structure of rutherfordine has all of Miscellaneous topologies the hexagons of the anion topology populated by uranyl ions, and half of the triangles correspond to CO3 groups, The compounds listed in Table 9 are based upon a whereas the remaining triangles are vacant (Fig. 13d). variety of sheets with miscellaneous topologies. Each Note that all of the CO3 triangles are in the same orienta- sheet is shown in Figure 14. The structure of the uranyl tion. The structure of [(UO2)(SeO3)] contains sheets that carbonate bijvoetite is unusual in that it contains uranyl are topologically identical to those in rutherfordine, but carbonate sheets (Fig. 14a), whereas the majority of 4+ the triangles of the topology are populated by Se O3 uranyl carbonates contain isolated uranyl tricarbonate polyhedra that are distorted owing to the lone-electron clusters. The sheet contains both uranyl pentagonal pairs on the Se4+ cations. Neither rutherfordine or and hexagonal bipyramids, and there are dimers of [(UO2)(SeO3)] have interlayer constituents. each that result from the sharing of equatorial edges. K2[(UO2)(W2O8] anion-topology: Four structures The dimers of hexagonal bipyramids are oriented have recently been reported (Table 8) that contain the perpendicular to the length of chains of uranyl poly- anion topology shown in Figure 13e. The corresponding hedra, and are connected through the dimers of uranyl sheet contains chains of edge-sharing uranyl pentagonal pentagonal bipyramids, which extend along the length bipyramids. Two equatorial edges of the bipyramids are of the chains. As such, each of the equatorial vertices shared with tungstate octahedra, such that octahedra are of the pentagonal bipyramids are shared with other attached to either side of the chain (Fig. 13f). Adjacent uranyl polyhedra, but two of the equatorial vertices of uranyl tungstate chains are connected by the sharing of each hexagonal bipyramid are bonded to only one U6+ vertices between the tungstate octahedra. Low-valence cation. The resulting chain of polyhedra is unique in cations provide linkages between the sheets, and each uranyl compounds. Carbonate groups are attached to the of these structures is anhydrous. sides of the chains by sharing edges with the hexagonal [(UO2)(BO3)O] anion-topology: The uranyl borate bipyramids, such that each bipyramid is linked to two sheet in the structure of [(UO2)(B2O3)O] (Fig. 13h) carbonate triangles. The chains are linked into sheets contains uranyl hexagonal bipyramids and borate through irregular coordination-polyhedra about trivalent triangles. The anion topology of the corresponding sheet cations, which correspond to Y and various rare-earth (Fig. 13g) is similar to that of rutherfordine (Fig. 13c) elements, especially Dy. The sheets are linked through in the presence of chains of edge-sharing hexagons that H bonds to H2O groups located in the interlayer of the are separated by triangles, but in this case the chains are structure. not directly linked, but rather are separated by chains The structure of haiweeite contains sheets of uranyl of edge-sharing triangles that are two triangles wide. pentagonal bipyramids and silicate tetrahedra (Fig. The uranyl borate sheets are linked through van der 14b). The bipyramids share edges, giving a chain that is Waals forces only. one bipyramid wide, and silicate tetrahedra share equa- [(UO2)Nb3O8] anion-topology: Two uranyl niobates torial edges of the bipyramids along both sides of the and one uranyl vanadate contain the sheet shown in chain. The resulting uranyl silicate chain has the same Figure 13j (Table 8), which is based upon the anion anion-topology as that found in the uranyl silicates of topology in Figure 13i. The squares in the anion topology the uranophane group (Fig. 10). However, the way these are populated by NbO6 octahedra in [(UO2)Nb3O8], by chains are connected into sheets distinguishes haiweeite both NbO6 and TiO6 octahedra in [(UO2)TiNb2O8], from the uranophane group. The chains are directly and by VO5 square pyramids in CsUV3O11. In all three linked in the uranophane group, as silicate tetrahedra structures, the sheets are directly linked, with the OUr bridge between adjacent chains of bipyramids. In the atoms of one sheet being the apical ligands of the octa- haiweeite sheet, the uranyl silicate chains are linked hedra or square pyramids of the adjacent sheet. through a complex crankshaft-like chain of silicate [(UO2)(Sb2O4)] anion-topology: The compound tetrahedra, and two such chains in different orientations [(UO2)(Sb2O4)] possesses a novel sheet of uranyl are superimposed in the long-range structure, which may 1872 THE CANADIAN MINERALOGIST be consistent with a domain structure or true disorder of hexagonal bipyramids share an edge, forming a chain the chain orientations. The resulting complex sheets are one polyhedron wide that is identical to that found in linked by Ca cations in the interlayer, as well as through rutherfordine and other structures shown in Figure 13. H bonds to interlayer H2O groups. However, unlike the structures in Figure 13, pentagonal The structure of Ag4[(UO2)4(IO3)2(IO4)2O2] contains bipyramids are attached to each side of this chain by a sheet of uranyl pentagonal and hexagonal bipyra- sharing two of their equatorial edges with two different 5+ 5+ mids, as well as both I O4 and I O3 polyhedra. The hexagonal bipyramids. The resulting chain is unique

FIG. 13. Miscellaneous sheets of uranyl polyhedra and other polyhedra containing higher-valence cations that contain hexagons. The corresponding compounds are listed in Table 8. HIERARCHY OF U6+ MINERALS AND INORGANIC COMPOUNDS 1873

FIG. 14. Miscellaneous sheets of uranyl polyhedra and other polyhedra containing higher-valence cations. The corre- sponding compounds are listed in Table 9.

4+ 4+ amongst uranyl compounds. Distorted I O4 pyramids pyramids shown in Figure 14e. The Te O4 pyramids, are attached to both sides of the chains of uranyl poly- which are distorted because of a stereoactive lone-elec- hedra by sharing three of their vertices with uranyl poly- tron pair, share vertices to form a chain. These chains 4+ hedra. The I O4 pyramids, which were described for are linked by the sharing of equatorial vertices of the 4+ the fi rst time from this structure, are strongly distorted bipyramids with the Te O4 pyramids. The sheets are by the presence of a stereoactive lone-electron pair. The linked through bonds to the K cations, which are located 5+ complex chains are linked through I O3 pyramids that within the voids in the sheets. also are distorted because of a stereoactive lone-electron The structure of Cs4[(UO2)2(V2O7)O2] contains the pair, and these pyramids share one vertex with the chain sheets of uranyl square bipyramids and vanadate tetra- on either side. The sheets are linked through interlayer hedra shown in Figure 14f. The uranyl square bipyra- Ag cations. mids share equatorial vertices, resulting in a chain one The structure of Ag6[(UO2)3O(MoO4)5] contains polyhedron wide that is identical to those found isolated unusual sheets of uranyl pentagonal bipyramids and as chains in several anhydrous compounds (Fig. 5a). molybdate tetrahedra (Fig. 14d). Trimers of uranyl The chains are linked into sheets by sharing equato- pentagonal bipyramids result from the sharing of edges, rial vertices with vanadate tetrahedra of pyrovanadate and these polyhedra share one common O atom. The groups. Sheets are linked through Cs cations in the trimers are connected by sharing vertices with molyb- interlayer. date tetrahedra that share one ligand with each of two The structure of Tl3{(UO2)2[Te2O5(OH)](Te2O6)}• such trimers. There is also a molybdate tetrahedron 2H2O has the sheets of uranyl pentagonal bipyramids 4+ that bridges between two uranyl polyhedra of the same and Te O4 pyramids shown in Figure 14g. The 4+ trimer. This sheet is unusual in that it has the only occur- distorted Te O4 pyramids share vertices, resulting in rence of an isolated trimer of uranyl polyhedra. Similar chains similar to those in K[UO2Te2O5(OH)] (Fig. 14e). 4+ trimers exist in other sheets, but in all other cases they The chains of Te O4 pyramids are linked by sharing are linked directly by the sharing of vertices and edges equatorial vertices of the uranyl pentagonal bipyra- between the uranyl polyhedra. The sheets are connected mids, and sheets are linked through the Tl cations in through bonds to interlayer Ag cations. the interlayer. The structure of K[UO2Te2O5(OH)] contains the The structure of Cs6[(UO2)4(W5O21)(OH)2(H2O)2] 4+ sheets of uranyl square bipyramids and distorted Te O4 contains unusual sheets of uranyl pentagonal bipyra- 1874 THE CANADIAN MINERALOGIST mids, tungstate octahedra, and tungstate square polyhedron wide (Fig. 15e). Identical chains are known pyramids (Fig. 14h). Four pentagonal bipyramids from the structures of moctezumite and (UO2)Cl2(H2O). share vertices, forming a ring, with a tungstate square Linkage of the chains is through a cation–cation inter- pyramid located at the center of the ring, such that its action in which some of the O atoms of the uranyl equatorial edges are also equatorial edges of the uranyl ion also are equatorial ligands of the bipyramids of polyhedra. The resulting clusters are linked to four adjacent chains. identical clusters through sharing edges and vertices The structure of (NH4)3(H2O)2{[(UO2)10O10(OH)] with dimers of tungstate octahedra that are formed by [(UO4)(H2O)2]} is an infi nite framework of uranium sharing an edge. polyhedra (Fig. 15f), fi ve of which are uranyl square or pentagonal bipyramids, whereas the sixth is better STRUCTURES CONTAINING INFINITE described as a distorted octahedron. Uranyl square FRAMEWORKS OF POLYHEDRA and pentagonal bipyramids are linked by sharing vertices, resulting in a sheet based upon the -U3O8 Burns et al. (1996) included only 26 structures in anion-topology that is topologically identical to those their hierarchy that contained frameworks with uranyl in ianthinite and spriggite (Fig. 11d). The square sites polyhedra. Through the detailed work of several groups of the -U3O8 anion-topology contain both the uranyl 6+ in synthesizing new structures, this structural class has square bipyramids and the distorted U 6 octahedra. now ballooned to 56 (Table 10). Two additional uranyl pentagonal bipyramids share an equatorial edge, giving a dimer. The sheets are stacked Frameworks of uranium polyhedra along [001], and are connected into a framework by linkages to the dimer of bipyramids located between Seven framework structures are based solely on the sheets. The NH4 and H2O groups are located uranium polyhedra. Four of these are polymorphs of within channels that extend parallel to the sheets of UO3 (Figs. 15a–d). These structures involve a range polyhedra. 6+ of U coordination polyhedra, and two (-UO3 and The compound Pb2(H2O)[(UO2)10UO12(OH)6 -UO3) do not contain uranyl ions. The structure of (H2O)6] is the fi rst Pb uranyl oxide hydrate reported to (UO2)Cl2 does contain a uranyl ion, but four of the possess a framework structure (Fig. 15g). It has both equatorial vertices of the pentagonal bipyramids in the uranyl square and pentagonal bipyramids, as well as structure are Cl atoms. The bipyramids are linked by one U6+ site that is in somewhat distorted octahedral the sharing of edges, resulting in chains that are one coordination. The structure contains complex chains HIERARCHY OF U6+ MINERALS AND INORGANIC COMPOUNDS 1875 1876 THE CANADIAN MINERALOGIST

FIG. 15. Frameworks of uranyl polyhedra and other polyhedra containing higher-valence cations. The corresponding compounds are listed in Table 10. HIERARCHY OF U6+ MINERALS AND INORGANIC COMPOUNDS 1877 1878 THE CANADIAN MINERALOGIST of vertex- and edge-sharing uranium polyhedra that (001) by sharing vertices with uranyl square bipyramids, are closely related to the chains of polyhedra that make such that each of the equatorial ligands of the square up the curite sheet (Fig. 11l). The Pb2+ cations are bipyramid is shared with a tetrahedron from a different located in channels extending through the framework ring. The sheets are linked into a framework through of polyhedra. additional uranyl square bipyramids, such that the four vertices of the silicate tetrahedra are shared with two Frameworks of uranium polyhedra tetrahedra within the ring, and two bipyramids. Note and silicate tetrahedra that similar four-membered rings of silicate tetrahedra occur in KNa3[(UO2)2(Si4O10)2](H2O)4, in which they The structures of six framework uranyl silicates, two are linked directly to form silicate sheets (Fig. 15h). of which are minerals, and one uranyl germanate, have been described. In each structure, uranyl polyhedra share Frameworks of uranyl pentagonal bipyramids vertices or edges with silicate (or germanate) tetrahedra, and phosphate, arsenate, or vanadate tetrahedra and the frameworks are composed entirely of uranyl polyhedra and silicate (or germanate) tetrahedra. The compounds [(UO2)3(PO4)2](H2O)4, (UO2) The compound KNa3[(UO2)2(Si4O10)2](H2O)4 was [(UO2)(AsO4)]2(H2O)4, (UO2)[(UO2)(AsO4)]2(H2O)5, fi rst found as an alteration phase of actinide-bearing and (UO2)[(UO2)(VO4)]2(H2O)5 each contain topologi- borosilicate glass under consideration as a contain- cally identical sheets of uranyl pentagonal bipyramids ment material for nuclear waste (Burns et al. 2000). and tetrahedra, as shown in Figure 15m. The sheet is The structure contains sheets of vertex-sharing silicate based upon the uranophane anion-topology (Fig. 10a), tetrahedra involving both four- and eight-membered but is a novel graphical isomer. Note that all of the rings of tetrahedra (Fig. 15h). Adjacent silicate sheets tetrahedra that share edges with bipyramids of any given are connected through uranyl square bipyramids, such chain of bipyramids are oriented in the same direction, that apical ligands of the silicate tetrahedra correspond in contrast to the uranyl silicate sheets shown in Figure to all four of the equatorial vertices of the bipyramid 10. In all four structures, adjacent sheets are connected (Fig. 15h). Na4[(UO2)2(Si4O10)2](H2O)4 was later through uranyl pentagonal bipyramids located between synthesized, and has an identical uranyl silicate frame- the sheets, resulting in frameworks. Each of the bipyra- work (Li & Burns 2001a). mids between the sheets shares one of its equatorial Soddyite and the isostructural compound [(UO2)2 vertices with a tetrahedron in the sheet on either side, (GeO4)(H2O)2] contain uranyl pentagonal bipyramids and the remaining three equatorial vertices are H2O. In that form chains by sharing equatorial edges (Fig. the cases of [(UO2)3(PO4)2](H2O)4 (Fig. 15m), (UO2) 15i). Each bipyramid also shares an edge with a tetra- [(UO2)(VO4)]2(H2O)5, and (UO2)[(UO2)(AsO4)]2(H2O)4 hedron, and the single unshared equatorial vertex of (Fig. 15n), the sheets of polyhedra are relatively fl at. In the bipyramid is occupied by H2O. Adjacent uranyl the case of (UO2)[(UO2)(AsO4)]2(H2O)5, the sheets are silicate (germanate) chains are directly linked as each strongly corrugated (Fig. 15o). tetrahedron shares two edges with bipyramids from The compounds Cs2(UO2)[(UO2)(PO4)]4(H2O)2, two different chains. Additional linkages arise through Rb2(UO2)[(UO2)(PO4)]4(H2O)2, K2(UO2)[(UO2) H bonds. (PO4)]4(H2O)2, Cs2(UO2)[(UO2)(AsO4)]4(H2O)2, and The structure of weeksite (Fig. 15j) contains sheets Rb2(UO2)[(UO2)(AsO4)]4(H2O)4.5 possess homeotypic of uranyl pentagonal bipyramids and silicate tetrahedra framework structures (Fig. 15p). Each has sheets of that are topologically identically to those found in uranyl pentagonal bipyramids and tetrahedra that are haiweeite. In the case of haiweeite, adjacent sheets are based upon the underlying uranophane anion-topology connected through bonds to Ca cations located in the (Fig. 10a). In each structure, the sheet is the same interlayer. In weeksite, the sheets are connected into a graphical isomer as found in -uranophane. Adjacent framework through the apical O atoms of the silicate sheets are connected into a framework by sharing tetrahedra, which provides for channels extending vertices with uranyl pentagonal bipyramids located parallel to the sheets that contain lower-valence cations between the sheets. Each interlayer bipyramid shares and H2O. four of its equatorial vertices with tetrahedra, two from 6+ The structure of Na2[(UO2)(SiO4)] contains U in each of the adjacent sheets. The remaining equatorial distorted octahedral coordination, with four short (~2 vertex of the bipyramid is H2O. The low-valence cations 6+ 6+ Å) and two longer (~2.3 Å) U –O bonds. Each U O6 and additional H2O are located in channels extending octahedron shares four of its vertices with silicate through the framework. tetrahedra, and Na cations are located in channels in The structure of [(UO2)2V2O7] (Fig. 15q) is a frame- the framework (Fig. 15k). work of uranyl pentagonal bipyramids and vanadate The compound RbNa[(UO2)(Si2O6)](H2O) contains tetrahedra. It contains sheets that are based upon the rings of four silicate tetrahedra formed by the sharing uranophane anion-topology, with the same graphical of tetrahedral vertices (Fig. 15l). Identical rings of isomer as found for [(UO2)3(PO4)2](H2O)4, (UO2) tetrahedra are linked into sheets that are parallel to [(UO2)(AsO4)]2(H2O)4, (UO2)[(UO2)(AsO4)]2(H2O)5, HIERARCHY OF U6+ MINERALS AND INORGANIC COMPOUNDS 1879 and (UO2)[(UO2)(VO4)]2(H2O)5 (Fig. 15m). However, these two types of polyhedra, yet the structural connec- in the structure of [(UO2)2V2O7], there are no uranyl tivities are remarkably diverse. In Ba(UO2)3(MoO4)4 ions located between the sheets; rather, direct sharing (H2O)4 and Mg(UO2)3(MoO4)4(H2O)8, only four of the of apical ligands of the vanadate tetrahedra connects the equatorial vertices of the bipyramids are shared with sheets into a framework. tetrahedra, with the fi fth position occupied by H2O. In The structure of Na[(UO2)4(VO4)3] is an unusually -(UO2)(MoO4)(H2O)2 and Sr(UO2)6(MoO4)7(H2O)19, complex framework composed of uranyl pentagonal all of the equatorial vertices of the bipyramids are bipyramids and vanadate tetrahedra (Fig. 15r). The shared with tetrahedra. The ratios of bipyramids to bipyramids are linked by sharing edges into chains tetrahedra range considerably in these four structures; that are one bipyramid wide. Vanadate tetrahedra share in those with more tetrahedra relative to bipyramids, equatorial edges of the bipyramids on either side of the fewer of the vertices of the tetrahedra are linked chain, in the same fashion as occurs in sheets based to bipyramids. In Mg(UO2)3(MoO4)4(H2O)8 and upon the uranophane anion-topology. However, the Ba(UO2)3(MoO4)4(H2O)4, channels through the frame- uranyl vanadate chains in Na[(UO2)4(VO4)3] are not work are bounded by O atoms of the uranyl ions, as well aligned to form sheets, but rather occur in the structure as by some apical (unshared) ligands of the tetrahedra. in two mutually perpendicular orientations, and are In Sr(UO2)6(MoO4)7(H2O)19 and Ca[(UO2)6(MoO4)7 linked into a framework with channels that accommo- (H2O)2](H2O)n, channels in the framework are delin- date the Na cations. eated by apical O atoms of tetrahedra. The structure of (NH4)2[(UO2)5(MoO4)7](H2O)5 Frameworks of uranyl polyhedra warrants special attention owing to the presence of large and molybdate tetrahedra channels through the framework of uranyl pentagonal bipyramids and molybdate tetrahedra (Fig. 15y). Each Twelve uranyl molybdate compounds, corre- of the fi ve symmetrically distinct uranyl pentagonal sponding to seven distinct crystal structures, are frame- bipyramids share all fi ve of their equatorial vertices works of uranyl pentagonal bipyramids and molybdate with tetrahedra, whereas four of the seven distinct tetrahedra. The considerable diversity of this chemical tetrahedra share each of their vertices with bipyra- group is related to the fl exibility of the linkage between mids, and the remaining are each connected to three the bipyramids and tetrahedra, which show a range bipyramids. The considerable range of U–O–Mo bond of U–O–Mo bond angles (e.g., Krivovichev & Burns angles in the structure, from 123.7 to 166.7°, permits 2003b). the unprecedented connectivity observed in this struc- The compounds Cs2[(UO2)6(MoO4)7(H2O)2], ture (Krivovichev et al. 2003). The structure contains (NH4)2[(UO2)6(MoO4)7(H2O)2], and Rb2[(UO2)6 strips of bipyramids and tetrahedra that spiral about the (MoO4)7(H2O)2] are isostructural (Fig. 15s). There are tubular openings along [001], with a primitive repeat- two types of uranyl pentagonal bipyramids, those that distance of 70.6 Å. The resulting framework contains share each of their equatorial vertices with molybdate several channels, the largest of which extends along tetrahedra, and those that share only four of their equa- [001] with a cross-section of 7.5 by 7.5 Å. The (NH4) torial vertices with molybdate tetrahedra, with the fi fth cations and H2O groups are located within the various vertex corresponding to H2O. All of the molybdate channels. tetrahedra link to four different bipyramids. The lower- valence cations are located in irregular voids within the Miscellaneous frameworks framework of polyhedra. The compounds -Cs2[(UO2)2(MoO4)3], Rb2[(UO2)2 Pyrophosphate P2O7 groups link uranyl pentagonal (MoO4)3] and Tl2[(UO2)2(MoO4)3] are isostructural bipyramids into a framework in the structure of (Fig. 15t). Each bipyramid shares all fi ve of its equato- Na2[(UO2)(P2O7)] (Fig. 15z). One phosphate tetrahe- rial vertices with molybdate tetrahedra, and the tetra- dron of the pyrophosphate group shares an equatorial hedra are either linked to three or four bipyramids. edge of a bipyramid, a vertex with another adjacent The structure contains unusually large channels that bipyramid, and one vertex with the other tetrahedron extend along [001] and [100], with dimensions 3.5 of the pyrophosphate group. The other phosphate 10.5 Å and 4.8 4.8 Å, respectively. The Cs and Rb tetrahedron shares a vertex with one bipyramid, and cations are located within the channels through the the remaining two vertices are unshared within the framework. framework. The Na cations are located within voids in The structures of Ba(UO2)3(MoO4)4(H2O)4 the framework. (Fig. 15u), -(UO2)(MoO4)(H2O)2 (Fig. 15v), Sr(UO2)6 The compounds -(UO2)(SeO4), -(UO2)(SO4), (MoO4)7(H2O)19 (Fig. 15w), Mg(UO2)3(MoO4)4 (H2O)8 and (UO2)(MoO4) have unusual frameworks that (Fig. 15x) and Ca[(UO2)6(MoO4)7(H2O)2](H2O)n include cation–cation interactions (Fig. 15aa). Uranyl (Fig. 15w) further illustrate the fl exibility of the linkage pentagonal bipyramids are linked into single-width between uranyl pentagonal bipyramids and molybdate chains extending along [010]. Each linkage between the tetrahedra. Each involves only vertex-sharing between bipyramids occurs through a cation–cation interaction, 1880 THE CANADIAN MINERALOGIST meaning that one of the uranyl ion O atoms of each The lithium uranyl tungstates Li(UO2)(WO4)2 bipyramid is also an equatorial vertex of an adjacent and Li2(UO2)4(WO4)4O both contain distorted WO6 bipyramid within the chain. The chains of bipyramids octahedra and uranyl pentagonal bipyramids, and are linked through tetrahedra, and each of the tetrahedra Li2(UO2)4(WO4)4O also has uranyl square bipyramids. shares all four of its vertices with bipyramids of the The structure of Li(UO2)(WO4)2 contains sheets of chains. In the case of the bipyramids, one of the equa- vertex-sharing WO6 octahedra that are linked through torial positions corresponds to the cation–cation inter- vertex-sharing with the uranyl polyhedra (Fig. 15ag). action, and each of the others is linked to tetrahedra. Note that uranyl polyhedra are linked into chains by The structure of [(UO2)3(PO4)O(OH)(H2O)2](H2O) vertex-sharing. These chains are identical to those has structural elements in common with the phos- found in sheets based upon the uranophane anion- phuranylite group of minerals (Fig. 15ab). The topology (Fig. 10). As noted by Obbade et al. (2004c), framework contains uranyl pentagonal and hexagonal insertion of additional chains of uranyl pentagonal bipyramids, as well as phosphate tetrahedra. Pentagonal bipyramids in the voids occupied by Li cations would bipyramids share an equatorial edge, forming a dimer. result in uranyl tungstate sheets with the uranophane The dimers are linked into chains by sharing edges with anion-topology. The structure of Li2(UO2)4(WO4)4O hexagonal bipyramids, such that four of the equatorial is composed of slabs of uranyl pentagonal bipyramids edges of the hexagonal bipyramids are shared with and WO6 octahedra that are based upon the uranophane pentagonal bipyramids, two on either side. The two anion-topology (Fig. 15ah). These slabs are linked into equatorial edges of the hexagonal bipyramids that are a framework by sharing vertices with uranyl square not shared with pentagonal bipyramids are shared with bipyramids. phosphate tetrahedra, resulting in uranyl phosphate The perovskite-like structures of Ba2MgUO6 and chains that are topologically identical to those found as K9BiU6O24 (Figs. 15ai, aj) can be described as deriva- parts of sheets in the phosphuranylite group of minerals. tives of the -UO3 structure-type (Fig. 15d). In both 6+ The H2O groups are located at the non-chain equato- cases, the U cations are in holosymmetric octahedral rial vertices of the uranyl pentagonal bipyramids. The coordination, with no uranyl ion present. These struc- uranyl phosphate chains are in two mutually perpen- tures are derived from that of -UO3 by doubling the dicular orientations, and are linked directly through the unit-cell edges, replacing some of the UO6 octahedra phosphate tetrahedra, as each tetrahedron shares one with either MgO6, BiO6 or KO6 octahedra, and with edge with each of two different chains (Fig. 15ab). The Ba or K cations inserted into the 12-coordinated sites resulting framework has channels along [001] that are of the channels. delineated by uranyl phosphate chains on either side. The structures of -Cd(UO2)O2, Cu(UO2)O2, and The framework is electroneutral, with only H2O groups Mn(UO2)O2 (Fig. 15ak) are closely related, although located in the channels. they are not strictly isostructural, as they crystallize The compounds Pb2[(UO2)(TeO3)3] (Fig. 15ad), in different space-groups. Each contains uranyl square Na8[(UO2)6(TeO3)10] (Fig. 15ae) and cliffordite (Fig. bipyramids and octahedra that are occupied by the diva- 4+ 15ad) all contain Te cations that have stereoactive lent cations. The OUr atoms are part of the octahedral lone-electron pairs. Typically, the Te4+ cation is strongly coordination of the divalent cations, and as a result the bonded to three atoms of O with bond lengths of ~1.9 U6+–O bond lengths are longer than usual. Å, as well as some additional anions located about 2.5 The structure of UCr2O6 involves a framework of to 3.0 Å from the cation. The three short bonds about uranyl square bipyramids and chromate octahedra (Fig. the cation are on one side, and the coordination poly- 15al). The chromate octahedra share edges, resulting in hedron is a pyramid that is capped by the cation. The brucite-like sheets, but one-third of the octahedral sites structures of Pb2(UO2)(TeO3)3 and Na8[(UO2)6(TeO3)10] are vacant. The uranyl square bipyramids are located each contain uranyl pentagonal bipyramids that share between the chromate sheets, in correspondence with equatorial vertices and edges with TeO3 pyramids to the vacant sites of the sheets of octahedra. 4+ 4+ form the framework. Cliffordite is unusual in that it is The compound U (UO2)(PO4)2 contains both U the only framework structure for which all of the U is coordinated by seven O atoms, and U6+ present as the in hexagonal bipyramidal coordination. typical uranyl pentagonal bipyramid. The uranium poly- The structure of K2[(UO2)2(VO)2(IO6)2O](H2O) is a hedra share edges, giving chains that are cross-linked rather open framework of uranyl square bipyramids and into a framework by sharing edges and corners with distorted VO6 and IO6 octahedra (Fig. 15af). The octa- PO4 tetrahedra (Fig. 15am). hedra share edges, forming a chain that extends along The compound Cs[(UO2)3(HIO6)(OH)(O)(H2O)] [001]. The chains are linked into ribbons by sharing (H2O)15, as well as the isostructural Li, Na, K and Rb polyhedron vertices, and these ribbons are linked into compounds, have the unusual framework of uranyl the framework through the equatorial vertices of the pentagonal bipyramids and periodate octahedra shown uranyl square bipyramids. The K cations and H2O in Figure 15an. The most unusual aspect of this rela- groups are located in channels that extend through the tively open framework is that some of the linkages framework (Fig. 15af). between uranyl pentagonal bipyramids are through HIERARCHY OF U6+ MINERALS AND INORGANIC COMPOUNDS 1881 cation–cation interactions, making this one of the few uranyl compounds to show this type of connectivity. Sullens et al. (2004) suggested that the chelation of the U(3) cation, which is involved in the cation–cation inter- 4– action, by the HIO6 ion results in transfer of signifi cant 4– electron-density from the HIO6 ion to the uranyl ion, thus stabilizing the cation–cation interaction.

NANOSTRUCTURE MATERIALS

Nanostructure materials exhibit important properties with promise for many potential applications. Such materials also provide unique opportunities to study fundamental structure–property relationships, and the properties of many nanoscale materials are measurably particle-size dependent. Self-assembling small clusters have been reported based on main-group elements, most notably C-60-buckminsterfullerene (Kroto et al. 1985), and transition-metal oxide clusters, including a wide variety of polyoxometalates built on WO, MoO, and NbO framework structures (Müller et al. 1998). Following the discovery of carbon nanotubes, consider- able attention has also been devoted to synthesizing a FIG. 16. U–32 uranyl peroxide nanosphere formed by the linkage of 32 chemically identical uranyl peroxide poly- range of oxidic nanotubular materials, which also have hedra. From Burns et al. (2005).

FIG. 17. Nanoscale tubules in a uranyl selenate. From Krivovichev et al. (2005a). 1882 THE CANADIAN MINERALOGIST tremendous potential in materials applications. Prior to tures contain isolated uranyl polyhedra. In 1996, Burns 2005, no materials have been reported with nanoscale et al. placed 180 structures in their hierarchy, of which aggregates or tubules of higher-valence actinide ions. 56 are minerals, 106 contain sheets, 19 contain chains, Burns et al. (2005) described an unprecedented 22 contain clusters, 26 are frameworks, and seven family of actinyl peroxide nanoclusters. These chemi- contain isolated uranyl polyhedra. Structures discov- cally and structurally elegant spherical clusters were ered since 1996 have revealed many novel topological obtained from alkaline solutions containing peroxide arrangements of uranyl polyhedra and other polyhedra under ambient conditions. The clusters are assembled containing higher-valence cations. Many of the new from topologically identical uranyl peroxide polyhedra, structures also correspond to topological arrangements and contain 24, 28 or 32 uranyl peroxide polyhedra that were known in 1996, and they expand the range of within the clusters (e.g., Fig. 16). Burns et al. (2005) compositions known to adopt a given topology. provided evidence for the self-assembly and persistence Tremendous advances have been made in under- of the some of these clusters in alkaline solutions, standing the crystal chemistry of uranyl compounds, prior to crystallization in complex molecular crystals. including the structures of uranyl minerals. The struc- The assembly and persistence of these nanoclusters in tures of virtually all of the common uranyl minerals solution suggest that they may infl uence the mobility have been resolved, but the structures are still either of actinides in the environment, such as where highly unknown or unrefi ned for about half of the known uranyl alkaline nuclear wastes leak into the vadose zone. The minerals, many of which are extremely rare. There has complexity of the clusters, which constitute a new been extensive exploration of many of the chemical class of polyoxometalates, is in sharp contrast to other classes of inorganic uranyl compounds, and much of clusters of cation polyhedra of higher bond-valence in the recent work has been focused on uranyl compounds uranyl compounds that are illustrated in Figure 4. containing tetrahedrally coordinated hexavalent cations Krivovichev et al. (2005a, b) reported the synthesis such as Mo, S, and Cr, as well as cations that contain and structures of two remarkable new uranyl selenate stereoactive lone-electron pairs such as Te4+ and Se4+. compounds that contain nanoscale tubules (Fig. 17). In Despite the considerable effort expended on structural each case, uranyl pentagonal bipyramids share vertices studies of such chemical classes of uranyl compounds, with selenate tetrahedra, resulting in open sheets that are new structural units and topologies continue to be found rolled into single-walled tubes, with external diameters on a regular basis, which suggests that many more novel of 17 and 25 Å, and inner crystallographically free structural connectivities await discovery. diameters of 4.7 and 12.6 Å. This discovery presents In developing the current structural hierarchy, I the fascinating possibility that many of the myriad of have attempted to provide a cohesive framework for sheets containing uranyl polyhedra may be able to form the discussion and understanding of the structures of nanoscale tubules. The range of sheet topologies and inorganic uranyl compounds. I consider this step to compositions suggests many potential applications of be essential toward application of knowledge of these such materials. structures to mineralogical, geological, environmental and technological issues. Just one example of the impor- SUMMARY AND FUTURE OUTLOOK tance of understanding the structures and chemical composition of uranyl minerals and compounds lies in The convergence of greatly enhanced technologies the area of nuclear waste disposal. Alteration of nuclear for detection of X-rays in single-crystal experiments waste forms such as spent nuclear fuel and actinide- with a renewed interest in the crystal chemistry of U6+ bearing waste glasses in a geological repository under has resulted in an explosion of new structures containing moist oxidizing conditions will obviously result in U6+. During the past decade, known structures of uranyl complex suites of uranyl compounds. Some of these compounds has more than doubled. These studies have alteration phases will incorporate radionuclides into further revealed the tremendous structural elegance, their structures, which is important for the long-term beauty, and complexity of uranyl compounds, which are performance of the repository. Theoretical predictions refl ected in the many Figures of this manuscript. of such incorporation mechanisms, as well as experi- Of the 368 structures covered in this structural hier- mental verifi cation of incorporation require a detailed archy, 204 (55.4%) are based upon sheets of polyhedra understanding of the crystal structures. Determina- containing cations of higher valence, and of the 89 tion of the thermodynamic stabilities and solubilities minerals, 69 (77.5%) contain sheets. Structures based of uranyl compounds relies heavily on the structural upon infi nite chains and frameworks of polyhedra are formulae derived from crystallographic studies, as the about equally abundant, and correspond to 57 (15.5%) compositions of many of these phases were incorrectly and 56 structures, respectively. Ten uranyl minerals are assigned in the absence of knowledge of their structures. classed as infi nite-chain structures, whereas only three Synthesis and structural analysis have permitted the have frameworks of polyhedra. There are 43 uranyl discovery of many new uranyl compounds that may be compounds with structures based upon isolated clusters relevant to repository performance. The ultimate predic- of polyhedra, of which seven are minerals. Eight struc- tion of thermodynamic properties and occurrences HIERARCHY OF U6+ MINERALS AND INORGANIC COMPOUNDS 1883 of uranyl compounds on the basis of their structural tremendous contributions in the development of connectivities is a tantalizing prospect, but this goal our original hierarchy of uranyl structures that was requires a detailed understanding of the crystal chem- published in 1996. My research group at the Univer- istry of uranium. sity of Notre Dame has contributed in many ways to Reports of both nanoscale clusters containing uranyl my understanding of uranyl crystal chemistry; I am polyhedra and nanotubular uranyl compounds appeared grateful to all members of my group, past and present: in the spring of 2005, and represent a new direction post-doctoral fellows Sergey Krivovichev, Christopher in the crystal chemistry of uranium and actinides in Cahill, Robert Shuvalov, Francis Hill, Philip Almond, general. Discovery of these novel materials occurred Oksana Kurmakova, graduate students Yaping Li, after many years of research concerning uranyl crystal Andrew Locock, Karrie-Ann Hughes, Jennifer Jackson, chemistry, at a time when the casual observer might Tori Ziemann, Paul Giesting, Amanda Klingensmith, have considered studies of uranium crystal chemistry Cara Alexopoulos, Bridget McCollam, undergraduates to be a relatively mature fi eld. Now, there are many Erin Keppel, Alex Garza, Rebecca Glatz, Kara Tholen, research opportunities concerning nanoscale uranium- Andrew Irvine, Aaron Martin, Kathryn Deely, Leslie based materials, including their importance for under- Hayden, Jessica Callahan, Theodore Flynn, Lauren standing chemical processes involving actinides, the Sturm, Peter Hotchkiss, Virginia Kelly, Nathan Porter, potential for novel materials applications, and the Patrick Horan, Karen Carlaccini, Ginger Sigmon, Maha impact of such materials on transport of actinides in Jain, Jinalee Almes, and Erin Diminick. Leslie Hayden the environment. prepared many preliminary versions of the fi gures of Unlike U6+, relatively few crystal structures have uranyl compounds used in this manuscript. been analyzed that contain neptunium or plutonium I thank Prof. Thomas Albrecht-Schmitt and Prof. in higher oxidation states (5+ or 6+). The degree to Sergey Krivovichev for their reviews of this manuscript, which the considerable knowledge concerning U6+ and Prof. Krivovichev for providing preprints of several crystal chemistry may be transferable to Np and Pu of his papers. Thanks also to Prof. Frank C. Hawthorne compounds is unclear. The few structures of Np5+, Np6+, and Prof. Robert F. Martin for editing the manuscript. Pu5+ and Pu6+ compounds that are known do contain I acknowledge fi nancial support for several years the same types of coordination polyhedra as U6+, actinyl of study of uranyl crystal chemistry from the Environ- ions in square, pentagonal and hexagonal bipyramidal mental Management Sciences Program of the United polyhedra. As such, one might expect that Np and Pu States Department of Energy (DE–FG07–97ER14820), structures will in many ways be similar to those already the National Science Foundation for my studies of Pb known from uranyl compounds. This may largely be the uranyl oxide hydrate minerals (EAR98–04723), and case for Np6+ and Pu6+, although their slightly longer the National Science Foundation for the Environmental bond-lengths, as compared with U6+, may lead to some Molecular Science Institute at the University of Notre novel structural connectivities in structures where the Dame (EAR02–21966). I also acknowledge current sharing of edges between actinyl polyhedra and poly- funding from the Offi ce of Science, Technology and hedra containing higher-valence cations is important. International of the United States Department of In the cases of Np5+ and Pu5+, the actinyl ion has a Energy that will permit the continuation of this line formal valence of 1+, and the bond strengths within of research. the actinyl ions are likely to be somewhat lower than in the case where a hexavalent cation is present. As REFERENCES such, the O atoms of the actinyl ions containing Np5+ and Pu5+ will be more readily shared with other actinyl ÅBERG, M. (1969): The crystal structure of [(UO2)2(OH)2Cl2 polyhedra, thus cation–cation interactions are likely (H2O)4]. Acta Chem. Scand. 23, 791-810. to be much more important than in uranyl structures. ______(1976): The crystal structure of [(UO ) Cl O (OH) The few structures of Np5+ compounds that are in the 2 4 2 2 2 (H2O)6]•4H2O a compound containing a tetranuclear aqua- literature, as well as about a dozen structures my group chlorohydroxooxo complex of uranyl(VI). Acta Chem. and I have recently determined, commonly do contain Scand. A30, 507-514. cation–cation interactions, which results in dramatically different structures as compared to uranyl compounds ______(1978) The crystal structure of hexaaqua-tri- with similar chemical compositions. -hydroxo- 3-oxo-triuranyl(VI) nitrate tetrahydrate, [(UO2)3O(OH)3(H2O)6]NO3•4H2O. Acta Chem. Scand. 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ALCOCK, N.W., ROBERTS, M.M. & BROWN, D. (1982): Actinide I acknowledge the Mineralogical Association of structural studies. 3. The crystal and molecular structures Canada for support and encouragement throughout of UO2SO4•H2SO4•5H2O and (NpO2SO4)2•H2SO4•4H2O. my ongoing education and career. I am grateful to J. Chem. Soc., Dalton Trans., 869-873. Prof. Rodney Ewing and Dr. Mark Miller for their 1884 THE CANADIAN MINERALOGIST

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______(2001a): A new uranyl sulfate chain in the structure ______& HUGHES, K.-A. (2003): Studtite, [(UO2)(O2) of uranopilite. Can. Mineral. 39, 1139-1146. (H2O)2](H2O)2: the fi rst structure of a peroxide mineral. Am. Mineral. 88, 1165-1168. ______(2001b): A new uranyl silicate sheet in the structure of haiweeite and comparison to other uranyl silicates. Can. ______, HUGHES KUBATKO, K.-A., SIGMON, G., FRYER, B.J., Mineral. 39, 1153-1160. GAGNON, J.E., ANTONIO, M.R. & SODERHOLM, L. (2005): Actinyl peroxide nanospheres. Angewandte Chemie Inter- ______, ALEXOPOULOS, C.M., HOTCHKISS, P.J. & LOCOCK, national Edition 44, 2135-2139. A.J. (2004b): An unprecedented uranyl phosphate frame- work in the structure of [(UO2)3(PO4)O(OH)(H2O)2](H2O). ______& LI, YAPING (2002): The structures of becquerel- Inorg. Chem. 43, 1816-1818. ite and Sr-exchanged becquerelite. Am. Mineral. 87, 550-557. ______& DEELY, K.M. (2002): A topologically novel sheet of uranyl pentagonal bipyramids in the structure ______, MILLER, M.L. & EWING, R.C. (1996): U6+ miner- of Na[(UO2)4O2(OH)5](H2O)2. Can. Mineral. 40, 1579- als and inorganic phases: a comparison and hierarchy of 1586. structures. Can. Mineral. 34, 845-880.

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______, CAHILL, C.L. & BURNS, P.C. (2001b): Synthesis, and X-ray absorption spectra of carnotite-type uranyl structural characterization, and topological rearrangement chromium(V) oxides: A[(UO2)2Cr2O8](H2O)n (A = K2, of a novel open framework U–O material: (NH4)3(H2O)2 Rb2, Cs2, Mg; n = 0, 4). Chem. Mater. 16, 1384-1390. {[(UO2)10O10(OH)][(UO4)(H2O)2]}. Chem. Mater. 13, 4026-4031. LOOPSTRA, B.O. (1970): The structure of -U3O8. Acta Crys- tallogr. B26, 656-657. ______, KRIVOVICHEV, S.V. & BURNS, P.C. (2001a): The crystal structure of Na4(UO2)(CO3)3 and its relationship to ______(1977): On the crystal structure of -U3O8. J. Inorg. schröckingerite. Mineral. Mag. 65, 297-304. Nucl. Chem. 39, 1713-1714.

LINDE, S.A., GORBUNOVA, Y.E. & LAVROV, A.V. (1980): For- ______& BRANDENBURG, N.P. (1978): Uranyl selenite and mation of K4UO2(PO4)2 crystals. Zh. Neorg. Khim. 25, uranyl tellurite. Acta Crystallogr. B34, 1335-1337. 1992-1994 (in Russ.). ______& CORDFUNKE, E.H.P. (1966): On the structure of ______, ______, ______& KUZNETSOV, V.G. (1978): -UO3. Rec. Trav. Chim. Pays-Bas 85, 135-142. Synthesis and structure of crystals of CsUO2(PO3)3. Dokl. Akad. Nauk SSSR 242, 1083-1085 (in Russ.). ______& RIETVELD, H.M. (1969): The structure of some alkaline-earth metal uranates. Acta Crystallogr. B25, ______, ______, ______& POBEDINA, A.B. (1984): 787-791. The synthesis and structure of crystals of Na2UO2P2O7, sodium uranyl phosphate. Zh. Neorg. Khim. 29, 1533-1537 ______, TAYLOR, J.C. & WAUGH, A.B. (1977): Neutron (in Russ.). powder profi le studies of the gamma uranium trioxide phases. J. Solid State Chem. 20, 9-19. LOCOCK, A.J. & BURNS, P.C. (2002a): The crystal structure of triuranyl diphosphate tetrahydrate. J. Solid State Chem. MACASKIE, L.E., BONTHRONE, K.M., YONG, PING & GODDARD, 163, 275-280. D.T. (2000): Enzymically mediated bioprecipitation of ura- nium by a Citrobacter sp.: a concerted role for exocellular ______& ______(2002b): Crystal structures of three lipopolysaccharide and associated phosphatase in biomin- framework alkali metal uranyl phosphate hydrates. J. Solid eral formation. Microbiology-UK 146, 1855-1867. State Chem. 167, 226-236. MAKAROV, E.S. & ANIKINA, L.I. (1963): Crystal structure of ______& ______(2003a): The crystal structure of syn- umohoite [UMoO6(H2O)2]•2H2O. Geochemistry, 14-21. thetic autunite, Ca[(UO2)(PO4)]2(H2O)11. Am. Mineral. 88, 240-244. ______& IVANOV, V.I. (1960): The crystal structure of meta-autunite, Ca(UO2)2(PO4)2•6H2O. Dokl. Acad. Sci. ______& ______(2003b): The structure of hügelite, an USSR 132, 601-603. arsenate of the phosphuranylite group, and its relationship to dumontite. Mineral. Mag. 67, 1109-1120. MARSH, R.E. (1988): On the structure of the series of oxides M(II)UMo4O16. J. Solid State Chem. 73, 577-578. ______& ______(2003c): The crystal structure of ber- genite, a new geometrical isomer of the phosphuranylite MAYER, H. & MEREITER, K. (1986): Synthetic bayleyite, group. Can. Mineral. 41, 91-101. Mg2[UO2(CO3)3]•18H2O: thermochemistry, crystallogra- phy and crystal structure. Tschermaks Mineral. Petrogr. ______& ______(2003d): Crystal structures and syn- Mitt. 35, 133-146. thesis of the copper-dominant members of the autunite and meta-autunite groups: torbernite, zeunerite, metatorbernite MEREITER, K. (1982a): The crystal structure of liebigite, and metazeunerite. Can. Mineral. 41, 489-502. Ca2UO2(CO3)3•ë11H2O. Tschermaks Mineral. Petrogr. Mitt. 30, 277-288. ______& ______(2003e): Structures and synthesis of framework Rb and Cs uranyl arsenates and their relation- ______(1982b): The crystal structure of walpurgite, ships with their phosphate analogues. J. Solid State Chem. (UO2)Bi4O4(AsO4)2•2H2O. Tschermaks Mineral. Petrogr. 175, 372-379. Mitt. 30, 129-139.

______& ______(2003f): Structures and synthesis of ______(1982c): Die Kristallstruktur des Johannits, Cu framework triuranyl diarsenate hydrates. J. Solid State (UO2)2(OH)2(SO4)2•8H2O. Tschermaks Mineral. Petrogr. Chem. 176, 18-26. Mitt. 30, 47-57.

______, ______& FLYNN, T.M. (2005): The role of water ______(1986a): Crystal structure and crystallographic in the structures of synthetic hallimondite, Pb2[(UO2) properties of a schröckingerite from Joachimsthal. Tscher- (AsO4)2](H2O)n and synthetic parsonsite, Pb2[(UO2)(PO4)2] maks Mineral. Petrogr. Mitt. 35, 1-18. (H2O)n, 0 ≤ n ≤ 0.5. Am. Mineral. 90, 240-246. ______(1986b): Synthetic swartzite, CaMg[UO2(CO3)3]• ______, SKANTHAKUMAR, S., BURNS, P.C. & SODERHOLM, 12H2O, and its strontium analogue, SrMg[UO2(CO3)3]• L. (2004): Syntheses, structures, magnetic properties, HIERARCHY OF U6+ MINERALS AND INORGANIC COMPOUNDS 1891

12H2O: crystallography and crystal-structures. Neues MURAKAMI, T., OHNUKI, T., ISOBE, H & SATO, T. (1997): Jahrb. Mineral., Monatsh., 481-492. Mobility of uranium during weathering. Am. Mineral. 82, 888-899. ______(1986c): Neue kristallographische Daten ueber das Uranmineral Andersonit. Anz. Oesterr. Akad. Wiss. Math.- NAZARCHUK, E.V., KRIVOVICHEV, S.V. & BURNS, P.C. Naturwiss. Kl. 123(3), 39-41. (2005a): Crystal structure of Tl2[(UO2)2(MoO4)3] and crystal chemistry of the compounds with composition ______(1986d): Structure of strontium tricarbonatodio- M2[(UO2)2(MoO4)3] (M = Tl, Rb, Cs). Radiochem. 47, ouranate(VI) octahydrate. Acta Crystallogr. C42, 1678- 447-451. 1681. ______, ______& ______(2005b): Crystal structure ______(1986e): Crystal structure refinements of two and thermal transformations of Ca[(UO2)6(MoO4)7(H2O)2] francevillites, (Ba,Pb)[(UO2)2V2O8]•5H2O. Neues Jahrb. (H2O)n. Zap. Vseross. Mineral. Obshchest. (in press). Mineral., Monatsh., 552-560. NIINISTÖ, L., TOIVONEN, J. & VALKONEN, J. (1978): Uranyl (VI) ______(1988): Structure of caesium tricarbonatodioxo- compounds. I. The crystal structure of ammonium uranyl uranate(VI) hexahydrate. Acta Crystallogr. C44, 1175- sulfate dihydrate, (NH4)2UO2(SO4)2•2H2O. Acta Chem. 1178. Scand. A32, 647-651.

MEUNIER, G. & GALY, J. (1973): Structure cristalline de la ______, ______& ______(1979): Uranyl (VI) schmitterite synthétique UTeO5. Acta Crystallogr. B29, compounds. II. The crystal structure of potassium uranyl 1251-1255. sulfate dihydrate K2UO2(SO4)2•2H2O. Acta Chem. Scand. A33, 621-624. MIJLHOFF, F.C., IJDO, D.J.W. & CORDFUNKE, E.H.P. (1993): The crystal structure of - and -Cs2U2O7. J. Solid State OBBADE, S., DION, C., BEKAERT, E., YAGOUBI, S., SAADI, M. Chem. 102, 299-305. & ABRAHAM, F. (2003c): Synthesis and crystal structure of new uranyl tungstates M2(UO2)(W2O8) (M = Na, MIKHAILOV, YU.N., GORBUNOVA, YU.E., BAEVA, E.E., SEREZH- K), M2(UO2)2(WO5)O (M = K, Rb), and Na10(UO2)8 KINA, L.B. & SEREZHKIN, V.N. (2001b): Crystal structure (W5O20)O8. J Solid State Chem. 172, 305-318. of Na2(UO2(SeO4)2)•4(H2O). Zh. Neorg. Khim. 46, 2017- 2021. ______, ______, DUVIEUBOURG, L., SAADI, M. & ABRA- HAM, F. (2003a): Synthesis and crystal structure of - and ______, ______, KOKH, L.A., KUZNETSOV, V.G., GREV- -Rb6U5V2O23, a new layered compound. J. Solid State TSEVA, T.G., SOKOL, S.K. & ELLERT, G.V. (1977): Synthe- Chem. 173, 1-12. sis and crystal structure of potassium trisulfatouranylate K4(UO2(SO4)3). Koord. Khim. 3, 508-513. ______, ______, RIVENET, M., SAADI, M. & ABRAHAM, F. (2004a): A novel open-framework with non-crossing ______, ______, SHISHKINA, O.V., SEREZHKINA, L.B. & channels in the uranyl vanadates A(UO2)4(VO4)3 (A = Li, SEREZHKIN, V.N. (2001a): Crystal structure of Cs2((UO2) Na). J. Solid State Chem. 177, 2058-2067. (SeO4)2(H2O))(H2O). Zh. Neorg. Khim. 46, 1828-1832. ______, ______, SAADI, M. & ABRAHAM, F. (2004b): MILLER, M.L., FINCH, R.J., BURNS, P.C. & EWING, R.C. (1996): Synthesis, crystal structure and electrical characterization Description and classifi cation of uranium oxide hydrate of Cs4[(UO2)2(V2O7)O2], a uranyl divanadate with chains sheet anion topologies. J. Mater. Res. 11, 3048-3056. of corner-sharing uranyl square bipyramids. J. Solid State Chem. 177, 1567-1574. MILLER, S.A. & TAYLOR, J.C. (1986): The crystal structure of saleeite, Mg[UO2PO4]2•10H2O. Z. Kristallogr. 177, ______, YAGOUBI, S., DION, C., SAADI, M. & ABRAHAM, 247-253. F. (2003b): Synthesis, crystal structure and electrical characterization of two new potassium uranyl molybdates MISTRYUKOV, V.E. & MIKHAILOV, Y.N. (1983): The character- K2(UO2)2(MoO4)O2 and K8(UO2)8(MoO5)2O6. J. Solid istic properties of the structural function of the selenito- State Chem. 174, 19-31. group in the uranyl complex with neutral ligands. Koord. Khim. 9, 97-102 (in Russ.). ______, ______, ______, ______& ______(2004c): Two new lithium uranyl tungstates Li2(UO2) MOROSIN, B. (1978): Hydrogen uranyl tetrahydrate, a hydrogen (WO4)2 and Li2(UO2)4(WO4)4O with framework based on ion solid electrolyte. Acta Crystallogr. B34, 3732-3734. the uranophane sheet anion topology. J. Solid State Chem. 177, 1681-1694. MUELLER, M.H., DALLEY, N.K. & SIMONSEN, S.H. (1971): Neutron diffraction study of uranyl nitrate dihydrate. ONDRUˇS, P., SKÁLA, R., VESELOVSK´Y, F., SEJKORA, J. & Inorg. Chem. 10, 323-328. VITTI, C. (2003): Čejkaite, the triclinic polymorph of Na4(UO2)(CO3)3 – a new mineral from Jachymov, Czech MÜLLER, A., PETERS, F., POPE, M.T. & GATTESCHI, D. (1998): Republic. Am. Mineral. 88, 686-693. Polyoxometalates: very large clusters – nanoscale magnets. Chem. Rev. 98, 239-271. 1892 THE CANADIAN MINERALOGIST

PADEL, L., POIX, P. & MICHEL, A. (1972): Préparation et étude ______, ______& DECLERCQ, J.-P. (1979): Structure of cristallographique du système Ba2(UMg)O6–Ba2(U2/3 phuralumite. Acta Crystallogr. B35, 1880-1882. Fe4/3)O6. Rev. Chim. Minérale 9, 337-350. ______, ______& DELIENS, M. (1990): Composition PAGOAGA, M.K., APPLEMAN, D.E. & STEWART, J.M.(1987): chimique et structure cristalline de la dewindtite Pb3[H Crystal structures and crystal chemistry of the uranyl (UO2)3O2(PO4)2]2•12H2O. Eur. J. Mineral. 2, 399-405. oxide hydrates becquerelite, billietite, and protasite. Am. Mineral. 72, 1230-1238. PUSHCHAROVSKY, D. YU, RASTSVETAEVA, R.K. & SARP, H. (1996): Crystal structure of deloryite, Cu4(UO2)[Mo2O8] PEARCY, E.C., PRIKRYL, J.D., MURPHY, W.M. & LESLIE, B.W. (OH)6. J. Alloys Comp. 239, 23-26. (1994): Alteration of uraninite from the Nopal I deposit, Peña Blanca District, Chihuahua, Mexico, compared to REIS, A.H., JR., HOEKSTRA, H.R., GEBERT, E. & PETERSON, degradation of spent nuclear fuel in the proposed U.S. S.W. (1976): Redetermination of the crystal structure of high-level nuclear waste repository at Yucca Mountain, barium uranate. J. Inorg. Nucl. Chem. 38, 1481-1485. Nevada. Appl. Geochem. 9, 713-732. RESHETOV, K.V. & KOVBA, L.M. (1966): Structures of PERRIN, A. (1976): Structure cristalline du nitrate de dihy- SrUO(4–x), CdUO(4–x) and Li4UO5. J. Struct. Chem. 7, droxo diuranyle tétrahydraté. Acta Crystallogr. B32, 589-590. 1658-1661. ROH, Y., LEE, S.R., CHOI, S.-K., ELLESS, M.P. & LEE, S.Y. ______(1977): Preparation, étude structurale et vibra- (2000): Physicochemical and mineralogical characteriza- tionnelle des complexes M2U2O5Cl4.2H2O (M = Rb, Cs): tion of uranium-contaminated soils. Soil Sed. Contam. 9, mise en évidence d’un anion tétranucléaire [(UO2)4O2 463-486. 4– Cl8(H2O)2] . J. Inorg. Nucl. Chem. 39, 1169-1172. ROSENZWEIG, A. & RYAN, R.R. (1975): Refi nement of the ______& LE MAROUILLE, J.-Y. (1977): Structure cristalline crystal structure of cuprosklodowskite, Cu[(UO2)2 et moléculaire du complexe tétranucléaire K2(UO2)4O2(O (SiO3OH)2]•6H2O. Am. Mineral. 60, 448-453. H)2Cl4(H2O)6. Acta Crystallogr. B33, 2477-2481. ______& ______(1977a): Kasolite, Pb(UO2)(SiO4)• PIRET, P. (1985): Structure cristalline de la fourmariérite, H2O. Cryst. Struct. Commun. 6, 617-621. Pb(UO2)4O3(OH)4•4H2O. Bull. Minéral. 108, 659-665. ______& ______(1977b): Vandenbrandeite CuUO2 ______& DECLERCQ, J.-P. (1983): Structure cristalline de (OH)4. Cryst. Struct. Commun. 6, 53-56. l’upalite Al[(UO2)3O(OH)(PO4)2]•7H2O. Un exemple de macle mimétique. Bull. Minéral. 106, 383-389. ROSS, M. & EVANS, H.T., JR. (1960): The crystal structure of cesium biuranyl trisulfate, Cs2(UO2)2(SO4)3. J. Inorg. ______, ______& WAUTERS-STOOP, D. (1980): Structure Nucl. Chem. 15, 338-351. cristalline de la sengiérite. Bull. Minéral. 103, 176-178. ______& ______(1964): Studies of the torbernite ______& DELIENS, M. (1982): La vanmeersscheite minerals. I. The crystal structure of abernathyite and the U(UO2)3(PO4)2(OH)6•4H2O et la méta-vanmeersscheite structurally related compounds NH4(UO2AsO4)•3H2O and U(UO2)3(PO4)2(OH)6•2H2O, nouveaux minéraux. Bull. K(H3O)(UO2AsO4)2•6H2O. Am. Mineral. 49, 1578-1602. Minéral. 105, 125-128. RYAN, R.R. & ROSENZWEIG, A. (1977): Sklodowskite, MgO• ______& ______(1987): Les phosphates d’uranyle et 2UO3•2SiO2•7H2O. Cryst. Struct. Commun. 6, 611-615. d’aluminium de Kobokobo. IX. L’althupite AlTh(UO2) [(UO2)3O(OH)(PO4)2]2 (OH)3•15H2O, nouveau minéral; SAADI, M., DION, C. & ABRAHAM, F. (2000): Synthesis and propriétés et structure cristalline. Bull. Minéral. 110, crystal structure of the pentahydrated uranyl orthovanadate 65-72. (UO2)3(VO4)2•5H2O, precursor for the new (UO2)3(VO4)2 uranyl-vanadate. J. Solid State Chem. 150, 72-80. ______, ______& PIRET-MEUNIER, J. (1988): La fran- çoisite-(Nd), nouveau phosphate d’uranyle et de terres SADIKOV, G.G., KRASOVSKAYA, T.I., POLYAKOV, Y.A. & NIKO- rares; propriétés et structure cristalline. Bull. Minéral. 111, LAEV, V.P. (1988): Structural and spectral studies on potas- 443-449. sium dimolybdatouranylate. Inorg. Mater. 24, 91-96.

______, ______, ______& GERMAIN, G. (1983): La SAINE, M.-C. (1989): Synthèse et structure de KU2O7 monocli- sayrite, Pb2[(UO2)5O6(OH)2]•4H2O, nouveau minéral; nique. J. Less-Common Metals 154, 361-365. propriétés et structure cristalline. Bull. Minéral. 106, 299-304. SARIN, V.A., LINDE, S.A., FIKIN, L.E., DUDAREV, V.Y. & GORBUNOVA, Y.E. (1983): Neutronographic study of ______& PIRET-MEUNIER, J. (1988): Nouvelle détermina- UO2H(PO3)3 monocrystals. Zh. Neorg. Khim. 28, 1538- tion de la structure cristalline de la dumontite Pb2[(UO2)3 1541 (in Russ.). O2(PO4)2]•5H2O. Bull. Minéral. 111, 439-442. HIERARCHY OF U6+ MINERALS AND INORGANIC COMPOUNDS 1893

SEREZHKIN, V.N., BOIKO, N.V. & MAKAREVICH, L.G. (1980a): SWIHART, G.H., GUPTA, P.K.S., SCHLEMPER, E.O., BACK, M.E. The refi ned crystal-structure of uranyl molybdate. Kristal- & GAINES, R.V. (1993): The crystal structure of moctezu- lografi ya 25, 858-860 (in Russ.). mite [PbUO2](TeO3)2. Am. Mineral. 78, 835-839.

______, ______& TRUNOV, V.K. (1982): Crystal SYKORA, R.E. & ALBRECHT-SCHMITT, T.E. (2003): Self- structure of Sr[UO2(OH)CrO4]2•8H2O. J. Struct. Chem. assembly of a polar open-framework uranyl vanadyl 23, 270-273. hexaoxoiodate(VII) constructed entirely from distorted octahedral building units in the fi rst uranium hexaoxo- ______, EFREMOV, V.A. & TRUNOV, V.K. (1980b): Crystal iodate: K2[(UO2)2(VO)2(IO6)2O]•H2O. Inorg. Chem. 42, structure of alpha-UO2MoO4•2H2O. Kristallografi ya 25, 2179-2181. 861-865 (in Russ.). ______& ______(2004): Hydrothermal synthesis and ______, SOLDATKINA, M.A. & BOIKO, N.V. (1983): Refi ne- crystal structure of Cs4[(UO2)4(W5O21)(OH)2(H2O)2]: ment of the crystal-structure (NH4)4[UO2(CO3)3]. J. Struct. a new polar uranyl tungstate. J. Solid State Chem. 177, Chem. 24, 770-774. 3729-3734.

______, ______& EFREMOV, V.A. (1981a): Crystal- ______, KING, J.E., ILLIES, A.J. & ALBRECHT-SCHMITT, T.E. structure of the uranyl selenate tetrahydrate. J. Struct. (2004a): Hydrothermal synthesis, structure, and catalytic Chem. 22, 451-454. properties of UO2Sb2O4. J. Solid State Chem. 177, 1717- 1722. ______, ______& ______(1981b): Crystal-structure of MgUO2(SO4)2•11H2O. J. Struct. Chem. 22, 454-457. ______, MCDANIEL, S.M., WELLS, D.M. & ALBRECHT- SCHMITT, T.E. (2002a): Mixed-metal uranium(VI) iodates: ______& TRUNOV, V.K. (1981): The crystal structure hydrothermal syntheses, structures, and reactivity of of UO2CrO4•5.5H2O. Kristallografi ya 26, 301-304 (in Rb[UO2(CrO4)(IO3)(H2O)], A2[UO2(CrO4)(IO3)2] (A = Russ.). K, Rb, Cs), and K2[UO2(MoO4)(IO3)2]. Inorg. Chem. 41, 5126-5132. SEREZHKINA, L.B., TRUNOV, V.K., KHOLODKOVSKAYA, L.N. & KUCHUMOVA, N.V. (1990): Crystal structure of KUO2 ______, WELLS, D.M. & ALBRECHT-SCHMITT, T.E. (2002b): CrO4(OH)•1.5(H2O). Koord. Khimiya 16, 1288-1291 (in Hydrothermal synthesis and structure of a new one- Russ.). dimensional, mixed–metal U(VI) iodate, Cs2[(UO2)(CrO4) (IO3)2]. Inorg. Chem. 41, 2304-2306. SHASHKIN, D.P., LUR’E, E.A. & BELOV, N.V. (1974): Crystal- structure of Na2[(UO2)SiO4]. Kristallografi ya 19, 958-963 TABACHENKO, V.V., BALASHOV, V.L., KOVBA, L.M. & SEREZH- (in Russ.). KIN, V.N. (1984a): Crystal-structure of barium molybdate- uranylate Ba(UO2)3(MoO4)4.4H2O. Koord. Khim. 10, 854- SHUVALOV, R.R. & BURNS, P.C. (2003): A monoclinic poly- 857 (in Russ.). morph of uranyl dinitrate trihydrate, [(UO2)(NO3)2(H2O)2]• (H2O). Acta Crystallogr. C59, i71-i73. ______, KOVBA, L.M. & SEREZHKIN, V.N. (1979): Crystal structure of manganese sulfatouranylate MnUO2(SO4)2 SIEGEL, S., HOEKSTRA, H. & SHERRY, E. (1966): The crystal (H2O)5. Koord. Khim. 5, 1563-1568. structure of high-pressure UO3. Acta Crystallogr. 20, 292-295. ______, ______& ______(1983): The crystal struc- ture of molybdatouranylates of magnesium and zinc of ______& ______(1968): The crystal structure of copper composition M(UO2)3(MoO4)4(H2O)8 (M = Mg,Zn). uranium tetroxide. Acta Crystallogr. B24, 967-970. Koord. Khim. 9, 1568-1571 (in Russ.).

______, ______& GEBERT, E. (1972b): The structure of ______, ______& ______(1984b): Crystal structure of -uranyl dihydroxide, UO2(OH)2. Acta Crystallogr. B28, Mg(UO2)6(MoO4)7•18H2O and Sr(UO2)6(MoO4)7•15H2O. 3469-3473. Koord. Khim. 10, 558-562 (in Russ.).

______, VISTE, A., HOEKSTRA, H.R. & TANI, B.S. (1972a): ______, SEREZHKIN, V.I., SEREZHKINA, L.B. & KOVBA, L.M. The structure of hydrogen triuranate. Acta Crystallogr. (1979): Crystal structure of manganese sulfatouranylate B28, 117-121. MnUO2(SO4)2•5H2O. Koord. Khim. 5, 1563-1568 (in Russ.). STERNS, M., PARISE, J.B. & HOWARD, C.J. (1986): Refi nement of the structure of trilead(II) uranate(VI) from neutron TALI, R., TABACHENKO, V.V. & KOVBA, L.M. (1993): Crystal powder diffraction data. Acta Crystallogr. C42, 1275- structure of Cu4UO2(MoO4)2(OH)6. Zh. Neorg. Khim. 38, 1277. 1450-1452 (in Russ.).

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WANG, XIQU, HUANG, JIN, LIU, LUMEI & JACOBSON, A.J. (2002): The novel open-framework uranium silicates Na2(UO2)(Si4O10)•2.1(H2O) (USH–1) and RbNa(UO2) Received January 24, 2005, revised manuscript accepted (Si2O6)•(H2O) (USH–3). J. Mater. Chem. 12, 406-410. June 7, 2005.