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., andChemistry Division, Argonne National Laboratory, 9700 South Cass, Argonne, Illinois 60439, U.S.A.

ABSTRACTThe 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, theperformance 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 thandoubled, and reconsideration of the structural hierarchy of uranyl compounds is warranted. Here, 368 inorganic crystal structuresthat contain essential U6+ are considered (of which 89 are minerals). They are arranged on the basis of the topological detailsof 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), finite clusters (43), chains (57), sheets (204), andframeworks (56) of polyhedra. Within these categories, structures are organized and compared upon the basis of either theirgraphical representations, or in the case of sheets involving sharing of edges of polyhedra, upon the topological arrangement ofanions 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’efficacité des sites d’enfouissement des déchêts nucléaires, et pour développerde matériaux nouveaux ayant des applications de haute technologie. Au cours de la dernière décennie, le nombre de composésinorganiques contenant le groupe uranyle (y inclus les minéraux) dont la structure est connue a plus que doublé, et une reconsidération de la hiérarchie structurale des composés uranylés est appropriée. Ici, 368 structures de cristaux inorganiques contenantle 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 finis (43), chaînes
(57), feuillets (204), et trames (56) de polyèdres. A l’intérieur de ces catégories, les structures sont organisées et comparées enfonction de soit leurs représentations graphiques, ou bien dans le cas de feuillets impliquant le partage d’arêtes de polyèdres, enfonction 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: pburns@ndu.edu

1840

THE CANADIAN MINERALOGISTINTRODUCTION
Crystals containing U6+ have been the focus ofextensive research over several decades. The first crystal
structures of inorganic U6+ (uranyl) compounds appearedin the literature several decades ago (e.g., Evans 1963),
and by 1996, about 180 structures were known (Burns
et al. 1996). The introduction of CCD-based detectorsof X-rays to mineralogy (Burns 1998a) greatly extended
the reach of single-crystal X-ray diffraction into thestudy of complex U6+ compounds. Today, at least 368
structures of inorganic uranyl compounds are known,of which 89 correspond to minerals. These materials
exhibit fascinating and complex atomic arrangements,
owing to the myriad of possible types of connections
between uranyl polyhedra and other uranyl polyhedra,
as well as polyhedra containing other higher-valence
cations. Current research concerning uranyl compounds
is driven by the search for novel solids with importantmaterials properties (e.g., Locock & Burns 2002a,
Krivovichev et al. 2002a, Krivovichev & Burns 2002a,
Almond et al. 2002a, Burns et al. 2005, Krivovichev etal. 2005a,b), and their importance in the environment(e.g., Li & Burns 2001a, Burns et al. 2000, 2004a,
Hughes Kubatko et al. 2003). Knowledge of the structures of uranyl minerals is important for understanding
the genesis of U deposits (Frondel 1958), and uranyl
minerals exhibit substantial structural and chemical
diversity, reflecting geochemical conditions dominantduring their formation.
Uranyl minerals and compounds are of considerable environmental importance. They are significant
for understanding water–rock interactions in U-richrocks (Frondel 1958). Uranyl minerals exert an impact
on the mobility of actinides in contaminated soils (e.g.,
Buck et al. 1996, Roh et al. 2000) and in vadose-zone
sediments polluted with actinides, such as the Hanford
and Savanna River sites in the U.S.A. (e.g., Yamakawa& Traina 2001). Precipitation of uranyl phosphateminerals in the vadose zone of contaminated sites by theaddition of phosphate has been proposed as a means to
mitigate U6+ plumes in groundwater (Fuller et al. 2002).
Uranyl minerals can be bio-precipitated (Macaskie etal. 2000), and potentially provide sources of phosphate
for biological consumption, as well as redox-active U
to serve as an electron acceptor needed for metabolismof bacteria. Uranyl minerals are important products ofalteration of nuclear waste forms under simulated conditions of geological repositories, such as those expectedin the proposed repository at Yucca Mountain, Nevada(e.g., Finch et al. 1999a, Finn et al. 1996, Wronkiewicz
et al. 1996).
About 200 uranyl minerals have been describedfrom nature. They usually occur in the oxidized zones
of U deposits, where it is common for several uranyl
mineral species to coexist, commonly in intimate intergrowths. In many cases, uranyl minerals are secondaryand result from the alteration of UO2+x, but they are

primary minerals in some occurrences. Owing to theirstructural and chemical complexities, and the experimental difficulties associated with their characterization, understanding of uranyl minerals as a whole lags
behind the current level of understanding of most major
mineral groups. The structures are fully characterized
for less than 50% of described uranyl minerals, andmany aspects of their chemical composition, stability
and occurrence still require study.
Burns et al. (1996) presented a structural hierarchyof 180 inorganic synthetic and natural uranyl phases.
Burns (1999a) expanded the hierarchy in the caseof minerals only. It is based upon the polymerization of those polyhedra that contain higher-valence
cations. Low-valence cations, including H atoms andH bonds, were ignored for the purpose of erecting thehierarchy.
Here I provide a revised and greatly expanded hierarchy of 368 structures of inorganic uranyl compounds(including 89 minerals). The volume of known structures mandates a concise description of structures.
Illustrations of all of the structural units are provided,and formulae and crystallographic parameters are givenin the tables for each of the 368 structures. I hope thatthe reader will gain an appreciation of the complex
beauty of this chemical class of compounds, and thatthe structural hierarchy will provide a basis for further
studies in actinide crystal chemistry.
I am delighted that this contribution appears in the50th Anniversary edition of The Canadian Mineralogist,
not only because it is one of the premier mineralogicaljournals in the world, but also because of the tremendous
impact that the Mineralogical Association of Canada has
had on my continuing education and career.

COORDINATION POLYHEDRA
Almost all U6+ in crystal structures is present as
an approximately linear UO22+ uranyl ion. The formal
valence of the uranyl ion is 2+, so it must be coordinated by anions in a stable crystal structure. The
uranyl ion is typically coordinated by four, five or six
ligands, arranged at the equatorial vertices of square,pentagonal and hexagonal bipyramids, respectively(Fig. 1). The bipyramids are capped by the O atoms of
the uranyl ions.
Burns et al. (1997a) provided U6+–O bond-lengthand uranyl ion bond-angle data for ~100 crystal structures. Only well-refined structures analyzed by singlecrystal X-ray diffraction, with final R indices less than
7%, were included. The bond and angle distributions
for uranyl polyhedra have been revised to incorporate
data for structures reported since 1997, using the same
criteria adopted by Burns et al. (1997a). A total of 222
structures have been included. The U6+– (: O, OH,
H2O) bond-length distributions for [6]U6+, [7]U6+ and[8] 6+
U are provided in Figure 2.

HIERARCHY OF U6+ MINERALS AND INORGANIC COMPOUNDS

Consider first the case of [7]U6+ (U6+ coordinated byseven atoms, including the uranyl ion); the distributionof U6+–O bond lengths in 270 polyhedra containing[7]U6+ in 143 structures is presented in Figure 2b. The
distribution is bimodal, as each polyhedron contains auranyl ion. The average U6+–OUr (Ur: uranyl) bondlength is 1.793 Å, with a standard deviation of 0.035
Å, whereas for the U6+–Oeq bonds, the average is 2.368
Å with a standard deviation of 0.100 Å. The distributions of both the U6+–OUr and U6+–Oeq bonds are rather
regular.
The bond-length distribution for [8]U6+ is shown in
Figure 2a. Although the sample size is smaller than thatof [7]U6+, it is still apparent that all [8]U6+ polyhedra inwell-refined structures contain a uranyl ion, and theaverage U6+–OUr bond length for 35 polyhedra in 32
structures is 1.783 Å, with a standard deviation of 0.030
Å. The distribution for the U6+–Oeq bonds is rather irregular, with an average of 2.460 Å and a standard deviation of 0.107 Å. There are few [8]U–Oeq bonds less than
~2.4 Å, causing the asymmetric distribution of bondlengths apparent in Figure 2a. This feature presumably
is related to the geometrical limitations of packing six
coplanar O atoms around a single uranyl ion.
The bond-length distribution for [6]U6+ is shownin Figure 2c. As discussed in detail by Burns et al.
(1997a), not every [6]U6+ polyhedron in well-refinedstructures has an associated uranyl ion. Several reported
structures contain [6]U6+ in octahedral or distorted
octahedral coordination, thus there are three modes inthe bond-length distribution for [6]U6+ that correspondto U6+–OUr and U6+–Oeq bond lengths of uranyl squarebipyramids, and the more regular bond-lengths of octahedral coordination (centered at ~2.1 Å). Considering
only the 54 uranyl square bipyramids in 47 well-refinedstructures, the average U6+–OUr bond-length is 1.816
Å, with a corresponding standard deviation of 0.050
Å, and the U6+–Oeq bond-length is 2.264 Å, standard
deviation 0.064 Å.
Using the bond-valence parameters provided by
Burns et al. (1997a), the typical bond-valences of the
U6+–OUr and U6+–Oeq bonds of each coordination polyhedron are: [6]U6+–OUr = 1.59 valence units (vu), [6]U6+–
Oeq = 0.71 vu, [7]U6+–OUr = 1.64 vu, [7]U6+–Oeq = 0.53
vu, [8]U6+–OUr = 1.67 vu, [8]U6+–Oeq = 0.44 vu (Fig. 3).
These values are consistent with extensive linkage of
uranyl polyhedra through equatorial ligands, either withother uranyl polyhedra or other polyhedra containing
higher-valence cations. The bond-valences incident
upon the OUr atoms, due to the bond to U6+ alone,
range from 1.59 to 1.67 vu. Thus, the apical (uranyl)
vertices of the uranyl bipyramids are seldom sharedwith polyhedra containing higher-valence cations, as
this would overbond the O position. The two-dimensional polymerization of uranyl polyhedra mandated by
the distribution of bond strengths within the polyhedra
favors the formation of sheets of polyhedra.

1841

In some cases, the solid state and solution chemistryof actinyl ions involves significant cation–cation interactions in which an O atom of an actinyl ion is also an
equatorial ligand of a neighboring polyhedron (Krot &Grigoriev 2004). The cation–cation designation arises
from the formal valences of the actinyl ions, both ofwhich are cations. Such interactions are relatively
common for Np5+, but are rather unusual in the caseof U6+ (Sullens et al. 2004), presumably owing to thefact that the bonding requirements of the O atoms of
the uranyl ions are nearly met by the bonds within theuranyl ion alone. To date, there are five inorganic uranyl
compounds that exhibit cation–cation interactions(1.4% of known structures), and each of these structures
is composed of a framework of polyhedra.

STRUCTURAL HIERARCHY OF URANYL PHASES
Structural hierarchies are a means to organize awealth of complex and strikingly different structures
into a cohesive framework. They provide considerable insight into the crystal chemistry of the focus
compounds, as the hierarchical arrangement is usuallybased upon structural connectivity. Structural hierarchies permit recognition of structural trends withinlarge groups of compounds that are generally obscuredby the complexity of the class of compounds. They canbe the basis for understanding the linkages between
mineral occurrences and underlying mineral structures,
especially in low-temperature geochemical environments that present complex assemblages of hydrous
minerals. However, consideration of minerals alone
in the development of a structural hierarchy is akin toviewing a ballet with tunnel vision, as minerals represent only the subset of a chemical class that is stable
over geological times under geochemical conditions. As
such, the structural hierarchy presented herein includes
all inorganic compounds that contain U6+ as a necessary
structural constituent.
Following the approach of Burns et al. (1996) and
Burns (1999a), structures of uranyl compounds arearranged here on the basis of their structural units
consisting of those cation-centered polyhedra of higher
bond-valence. Structures naturally fall into five categories corresponding to isolated polyhedra, finite clustersof polyhedra, chains of polyhedra, sheets of polyhedra,and frameworks of polyhedra. In each case the structural unit is illustrated, and pertinent crystallographic
information is provided in the Tables.
In most cases, assignment of a structure to one of
the five classes is straightforward. In a few structures,
sheets of polyhedra are linked together by sharing
anions, forming a framework, or the sheets are linked
through uranyl ions located in interlayer positions,which also results in a framework. Such structures
usually contain sheets that are similar or identical to thesheets that occur isolated in other structures; as such,

1842

THE CANADIAN MINERALOGIST

FIG. 1. Uranyl coordination polyhedra in uranyl compounds. Shown are (a) the (UO2)2+ uranyl ion, (b) uranyl square bipyramid, (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
despite their three-dimensional connectivity.
In many cases, the chemical formulae used in the
text and tables have been re-arranged from those givenby the original authors to emphasize structural features
and to facilitate comparisons. In Tables that list clusters
and chains, structures based upon similar structuralunits are grouped, and double lines are used to separate
those that have different graphical representations. In
Table 4 (see below), structures based upon sheets withidentical graphical representations are grouped, andthose involving different graphs are separated by doublelines. In Tables 5 through 8 (see below), all of which
list structures that contain sheets of polyhedra, doublelines separate structures with different anion-topologies,
and dotted lines separate structures with distinct sheets,
but the same underlying anion-topology.

STRUCTURES CONTAINING ISOLATED POLYHEDRA
Eight structures contain isolated U6+ polyhedra(Table 1). In these structures, there are no direct linkages between the U6+ polyhedra; linkages all takeplace through low-valence cations. In the structures of
Ca3[UO6], Sr3[UO6], K2Li4[UO6] and Li6[UO6], theU6+ cation is octahedrally coordinated, with no uranyl

ion present. The structure of Cs2[(UO2)Cl4] contains
UrCl4 square bipyramids, with Cl atoms at the equatorial positions. The structure of [(UO2)Cl2(H2O)3] hasa Ur(H2O,Cl)5 polyhedron, with two equatorial Cl
atoms. The structures of [(UO2)(H2O)5](ClO4)2 and[(UO 2)(H 2O) 5](ClO 4) 2•2H 2O each contain isolatedUr(H2O)5 pentagonal bipyramids, as well as isolated
perchlorate tetrahedra.

STRUCTURES CONTAINING FINITE CLUSTERS
OF POLYHEDRA
There are 43 compounds based upon finite clustersof polyhedra of higher bond-valence, which represents
a significant growth from the 22 structures in thisclass reviewed by Burns et al. (1996). Of the 21 new
structures reported since 1996, only two correspond tominerals, and only seven of the total in this class areminerals. The structures are arranged in Table 2 on thebasis of their graphs; those with identical graphs aregrouped together, regardless of the chemical identitiesof the constituents of the clusters.
Graphical representations of crystal structures are a
powerful approach to reducing structural complexity forthe establishment of underlying relationships amongst
groups of structures. In the case of uranyl compounds,

HIERARCHY OF U6+ MINERALS AND INORGANIC COMPOUNDS

FIG. 2. Bond-length distributions in uranium-bearing polyhedra in well-refined structures.
(a) Uranyl hexagonal bipyramids, (b) uranyl pentagonal bipyramids, (c) uranyl squarebipyramids 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.

1843

1844

THE CANADIAN MINERALOGIST

this approach works best where most of the connections between polyhedra containing higher-valence
cations occur through the sharing of vertices. The graphis obtained from the structure by representing eachdistinct type of polyhedron with a colored circle, and thenumber of vertices shared between adjacent polyhedra
are shown by connectors between the colored circles.
Throughout this paper, black circles are used to represent
uranyl polyhedra, and white circles represent tetrahedra
or triangles containing higher-valence cations. Consider
for example the clusters shown in Figures 4c and 4i. The
graph corresponding to the cluster in Figure 4c has two
black circles and eight white circles, which corresponds
to the number of uranyl polyhedra and tetrahedra in thecorresponding cluster. It is also immediately apparent
that there are two types of white circles, those that areconnected to two black circles, and those that are linked
to only one black circle. These white circles correspondto the two- and one-connected tetrahedra of the cluster,
respectively. The graph also indicates that all linkages
between the polyhedra in the cluster arise by the sharingof single vertices only (the linkages are indicated bysingle connectors in the graph). In contrast, the graphcorresponding to the cluster shown in Figure 4i has
three types of white circles, those that are connected toa single black circle by a single connector, those thatare connected to two black circles by single connectors,
and those that are connected to a single black circle
by two connectors. The presence of two connectors in
the cases of two of the white circles indicates that anedge is shared between the uranyl polyhedron and thecorresponding tetrahedron.
Uranyl carbonate minerals that crystallize fromalkaline solutions contain the uranyl tricarbonate
cluster, [(UO2)(CO3)3]4– (Fig. 4a). A uranyl hexagonal
bipyramid shares three edges with carbonate triangles,
and all equatorial anions of the bipyramids constituteligands of the carbonate groups. This cluster is known
from seven mineral structures as well as 10 synthetic
compounds (Table 2). In each case, independent uranyl

tricarbonate clusters are linked only through bonds
to low-valence cations, including H. A topologically
identical cluster containing nitrate triangles instead ofcarbonate is found in the structures of Rb[(UO2)(NO3)3]and K[(UO2)(NO3)3].
A uranyl square bipyramid shares each equatorial vertex with a different tetrahedron in the clusterwith composition [(UO2)(XO4)4] shown in Figure 4b.
This cluster was recently found in three compounds(Table 2), with the X cation being Mo6+ in two structures and Cr6+ in the third case. A more complex uranyl
molybdate cluster composed of two uranyl pentagonal
bipyramids and eight MoO4 tetrahedra is known from
Na6[(UO2)(MoO4)4] and Na3Tl3[(UO2)(MoO4)4] (Fig.
4c, Table 2). Each equatorial ligand of each uranyl
pentagonal bipyramid is shared with a tetrahedron, andtwo tetrahedra provide a bridge between the uranyl
pentagonal bipyramids.
Remarkable clusters of uranyl pentagonal bipyramids and sulfate tetrahedra occur in Na 6 [(UO 2 )(SO 4 ) 4 ](H 2 O) 2 , Na 10 [(UO 2 )(SO 4 ) 4 ](SO 4 ) 2 (H 2 O) 3 ,and KNa5[(UO2)(SO4)4](H2O) (Fig. 4d). One sulfate
tetrahedron shares an edge with the bipyramid, aconnectivity that had not been previously observed ina hydrous uranyl sulfate. Three additional sulfate tetrahedra are linked to the other three equatorial vertices ofthe bipyramid. The sharing of edges between polyhedra
containing U6+ and S6+ cations presumably entails
significant cation–cation repulsion. However, crystalsof these compounds several millimeters in maximum
dimension were grown at 70°C in uranyl sulfate solutions, which implies cluster stability, even in solutions(e.g., Burns & Hayden 2002).
Six uranyl nitrates contain isolated clusters withuranyl polyhedra and nitrate triangles. The cluster inFigure 4e has a uranyl hexagonal bipyramid that shares
two of its trans equatorial edges with nitrate triangles,
and the remaining two vertices of the bipyramid areshared with two different nitrate triangles. This cluster
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 representations) of uranyl polyhedra and other polyhedra
containing higher-valence cations that occur inthe structures of uranyl compounds. The corresponding compounds are listed in Table 2.

which is the only structure known in which nitrate
triangles are linked to uranyl polyhedra by sharing
only a single vertex; in all other cases, edges areshared between uranyl and nitrate polyhedra. Twoclusters that are graphical isomers contain a single
uranyl hexagonal bipyramid that shares two equatorial
edges with nitrate triangles (Figs. 4f, g). The former
is found in the structure of [(UO2)(NO 3) 2](H 2O) 6,[(UO2)(NO3)2](H2O)2 and [(UO2)(NO3)2](H2O)3, thelatter is found in [(UO2)(NO3)2(H2O)2](H2O). In a more
complex cluster found in [(UO2)2(OH)2(NO3)2](H2O)4,
a uranyl hexagonal bipyramid shares an equatorial edge
with a uranyl pentagonal bipyramid, as well as twoequatorial edges with nitrate triangles (Fig. 4h). In all
but Rb2[(UO2)(NO3)4], the uranyl nitrate clusters arelinked only though H bonds.
The structure of the anhydrous compound K4[(UO2)(SO4)3] possesses a remarkable cluster (Fig. 4i), in
which two uranyl pentagonal bipyramids are bridged bysharing vertices with two sulfate tetrahedra, and eachbipyramid also shares an additional vertex with anothersulfate tetrahedron, and an edge with a fourth sulfate
tetrahedron. This is the only example of an anhydrous
phase in which a sulfate tetrahedron shares an edge witha uranyl bipyramid. This phase was grown at elevatedtemperature, and its stability under ambient conditions
is uncertain.
Four clusters contain only uranyl bipyramids thatshare equatorial edges (Figs. 4j, k, l, m). Two aresimple dimers of either square bipyramids or pentagonal
bipyramids (Figs. 4l, m). One is a trimer of uranyl
pentagonal bipyramids (Fig. 4k), and one contains foururanyl pentagonal bipyramids that share edges (Fig. 4j).
The latter two clusters each contain oxygen atoms thatare common to three uranyl polyhedra.
The uranyl borate cluster shown in Figure 4n was
synthesized by aging a boron-rich solution at room
temperature (Behm 1985). The cluster contains 16
borate triangles and tetrahedra that surround a uranylhexagonal bipyramid; four of the borate polyhedra
share edges with the uranyl bipyramid, and four additional borate polyhedra share a single vertex with the
bipyramid.
The cluster shown in Figure 4o contains a uranyl
pentagonal bipyramid that shares two vertices withperchlorate tetrahedra. The remaining equatorial
vertices of the bipyramid are H2O groups, and theclusters are linked through H bonding only.

HIERARCHY OF U6+ MINERALS AND INORGANIC COMPOUNDSSTRUCTURES CONTAINING INFINITE CHAINS
OF POLYHEDRA
Burns et al. (1996) listed 19 uranyl compoundswith structures based upon infinite chains of polyhedraof higher bond-valence, of which five correspond tominerals. Currently, 57 such structures are known,of which 10 are minerals (Table 3). The remarkable
expansion of this structural class is largely attributableto research focused on understanding linkages between
uranyl polyhedra and tetrahedra containing Mo6+, S6+
or Cr6+, as well as various polyhedra containing cations
with stereoactive lone-electron pairs such as Se4+ andI5+, which occur in 16 of the new structures. Thirtynine structures containing uranyl polyhedra and any ofthe lone-pair stereoactive cations Te4+, Se4+ and I5+ are
known; remarkably, 41% of these contain infinite chains,
whereas only 16% of all inorganic uranyl compoundswith known structures are based upon chains. Presumably, the irregular polyhedra surrounding lone-pairstereoactive cations, and the existence of the lone pairof electrons rather than an additional ligand, somewhat
favor polymerization in one dimension only.
Chains of polyhedra in uranyl compounds arearranged in Figure 5 on the basis of their graphs,
with chains dominated by the sharing of only vertices
between polyhedra provided first, followed by chainsthat also involve sharing of polyhedron edges. Crystallographic parameters of corresponding structures are
given in Table 3.
The simple chain of uranyl square bipyramidsthat share vertices shown in Figure 5a occurs in
Na 4 [(UO 2 )O 3 ], Ca 2 [(UO 2 )O 3 ], Sr 2 [(UO 2 )O 3 ] andLi4[(UO2)O3]. Each of the structures is anhydrous, andlinkages between the chains involve lower-valence
cations only.
The chains illustrated in Figures 5b through 5f aregraphical isomers. The chains contain either uranylsquare bipyramids or pentagonal bipyramids, and each
is four-connected within the chain, as illustrated in thecorresponding graph. Bipyramids are linked througheither tetrahedra or pyramids, each of which is twoconnected within the chain. Chain 5b contains either
arsenate, molybdate, or phosphate tetrahedra. Chains 5c,
5d, and 5f have lone-pair stereoactive Se4+ or Te4+, asreflected by the one-sided pyramidal coordination polyhedra about these cations. The structures corresponding
to each chain are listed in Table 3.
The chains of uranyl pentagonal bipyramids andtetrahedra (or SeO3 pyramids) shown in Figures 5g, 5h
and 5i correspond to an identical graph, and are known
from seven compounds (Table 3). Uranyl pentagonal
bipyramids are bridged through two tetrahedra by thesharing of vertices, such that four equatorial vertices ofthe bipyramids are also tetrahedral vertices of bridging(two-connected) tetrahedra. The fifth equatorial vertexof the bipyramid is shared with a non-bridging tetrahedron that decorates the side of the chain.

1847

The chains of uranyl pentagonal bipyramids andtetrahedra shown in Figures 5j, 5k, 5l and 5m, which are
known from eight compounds (Table 3), are graphicalisomers. Each bipyramid shares equatorial vertices withthree tetrahedra, whereas the remaining two equatorial
vertices correspond to H2O groups. Each tetrahedron islinked to three bipyramids, and the chains differ in thedetails of the orientation of the tetrahedra.
Two chain types have been discovered that containuranyl pentagonal bipyramids, chromate tetrahedra, andiodate pyramids (Fig. 5n, four structures, Fig. 5o, twostructures). In these unusually complex chains, pairs ofuranyl pentagonal bipyramids are bridged by sharing
vertices with two chromate tetrahedra, and the resulting
tetramers of polyhedra are connected into chains by
sharing vertices with iodate pyramids. In both chains,
the tetrahedra are two-connected, and two vertices ofeach are terminal. In chain 5o, each bipyramid is linked
to two tetrahedra and two pyramids. In the case of chain
5n, iodate pyramids also decorate the chains by sharing
a single vertex with the uranyl pentagonal bipyramids,
resulting in bipyramids that are five-connected within
the chains.
The chains shown in Figures 5p (one structure) and5q (two structures) combine uranyl pentagonal bipyramids and molybdate tetrahedra. Pairs of bipyramids
share a vertex, resulting in dimers, and two tetrahedra
bridge the dimers on two sides by sharing vertices witheach bipyramid. The resulting tetramers are connected
into chains by sharing vertices with four different
tetrahedra. Thus, each bipyramid is five-connected
within the chain, whereas the tetrahedra are each
two-connected.
Two types of chain contain only uranyl polyhedrathat share edges. The structures of moctezumite and[(UO2)Cl2H2O] have simple chains of pentagonal bipyramids that share edges (Fig. 5r), whereas studtite has
chains of edge-sharing hexagonal bipyramids (Fig. 5s).
Studtite is one of only two known peroxide minerals,
and it is the only one with a known structure (Burns &Hughes 2003). Each hexagonal bipyramid contains two
peroxide groups that constitute edges of the bipyramid;
both of the O atoms of the peroxide group are bonded
to the central U6+ cation. The remaining two vertices ofeach bipyramid are H2O groups. Polyhedra are linked
by the sharing of peroxide groups, for which the corresponding edges of the bipyramids are ~1.45 Å. Recent
studies have shown that this unusual mineral probably
grows because of the buildup of peroxide that forms byalpha-radiolysis of water in nature (Hughes Kubatko
et al. 2003).
The chains shown in Figures 5t and 5u are based
upon the same graph. That in Figure 5t is from the structures of parsonsite and hallimondite, and is composed ofuranyl pentagonal bipyramids and phosphate or arsenate
tetrahedra. Bipyramids share an edge, forming a dimer,and dimers are linked into chains by sharing edges andvertices with tetrahedra. Additional tetrahedra decorate

1848

THE CANADIAN MINERALOGISTHIERARCHY OF U6+ MINERALS AND INORGANIC COMPOUNDS

the edges of the chains by sharing vertices with thepolyhedra. The structures of Ca[(UO2)(SeO3)2] andSr[(UO2)(SeO3)2] contain chains (Fig. 5u) in which
selenite pyramids assume the role of tetrahedra in the
parsonsite chain.
The chain shown in Figure 5v is known only fromSr[(UO2)(SeO3)2](H2O)2, and contains units similar
to those in Ca[(UO2)(SeO3)2] and Sr[(UO2)(SeO3)2].
These chains have the same dimers of edge-sharing
uranyl pentagonal bipyramids, with two selenite pyramids that share edges with the bipyramids, but thelinkages of these units into chains differ.
The backbone of the chain found in UO2(IO3)2 (Fig.
5w) is composed of uranyl hexagonal bipyramids thatshare edges, and is thus related to the chains found in
studtite (Fig. 5s). In UO2(IO3)2, the chains also containiodate pyramids that are attached to either side bysharing edges with the bipyramids.
The chain shown in Figure 5x occurs in three uranyl
iodates, and is unusual in that it is the only chain that
contains uranyl ions in two different coordinations.
The zig-zag chain is formed by the sharing of edges
between uranyl pentagonal bipyramids and squarebipyramids, arranged such that there are dimers of1849

pentagonal bipyramids that are bridged by the squarebipyramids. Additional rigidity of the chain is provided
by iodate pyramids that bridge adjacent dimers ofpentagonal bipyramids, and the chain is decorated byiodate pyramids that share only a single vertex withthe bipyramids.
The complex chain shown in Figure 5y is known
from three uranyl iodates (Table 3). The bulk of thechain is composed of uranyl pentagonal bipyramidsthat share equatorial edges, resulting in a chain that is
two bipyramids wide. Chain rigidity is enhanced byiodate pyramids that bridge between bipyramids, andadditional iodate pyramids decorate the chain by sharing
single vertices with the bipyramids.
The chain found in uranopilite (Fig. 5z) is the mostcomplex chain found to date in a uranyl mineral. It
is composed of uranyl pentagonal bipyramids thatare linked by sharing equatorial edges to form sixmembered clusters. Within these clusters, two O atoms
are bonded to three uranyl ions, and all but four of theequatorial anions that terminate the cluster are H2Ogroups. Identical clusters of six bipyramids are linked
by sharing vertices with sulfate tetrahedra, such thateach 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
directly between adjacent chains, and to interstitial H2Ogroups. Hydrogen bonds also extend between clustersof bipyramids within the same chain.

STRUCTURES WITH INFINITE SHEETS OF POLYHEDRA
Structures containing infinite sheets of polyhedraof higher bond-valence are the most abundant class inuranyl minerals and synthetic inorganic compounds,
accounting for 204 of the 368 structures examinedherein. The dominance of sheets of polyhedra arises
from the uneven distribution of bond strengths withinuranyl polyhedra. The bond-valence requirements of
the OUr atoms are nearly satisfied by the bond to theU6+ cation alone, thus these O atoms usually do notform bonds to other higher-valence cations. In contrast,
the equatorial ligands of the uranyl bipyramids must
participate in additional significant bonding for astable configuration to result. In some cases, equatorial
ligands correspond to H2O groups that are not bonded
to additional high-valence cations. However, it is morecommon for uranyl polyhedra to link to other uranyl
polyhedra or other polyhedra of higher bond-valence
through the equatorial ligands, which favors the formation of sheets.
Burns et al. (1996) and Burns (1999a) grouped
sheets on the basis of the topological arrangement ofanions within the sheet. In this approach, all anions thatare bonded to at least two cations within the sheet areconsidered, and those separated by less than ~3.5 Å areconnected by lines. Removal of all atoms from further
consideration results in the sheet anion-topology, which,when projected onto a plane, is a two-dimensional tilingof space that represents the topological arrangement ofanions within the sheet from which it was derived. The
utility of this approach is that sheets of polyhedra thatbear little immediate resemblance commonly have thesame underlying sheet anion-topology. The sheet aniontopology is a representation of an approximate closest

packing of anions, with smaller cations populatinginterstitial positions to produce the observed sheets.
As shown by Miller et al. (1996) and Burns (1999a),the sheet anion-topologies can be assessed as stacking
sequences of a small number of distinct chains of polygons, which provides a useful means of comparison of
the topologies as well as a short-hand notation. Thisapproach has demonstrated that some of the extraordinarily complex sheets recently found in minerals suchas vandendriesscheite and wölsendorfite are composedof modules of simpler sheets found in other minerals orsynthetic compounds (Burns 1997, 1999b).
Sheets dominated by the sharing of vertices
The sheet anion-topology approach works well for
most sheets that are dominated by the sharing of edges
between polyhedra of higher bond-valence. However,sheets connected only by the sharing of vertices
between polyhedra correspond to a myriad of complex
anion-topologies that defy usefulness. This was not a
substantial setback for Burns et al. (1996) or Burns(1999a), but the recent discovery of many such sheets
in synthetic compounds, especially those containing
hexavalent cations in tetrahedral coordination (such as
Mo6+, Cr6+, S6+), requires a new approach to their hierarchical organization. To further understanding of therelationships within sheets dominated by vertex sharing,
Krivovichev & Burns (2003j) developed a graphicalapproach for the analysis of such sheets. This approach
has considerable merit, as it is generally possible to findparent graphs that may be used to derive the graphs ofa diverse range of sheets by simple deletion of vertices
and connectors.
Krivovichev (2004) has greatly expanded the treatment of Krivovichev & Burns (2003j) to include agraphical approach to structures containing zero-, one-,and two-dimensional units with corner-sharing between
coordination polyhedra. He developed a method ofgraphical representation of these structures, and intro-

1852

THE CANADIAN MINERALOGISTHIERARCHY OF U6+ MINERALS AND INORGANIC COMPOUNDS

duced short-hand notations for the required graphs.
His study included structures in which two adjacentpolyhedra share no more than one common vertex, andwhere there is no linkage between polyhedra of the same
type. These criteria match many of the sheets containing
uranyl polyhedra that are dominated by vertex sharing(Table 4), and Krivovichev (2004) arranged these structures on the basis of their graphs, together with coverageof non-uranium structures.
Krivovichev (2004) defined the connectedness, s,of a polyhedron as the number of adjacent polyhedrawith which it shares common corners. Consider thegraph shown in Figure 6a. It contains black circles andwhite circles. Each black circle is connected to six whitecircles, and white circles are connected to three black
circles. The elementary unit of the graph is a rhomb that
contains two black and two white circles. The values ofs at the nodes of the rhomb written in cyclic order are{3.6.3.6}, which may be used to designate the graph.
Krivovichev (2004) denoted the {3.6.3.6} graph as a1853

parent graph because a series of graphs may be derived
from it, which correspond to known structures, by
simply deleting connectors or vertices, together withall of their associated connectors in the case of deleted
vertices. In the case of the {3.6.3.6} graph, Krivovichev(2004) found 34 derivative graphs that correspond to
known structures, with M:T ratios of 1:2, 2:3, 4:3, 13:18,
3:8, 5:8, 3:5, and 1:1 (where M represents non-tetrahedral polyhedra such as uranyl pentagonal bipyramids,
and T corresponds to tetrahedra). Krivovichev (2004)
provided a designation for the graphs of the generalform L M/T c, where L designates that the graph is asheet, M/T gives the ratio of non-tetrahedral polyhedra
to tetrahedra, and c is an alphabetic designator used to
distinguish various graphs with the same M/T ratio.
Several sheets considered by Krivovichev (2004)
and herein have either Se4+O3 or I5+O3 polyhedra. In
these cases, the cations contain stereoactive lone pairsof electrons that occur in the position of the fourthvertex 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

the current discussion, Se4+O3 and I5+O3 polyhedra aretreated as tetrahedra.
Twenty-nine uranyl compounds contain sheetswith corresponding graphs that are derived from the{3.6.3.6} parent graph (Table 4). Polyhedral representations of each distinct sheet, together with theircorresponding graphs, are shown in Figure 6.
Two sheets are known that correspond to thegraph L1/2b (Fig. 6b). The sheet in the structure of[UO2(IO3)2(H2O)] contains uranyl pentagonal bipyramids and I5+O3 pyramids. The sheet in Cs2[(UO2)(SeO4)2(H2O)](H2O) is composed of uranyl pentagonal
bipyramids and selenate tetrahedra. In each case thebipyramids are only connected to four tetrahedra, andthe fifth equatorial vertex of the bipyramid is occupied by H2O. Each tetrahedron is connected to two
bipyramids.
The structure of (NH4)2[(UO2)(SO4)2(H2O)](H2O)
contains the sheet shown in Figure 6f, which is based
upon the L1/2c graph shown in Figure 6e. The
bipyramids share equatorial vertices with four sulfate
tetrahedra, and H2O is present at the fifth equatorial
vertex of the bipyramid. As in L1/2b, each tetrahedron
is connected to two bipyramids. Sheets based upongraphs L1/2b and L1/2c differ in the orientation of the
bipyramids and the unshared equatorial vertices of the
bipyramids. Sheets based on L1/2b have all unsharedvertices of the bipyramids extending in the same direction, but these vertices alternate in opposite directions in
the case of sheets based upon L1/2c (Figs. 6d, f).
Twenty-three structures contain sheets with a M:
T ratio of 1:2; they contain twice as many tetrahedra
as uranyl pentagonal bipyramids. Sixteen correspondto the graph designated L1/2d by Krivovichev (2004)(Table 4). Of these, eight have molybdate tetrahedra(Fig. 6h) and seven contain selenite pyramids or
selenate tetrahedra (Fig. 6i), and all but three crystallizein space group P21/c or P21/n. Each uranyl pentagonal
bipyramid in the uranyl molybdate and uranyl selenite
or selenate sheet shares vertices with five distinct
tetrahedra, but tetrahedra are linked either to two or
three bipyramids. The connectivity of the tetrahedra
traversing around the bipyramids is 2–3–3–2–3 in eachcase. All known structures containing this sheet havemonovalent cations in the interlayer regions, where H2O
also occurs in some cases.
The structures of Na2[(UO2)(MoO4)2] and Ag2[(UO2)(SeO3)2] contain sheets based upon the L1/2e graphshown in Figure 6j. Examination of the sheets shown in
Figures 6k and 6l shows that each bipyramid is linked
to five tetrahedra, and that tetrahedra are connected
to either two or three bipyramids. The connectivityof the tetrahedra traversing around the bipyramids is
2–2–3–3–3 in both cases, which differs from L1/2d, forwhich the connectivity is 2–3–3–2–3.
The isostructural compounds Mg[(UO2)3(MoO4)4](H2O)8 and Zn[(UO2)3(MoO4)4](H2O)8 contain theuranyl molybdate sheet shown in Figure 6n that is based

1855

upon graph L1/2g (Fig. 6m). In these structures, theuranyl dimolybdate sheets are connected through uranyl
pentagonal bipyramids located in the interlayer, resulting
in frameworks. They are included here as sheets because
the fundamental portion of the structure corresponds toa graph whose parent is {3.6.3.6}. Within the sheet, eachbipyramid is linked to five tetrahedra, and tetrahedra
are either connected to two or three bipyramids. The
connectivity of the tetrahedra traversing around the
bipyramids is 2–3–3–2–3, which is identical to graph
L1/2d. The distinction between sheets based upongraphs L1/2g and L1/2d lies in the specific bipyramids
to which each tetrahedron bridges.
The uranyl chromate sheet in the structure ofK2[(UO2)2(CrO4)3(H2O)2](H2O)4 (Fig. 6p) is the only
example of a uranyl compound with a sheet based upon
the graph designated L2/3e (Fig. 6o). This is a relatively
open sheet in which each bipyramid is linked to four
tetrahedra, with the fifth vertex of the bipyramid corresponding to H2O. Of the four tetrahedra that are linked
to any bipyramid, two bridge between two bipyramids
and two are connected to three bipyramids.
Five structures contain sheets that are based upon
the graph designated L3/5b (Fig. 6q). Each contains
a uranyl chromate sheet with a bipyramid-to-tetrahedron ratio of 3:5 (Fig. 6r). All bipyramids share theirfive equatorial vertices with five tetrahedra, and all
five of the tetrahedra are connected to three different
bipyramids.
The structures of -Cs 2 [(UO 2 ) 2 (MoO 4 ) 3 ] andCs2[(UO2)2(SO4)3] contain identical sheets (Fig. 6t),
and are the only known uranyl compounds that are
based upon graphs derived from a parent graph withthe connectivity {5.3.5.3}{5.3.5.3} (Fig. 6s) (Krivovichev 2004). Each bipyramid in this sheet is linked to
five tetrahedra, of which three of the tetrahedra areconnected to three bipyramids and two are linked tofour bipyramids.
The {4.4.4.4} graph shown in Figure 6u corresponds
to the autunite anion-topology of Burns et al. (1996).
The autunite-type sheet shown in Figure 6v is formedby uranyl square bipyramids and tetrahedra that areconnected by sharing vertices, such that each bipyramid
is connected to four tetrahedra, and each tetrahedron islinked to four bipyramids. Structures of the autunite
and meta-autunite groups contain this sheet, with either
phosphate or arsenate tetrahedra. The sheet is based
upon the autunite anion-topology that is composed ofsquares that share vertices and edges (Burns et al. 1996).
This is a large family of structures, with 17 refined thus
far, of which 10 correspond to minerals. The structures
vary considerably in the occupation and connectivityof their interlayer constituents (e.g., Locock & Burns
2003a, d). The minerals of this group are commonly
associated with uranium deposits containing phosphate,and have a significant impact upon uranium mobility insuch situations due to their low solubilities (Murakami
et al. 1997). Current studies are addressing the possi-

1856

THE CANADIAN MINERALOGIST

bility that enhanced formation of autunite-groupminerals in the subsurface may reduce uranium mobility
in the vadose zones of contaminated sites.
The sheet of vertex-sharing uranyl square bipyramids
shown in Figure 6w is also based upon the {4.4.4.4}
graph presented in Figure 6u. This sheet occurs inseven structures (Table 4). In five cases, the structures
are anhydrous, and sheets are connected by bonds tointerlayer cations including Li, Ba, Sr, Na and Pb. In
-UO2(OH)2 and -UO2(OH)2, all of the equatorial
vertices of the uranyl square bipyramids are occupied byhydroxyl groups, and linkages between adjacent sheets
are assured through H bonds only.
The structure of K 4[(UO 2)(PO 4) 2] contains theunusual sheet of uranyl square bipyramids and phosphate tetrahedra shown in Figure 6y. This sheet can beobtained from the autunite sheet (Fig. 6v) by deleting
every second bipyramid. The sheet contains twice as
many tetrahedra as bipyramids, each bipyramid is linked
through its equatorial vertices to four tetrahedra, but the
tetrahedra are only connected to two bipyramids.
Miscellaneous sheets based upon uranyl pentagonal
bipyramids and tetrahedra (or Se4+O3 pyramids) dominated by single linkages between polyhedra are shownin Figure 7. The structures of Pb[(UO2)(SeO3)2] andBa[(UO2)(SeO3)2] contain sheets with identical compositions, but different connections between polyhedra(Figs. 7a, b). In both sheets, each bipyramid is linked
to four different SeO3 pyramids, three by the sharingof a single vertex, and one by sharing an edge. Halfof the SeO3 groups are linked to two bipyramids onlyby sharing single vertices (designated A), whereas theother half are connected to one bipyramid by sharing
a vertex, and another by sharing an edge (designatedB). The distinction between the two sheets lies in thearrangement of these two types of SeO3 groups. In the
sheet shown in Figure 7a, a circuit about any bipyramid
involves the sequence ABAB, whereas the circuits in the
sheet in Figure 7b involve the sequence AABB.
The structure of K[(UO 2)(CrO 4)(OH)](H 2O) 1.5
contains dimers of edge-sharing uranyl pentagonal
bipyramids, and all of the equatorial vertices of the
bipyramids that are not shared within the dimer are
linked to chromate tetrahedra (Fig. 7c). Each tetrahedron
is connected to three different dimers of bipyramids.
The structures of Cs[(UO2)(PO3)3] (Fig. 7d) and[(UO2)H(PO3)3] (Fig. 7e) are closely related in thateach contains sheets of uranyl pentagonal bipyramids
and phosphate tetrahedra, with groups of tetrahedra
linked by vertex sharing. In Cs[(UO2)(PO3)3], threephosphate tetrahedra are linked, and non-bridging O
atoms are shared with four different bipyramids. Allof the equatorial vertices of the bipyramids are sharedwith tetrahedra. In [(UO2)H(PO3)3], phosphate tetrahedra are linked into dimers and trimers, and only fourof the equatorial vertices of the bipyramids are sharedwith tetrahedra.

The structure of (NH 4)[(UO 2)F(SeO 4)](H 2O) isbased upon the uranyl selenate sheet shown in Figure 7f.
The sheet contains uranyl pentagonal bipyramidsthat are linked into chains by the sharing of a single
vertex between adjacent bipyramids. According toBlatov et al. (1989), the vertices shared between the
bipyramids correspond to F, which is uncommon in
uranyl compounds. The three equatorial vertices ofthe bipyramids that are not shared within the chain arelinked to selenate tetrahedra. Each selenate tetrahedron
is connected to three bipyramids of two chains, resulting
in the uranyl selenate sheet. It is interesting to notethat this is the only known sheet that involves uranyl
pentagonal bipyramids and tetrahedra in which the
bipyramids are linked by sharing single vertices.
The structure of Co(H2O)6[(UO2)5(SO4)8(H2O)](H2O)5 contains the complex sheet of uranyl pentagonal
bipyramids and sulfate tetrahedra shown in Figure 7g.
The structure has two types of bipyramids; those thatshare all five, and those that share only four of theirequatorial vertices with sulfate tetrahedra. In the latter
case, the fifth equatorial vertex of the bipyramid corresponds to H2O. There are three types of tetrahedra
in the sheet, which share two, three, or four of their
vertices with bipyramids. The inclusion of H2O in an
equatorial position of some of the bipyramids, as wellas the connection of some sulfate tetrahedra to only two
bipyramids, results in small pores within the sheets ofpolyhedra.
Sheet anion-topologies containing
triangles and pentagons
Those structures based upon sheet anion-topologies
containing only triangles and pentagons are presented
in Table 5 and Figure 8. Only five such topologies havebeen found in structures, and in all but one case, allof the pentagons of the anion topologies are occupied
with uranyl ions, giving uranyl pentagonal bipyramids,
whereas the triangles remain vacant. Thus, the sheet
anion-topologies in Figure 8 represent five distinct
types of sheets composed only of uranyl pentagonal
bipyramids.
Protasite anion-topology: The protasite aniontopology (Fig. 8a) is the basis for two distinct typesof sheet. Population of each pentagon of the anion
topology with a uranyl ion results in the well-known
-U3O8-type sheet of edge-sharing uranyl pentagonal
bipyramids (Fig. 8b). This sheet is the basis of seven
known uranyl oxide hydrate minerals, as well as foursynthetic phases. The sheet topology is compatible witha variety of interlayer constituents, including Pb, Na, K,
Ca, Sr, Ba and Cs. The net charge of the sheet is variable, as there are four distinct distributions of hydroxylions known for this anion topology (Fig. 9).
Three uranyl sulfates are known that contain a sheet
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 polyhedra 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

this sheet, uranyl pentagonal bipyramids are connectedby sharing vertices with sulfate tetrahedra, such thateach bipyramid and each tetrahedron is four-connected
within the sheet.
Na[(UO2)4O2(OH)5](H2O)2 anion-topology: The
structure of Na[(UO2)4O2(OH)5](H2O)2, which was
obtained by mild hydrothermal synthesis, represents
the first occurrence of its underlying sheet aniontopology (Fig. 8d). The composition of this sheet
is distinct from any known sheet with the protasiteanion-topology, despite the fact that it is composed
only of uranyl pentagonal bipyramids. The sheet in
Na[(UO2)4O2(OH)5](H2O)2 is closely related to that of
Ca[(UO2)4O3(OH)4](H2O)2, as outlined below.
Fourmarierite anion-topology: The fourmarierite
anion-topology (Fig. 8f) is the basis of topologically
identical sheets found in fourmarierite, schoepite, andmetaschoepite. The sheets in each mineral are arrangements of uranyl pentagonal bipyramids; in fourmarierite, the sheets have a net negative charge, whereas
those in schoepite and metaschoepite are electroneutral.

In fourmarierite, Pb2+ cations and H2O groups occupy
interlayer positions, but the interlayers of schoepite
and metaschoepite contain only H2O. In a study of thecrystal chemistry of fourmarierite, Li & Burns (2000a)found evidence for significant variability of both theamount of Pb in the interlayer, and the OH content
of the sheets. Li & Burns (2000a) also synthesized acrystal intermediate in composition between schoepite
and fourmarierite, suggesting that a solid-solution series
involving a coupled substitution is possible between
these minerals. Li & Burns (2000a) proposed thegeneral formula Pb(1–x)[(UO2)4O(3–2x)(OH)(4+2x)](H2O)4
for fourmarierite.
Vandendriesscheite anion-topology: The vandendriesscheite anion-topology is the most complex
topology known that contains only pentagons andtriangles (Figs. 8h, i). The sheet is truly extraordinary
in that it contains a 41 Å primitive repeat-distance, areflection of the complex arrangement of pentagons andtriangles in its corresponding anion-topology. Notably,
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 topologythat are not marked by circles correspond to O atoms.

upon pentagons and triangles, and factors that resultin the stabilization of the complex vandendriesscheite
sheet are unclear. However, the sheet is related to asimpler sheet that contains only uranyl pentagonal
bipyramids. Using an approach involving stacking
sequences of chains of polygons, Burns (1997) showedthat the vandendriesscheite anion-topology is builtof modules of the much simpler protasite (-U3O8)
anion-topology.
Cs2[(UO2)2O3] anion-topology: The structures of
the alpha and beta polymorphs of Cs2[(UO2)2O3] eachpossess sheets of uranyl pentagonal bipyramids (Fig. 8k)that are based upon the sheet anion-topology shown in
Figure 8j. Each of the pentagons in the anion topology
share three of their edges with adjacent pentagons, andall of the pentagons are populated by uranyl ions in theobserved sheets. The sheet contains a zig-zag chain ofedge-sharing pentagonal bipyramids that is two polyhedra wide (vertical in Fig. 8k), topologically identicalto that in the zippeite sheet (see below, Fig. 11v). Unlikein the zippeite sheet, these chains are linked by sharingof edges between bipyramids of adjacent chains. The

triangles of the topology are vacant, and the Cs cations
are located between the sheets of polyhedra.
The distinction between the various sheetscontaining only uranyl pentagonal bipyramids is mostapparent in the distribution of the triangles within
the sheet anion-topology, which remain vacant in the
populated sheets. In the case of the protasite aniontopology and derivative sheets, each triangle is isolated
from the others and points in the same direction. In
Na[(UO2)4O2(OH)5](H2O)2, the triangles occur in pairsthat share a vertex, resembling bowties, and there are
two distinct orientations of bowties. The fourmarierite
anion-topology also contains triangle bowties, but differsfrom Na[(UO2)4O2(OH)5](H2O)2 in that each is oriented
in the same direction. The vandendriesscheite aniontopology contains both isolated triangles and triangle
bowties; all bowties are aligned, whereas the isolated
triangles occur in two orientations. The Cs2[(UO2)2O3]
anion-topology contains pairs of triangles that share anedge, giving a flattened rhomb.
The uranyl oxide hydrates based upon sheets ofuranyl pentagonal bipyramids are the first minerals

1860

THE CANADIAN MINERALOGIST

to form where uraninite, UO2+x, is altered in a moist,
oxidizing environment (Finch & Ewing 1992). Studies
of natural analogues, as well as laboratory-scale simulations, have also demonstrated that several of theseminerals form where spent nuclear fuel is altered in anoxidizing environment in the presence of water, similar
to conditions expected in the proposed geological repository for nuclear waste at Yucca Mountain, Nevada (e.g.,
Pearcy et al. 1994, Finn et al. 1996, Finch et al. 1999a).
Recently, Burns et al. (2004a) showed that Np5+ isincorporated into synthetic powders of the Na analogueof compreignacite, which supports the hypothesis thatformation of such minerals in a geological repository
may impact the future mobilities of key radionuclides.
Sheet anion-topologies containing triangles,
squares and pentagons
Seventy-one uranyl compounds are known that
contains sheets that are based upon anion topologies
containing triangles, squares and pentagons. In all cases
at least some of the pentagons are populated by uranyl
ions; thus all sheets in this class contain uranyl pentagonal bipyramids. The triangles of the anion topologies
are occupied by BO3 groups in two structures, and by
the face of a tetrahedron in many structures. The squaresites of the anion topologies provide coordination environments for cations in square pyramidal or octahedral
coordination.
Uranophane anion-topology: The uranophane
anion-topology is elegant in its simplicity (Fig. 10a). It
contains chains of edge-sharing pentagons that are separated by chains of edge-sharing triangles and squares.
This anion topology is the basis for a chemically diverse
group of compounds that includes the important groupof the uranophane minerals. In all but two of the representative structures, the pentagons are all populated byuranyl ions, giving pentagonal bipyramids. In the caseof ulrichite, half of the pentagons are populated by Ca,
and half of the pentagons contain Na in Na5.5(UO2)3(H0.5PO4)(PO4)3, but the Na is not considered to be partof the structural unit for the purposes of illustration in
Figure 10h. There are 17 structures known that containsheets based upon the uranophane anion-topology, of
which 11 are minerals; they are grouped in Table 10(see below) according to the specific population of the
topology.
In the compounds Na[(UO2)(BO3)] and Li[(UO2)(BO3)], the triangles of the uranophane anion-topology
are populated by BO3 groups (Fig. 10b). These anhydrous compounds are readily synthesized at elevatedtemperatures, and contain topologically identicalsheets.
The uranophane-group minerals are uranyl silicatesthat have sheets based upon the uranophane aniontopology. In each case, the triangles of the topology
correspond to the faces of silicate tetrahedra, but threegraphical isomers of the uranyl silicate sheet are known

to occur because of different orientations of the silicate
tetrahedra. This may readily be seen by examining the
sheets given in Figures 10c–e, with emphasis on theorientation of the apical, non-bridging, O or OH groupsof the silicate tetrahedra. The structure of oursinite,which was only recently reported, represents a newgraphical isomer. With the exception of kasolite, all of
the uranophane-group minerals with known structurescontain acid silicate groups, where the hydroxyl groupis non-bridging within the sheet.
Studies of natural analogues (Pearcy et al. 1994),
as well as laboratory simulations (Wronkiewicz et al.
1992, 1996, Finch et al. 1999a) have shown that alteration of spent nuclear fuel in the proposed geologicalrepository at Yucca Mountain, Nevada, will likely resultin the formation of uranophane and boltwoodite. Burns
et al. (1997b) argued that these minerals may incorporate various radionuclides released from the nuclear
waste, and as such, they may have an impact upon rates
of radionuclide release from the proposed repository.
Using crystals of natural boltwoodite, Burns (1999c)ion-exchanged Cs into interlayer positions of thestructure by treating crystals with solutions containing
CsCl at elevated temperatures. Notably, ion exchange
was accomplished for single crystals that remained
intact even following exchange, and ion exchange wasdemonstrated by structure solution for a treated crystal.
This was the first demonstration of ion exchange in asingle crystal of a uranyl mineral, and demonstrated thatradioactive Cs can be accommodated in the interlayer
region of boltwoodite. Recently, Burns et al. (2004a)
showed that Np5+ is incorporated into powders of
uranophane synthesized at pH ~5 and 100°C, conditions
similar to those expected in the proposed repository
at Yucca Mountain where water may first contact thenuclear waste.
The structure of ulrichite was recently revisited byKolitsch & Giester (2001); this work confirmed manyof the earlier-reported aspects of the structure, but
provides a new space-group assignment and significantly improved precision of the structural parameters.
Ulrichite contains a sheet that is based upon the uranophane anion-topology, with half of the pentagons populated by uranyl ions, and the other half populated byCa8 polyhedra (Fig. 10f). The triangles of the topology
are populated by phosphate tetrahedra.
The structure of umohoite was once considered
to contain sheets of uranyl hexagonal bipyramids(Makarov & Anikina 1963). A redetermination of
the structure revealed a new uranyl molybdate sheetthat is based upon the uranophane anion-topology(Krivovichev & Burns 2000a). Each pentagon in theanion topology is populated by a uranyl ion, giving
uranyl pentagonal bipyramids, and the squares of the
topology are equatorial vertices of distorted Mo5
polyhedra (Fig. 10g), and the non-sheet ligand is H2O.
The interlayer of the structure contains only H2O, withthe 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 inTable 6.

The structures of K2[(UO2)2O3], [H2(UO2)3O4],[(UO 2)(CuO 4)], schmitterite, and K[(UO2)(NbO 4)]
all contain sheets based upon the uranophane aniontopology, in which all pentagons are occupied by uranyl
ions, and the squares of the topology are occupied byvarious cations (Figs. 10i–m). In K2[(UO2)2O3] (Fig. 10i)and [H2(UO2)3O4] (Fig. 10j), uranyl ions populate thesquares of the anion topology, resulting in uranyl squarebipyramids, with all squares occupied in K2[(UO2)2O3]
and only half of the squares occupied in [H2(UO2)3O4].
Schmitterite is unusual in that it is the only uranyl
tellurite with a structure based upon the uranophane
anion-topology (Fig. 10l). Each square contains Te4+
in a highly distorted pyramidal geometry owing to thestereoactive lone-electron pair on the cation.

A substantial family of synthetic uranyl phosphate
framework structures has recently been discoveredthat contains sheets based upon the uranophane aniontopology, with the triangles populated by the faces ofphosphate tetrahedra. Uranyl polyhedra in the interlayer regions provide direct connections between the
sheets, and this family of structures is included withthe framework structures discussed below (see below,
Figs. 15m–o).
Francevillite anion-topology: The francevillite
anion-topology (Fig. 11a) consists of pairs of edgesharing pentagons and edge-sharing squares as wellas triangles. Twelve structures are known in which
sheets are based upon this anion topology, of whichfour are minerals (Table 6). In all of the sheets, each

1862

THE CANADIAN MINERALOGISTHIERARCHY OF U6+ MINERALS AND INORGANIC COMPOUNDSof the pentagons of the anion topology is populated byuranyl ions, giving uranyl pentagonal bipyramids. The
francevillite group of minerals, as well as synthetic
uranyl vanadates, involves population of the squares of
the anion topology by VO5 square pyramids, resulting
in dimers of edge-sharing pyramids. Within each dimer,
the apical vertices of the two adjacent square pyramids
point in opposite directions. One structure is known

1863

that contains NbO5 square pyramids. Recently, fivestructures have been reported with the francevillite-type
sheet in which the squares are populated by CrO5 squarepyramids (Locock et al. 2004). These structures are
remarkable in that they contain pentavalent chromium,
which is uncommon in crystal structures. In each case,low-valence cations in the interlayers of the structures,

1864

THE CANADIAN MINERALOGIST

which also contain H2O groups in some cases, provide
linkages between the sheets.
-U 3 O 8 anion-topology: The -U 3 O 8 aniontopology (Fig. 11c) is the basis of the structure of
-U3O8, which contains U in multiple valence states,
and three minerals. This topology is similar to theuranophane anion-topology in that both contain identical chains of edge-sharing pentagons, as well as chainsof edge-sharing triangles and squares. The distinctionis that pentagons of adjacent chains share vertices onlyin the -U3O8 anion-topology, which also contains
more triangles. In the case of the sheet in -U3O8, allof the pentagons and squares of the anion topology are
populated by U. Wyartite is the only mineral known tocontain pentavalent uranium, although several synthetic
compounds contain uranium in this valence state(Burns & Finch 1999). The sheet of uranyl polyhedra
in wyartite is closely related to that in -U3O8, but oneof the U polyhedra contains pentavalent U, and the CO3group shares an edge with one U polyhedron such that it
extends into the interlayer. The sheets are linked throughbonds to Ca in the interlayer, as well as by a network of
H bonds. The structures of ianthinite and spriggite eachhave sheets of uranyl pentagonal and square bipyramidsthat are topologically identical to those in -U3O8; in
spriggite, all of the U is hexavalent, whereas the U in
the structure of ianthinite has multiple valence states.
On the basis of an imprecise refinement of the structure,
Burns et al. (1997c) reported that ianthinite probably
contains both U6+ and U4+, but the subsequent discovery
of wyartite suggests that ianthinite also may containpentavalent U.
Iriginite anion-topology: Iriginite, as well as two
synthetic uranyl molybdates, contain sheets based upon
the iriginite anion-topology, shown in Figure 11e. The
sheets are identical in these three structures, and containuranyl pentagonal bipyramids and distorted Mo 6octahedra 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 Mo6octahedra, two by sharing edges and two by sharing
only vertices. In the case of [Ca(UO 2)(Mo 2O 7) 2],
pentagons of the anion topology are populated by Ca
as well as U.
Sayrite anion-topology: Two structures are known
that contain sheets of uranyl square and pentagonal
bipyramids (Fig. 11h) that are based upon the sayrite
anion-topology (Fig. 11g). The squares and pentagonsof the anion topology are each populated by uranyl ions;thus the resulting sheet is composed of edge- and vertexsharing uranyl square and pentagonal bipyramids. The
sheet is anhydrous in K2[(UO2)5O8](UO2)2, and theinterlayer of the structure contains uranyl ions. In thecase of the rare mineral sayrite, the sheet contains H
present as OH groups, and adjacent sheets are linked
by bonds to Pb2+ cations and H2O groups located in
the interlayer.
[Pb3(UO2)11O14] anion-topology: The isostructuralcompounds [Pb 3 (UO 2) 11O 14] and [Sr 3(UO 2) 11O 14]
contain complex sheets of uranyl square and pentagonal bipyramids and either Pb7 or Sr7 pentagonal
bipyramids (Fig. 11j). In this case, inclusion of Pb andSr in the structural unit is warranted, as they assume
similar structural roles as U. Adjacent sheets are linked
directly through the Pb7 or Sr7 polyhedra. These
structures are remarkable in the presence of Pb2+ andSr2+ within the sheets of uranyl polyhedra, which is
not known to occur in any of the structures of uranyloxide hydrates.
Curite anion-topology: The curite anion-topology(Fig. 11k), which contains pentagons, distorted squares,
and triangles, is the basis of sheets of uranyl pentagonal
and square bipyramids in curite and its synthetic Srsubstituted analogue. Although the square bipyramid inthis sheet is strongly distorted from an ideal geometry,similarly distorted square bipyramids also occur in the
structures of K5[(UO2)10O8(OH)9](H2O) (Fig. 11n),
Ca[(UO2)4O3(OH)4](H2O)2 (Fig. 11p), and Pb2(H2O)[(
UO2)10UO12(OH)6(H2O)6] (see below, Fig. 15g). Li &Burns (2000b) studied the structures of several crystalsof curite, including a synthetic crystal, and found that thePb sites are partially occupied in each crystal studied.
The general formula Pb3+x(H2O)2 [(UO2)4O4+x(OH)3–x]2
was proposed, and Pb variability in the interlayer isaccompanied by changes of the hydroxyl content of
the sheet of uranyl polyhedra, which provides a chargebalance mechanism. Synthesis and structure determination of the Sr-substituted analogue of curite (Burns &Hill 2000a) demonstrate that the unusual sheet of uranyl
polyhedra found in curite is not a result of the distorted
coordination environment about the interlayer Pb2+
cation caused by a stereoactive lone-electron pair.
K5[(UO2)10O8(OH)9](H2O) anion-topology: Burns& Hill (2000b) reported the structure of a new K uranyloxide hydrate that contains complex sheets of uranyl
pentagonal bipyramids and highly distorted uranylsquare bipyramids (Fig. 11n). Despite its chemical simi-

larity to compreignacite, this phase has not been foundas a mineral. The sheet within its structure consists ofdouble-width chains of bipyramids that share equatorial
edges; adjacent chains are connected by the sharing ofvertices and edges between bipyramids. The chainsare similar to those of the curite-type sheet (Fig. 11l),
except that they contain fewer square bipyramids,
although these polyhedra are similarly distorted in thetwo structures.
Ca[(UO2)4O3(OH)4](H2O)2 anion-topology: Despite
its chemical similarity with becquerelite, the structureof Ca[(UO2)4O3(OH)4](H2O)2 is based upon a novel
sheet of uranyl pentagonal bipyramids and distorted
square bipyramids (Fig. 11p). The sheet contains two
distinct double-width chains of bipyramids, one that
contains only pentagonal bipyramids, and one that
contains equal proportions of square and pentagonal
bipyramids. The chains are linked into sheets only by
the sharing of vertices between bipyramids. The sheet
is similar to the curite-type sheet and the sheet found in
K5[(UO2)10O8(OH)9](H2O) in the dominance of doublewidth chains and the presence of highly distorted squarebipyramids. Ca[(UO2)4O3(OH)4](H2O)2 has not beenfound in nature, but it is readily synthesized under avariety of conditions, and it commonly occurs withbecquerelite in products of synthesis (Glatz et al. 2002).
Recently, Kubatko et al. (2005) determined the heatsof formation of several uranyl oxide hydrates using
drop-solution calorimetry. Those results demonstrated
that Ca[(UO2)4O3(OH)4](H2O)2 is unstable in terms ofenthalpy relative to the oxides; thus it appears to form
metastably in synthesis reactions.
The sheet in Ca[(UO2)4O3(OH)4](H2O)2 is closely
related to the sheet of uranyl pentagonal bipyramids foundin the structure of Na[(UO2)4O2(OH)5](H2O)2 (Fig. 8e).
The U(3) square bipyramid in Ca[(UO2)4O3(OH)4](H2O)2
has only four equatorial ligands; if this were a pentagonal
bipyramid, the sheet would be topologically identicalto that in Na[(UO2)4O2(OH)5](H2O)2. Displacement ofa single atom by ~0.6 Å is the topological difference
between these two sheets.
Cs4[(UO2)5O7] anion-topology: The Cs4[(UO2)5O7)]
anion-topology (Fig. 11q) is the basis for the sheet (Fig.
11r) found only in Cs4[(UO2)5O7]. The sheet contains
both uranyl square and pentagonal bipyramids that areconnected by sharing equatorial edges and vertices.
Adjacent sheets are linked through bonds to the Cs
cations in the interlayer of the structure.
Vandenbrandeite anion-topology: The vandenbrandeite anion-topology (Fig. 11s) is the basis for one sheet(Fig. 11t) that contains dimers of edge-sharing uranyl
pentagonal bipyramids that are linked by dimers ofedge-sharing Cu5 square pyramids. The OUr anionsof the pentagonal bipyramids are apical ligands of theCu5 square pyramids of adjacent sheets.
Zippeite anion-topology: The zippeite aniontopology and corresponding sheet are shown in Figures
11u and 11v. The sheet contains zig-zag chains of edge-

HIERARCHY OF U6+ MINERALS AND INORGANIC COMPOUNDS

sharing uranyl pentagonal bipyramids, and the chainsare connected by the sharing of vertices between the
bipyramids and sulfate tetrahedra, such that each sulfate
tetrahedron is linked to four different bipyramids. The
formulae and crystallographic parameters for zippeitegroup compounds are listed in Table 6. Vochten et al.
(1995) reported the first structure of a zippeite-structurematerial for the synthetic analogue of zippeite, theK-bearing mineral. Burns et al. (2003) presented arevised structure for zippeite, as well as structures forfour additional synthetic analogues of zippeite-groupminerals and three synthetic phases of the zippeite
group. Brugger et al. (2003) reported the structure of
the new zippeite-group mineral marecottite.
Burns et al. (2003) showed that although theuranyl sulfate sheets are topologically identical in eachzippeite-group structure studied, the sheets in someare anhydrous, whereas in others they may containhydroxyl groups located at equatorial vertices of theuranyl pentagonal bipyramids. The interlayer configurations of zippeite-group phases are diverse, and include
either monovalent or divalent cations as well as H2Ogroups. Burns et al. (2003) concluded that the distribution of interlayer constituents can be rationalized on
the basis of the bonding requirements of the uranyl
sulfate sheets.
K2[(UO2)2(WO5)O] anion-topology: Three topologically identical sheets (Fig. 11x) have recently beenfound that are based upon the anion-topology shown in
Figure 11w, which contains clusters of eight pentagons
linked by edge-sharing (Table 6). Each of these clusters
is connected to four identical clusters by sharing one
vertex with each, which results in both triangles andsquares in the anion topology. All pentagons of theanion topology are populated by uranyl ions in each of
the three structures, giving uranyl pentagonal bipyramids, and three out of four of the squares are the basesof square pyramids containing W in two structures, andMo in the other. One square pyramid shares three of its
edges with uranyl bipyramids, whereas the other two
share only one edge with a bipyramid, but two of their
vertices are shared with other bipyramids. Interlayer
cations provide linkage between the sheets.
K6[(UO2)5(VO4)2O5] anion-topology: Topologicallyand chemically identical uranyl vanadate sheets (Fig.
11z) based upon the same anion-topology (Fig. 11y)have recently been found in four anhydrous compounds(Table 6). The sheet contains three-polyhedron-wide
chains of uranyl square and pentagonal bipyramids,
identical to those found in the -U 3O 8-type sheet(Fig. 11d), that are connected into sheets by sharingedges and vertices with VO 4 tetrahedra. Linkages
between sheets are provided by monovalent cations
located in the interlayer regions of the structures.
K8[(UO2)8(MoO5)3O6] anion-topology: The uranyl
molybdate sheet contained in K8[(UO2)8(MoO5)3O6] isshown in Figure 11ab, with its corresponding aniontopology in Figure 11aa. The sheet contains groups of1867

six uranyl pentagonal bipyramids that are linked into acluster by the sharing of equatorial edges. The resulting
clusters are linked into a sheet both by sharing vertices
directly between clusters, and by sharing polyhedronedges with MoO5 groups.
[Mg(UO2)(B2O5)] anion-topology: The [Mg(UO2)(B2O5)] anion-topology, and the corresponding heteropolyhedral sheet, are shown in Figures 11ac and 11ad,
respectively. The sheet contains uranyl pentagonal
bipyramids that share two equatorial edges with BO3groups that are part of B2O5 dimers of BO3 groups.
Each bipyramid also shares two equatorial vertices
with other BO3 groups, and MgO6 octahedra share an
equatorial edge of a bipyramid, as well as vertices with
two other bipyramids.
Wölsendorfite anion-topology: The structure ofwölsendorfite contains an extremely complex sheet ofuranyl square and pentagonal bipyramids (Fig. 11af),
and has a primitive repeat-distance of 56 Å (Burns
1999b). Wölsendorfite contains the most topologicallycomplex sheet known in any uranyl compound. By
considering the sheet anion-topology of wölsendorfite(Fig. 11ae) as a stacking sequence of simpler chains,
Burns (1999b) demonstrated that the topology iscomposed of modules of the simpler protasite (-U3O8)
and -U3O8 anion topologies. Sheets are linked througha complex interlayer that contains Pb and Ba cations,as well as H2O groups.
Cs4[(UO2)3O(MoO4)2(MoO5)] anion-topology: The
uranyl molybdate sheet that occurs in the structure ofCs4[(UO2)3O(MoO4)2(MoO5)], and its corresponding
anion-topology, are shown in Figures 11ag and 11ah.
The uranyl molybdate sheet contains uranyl pentagonal
bipyramids, molybdate tetrahedra, and MoO5 polyhedra. Three bipyramids share edges and a common
O atom, resulting in trimers that are linked into chainsby the sharing of polyhedron vertices. The MoO5 polyhedron occupies a pocket in the chain of bipyramids,
where it shares two of its edges with bipyramids. The
chains of bipyramids and MoO5 polyhedra are linked
into sheets by sharing vertices with tetrahedra, and eachtetrahedron is linked to two bipyramids, one from thechain on either side.
Ag 10 [(UO 2 ) 8 O 8 (Mo 5 O 20 )] anion-topology: The
structures of Ag10[(UO2)8O8(Mo5O20)] and Na10[(UO2)8
O8(Mo5O20)] contain the sheet of uranyl pentagonal
bipyramids and MoO6 or WO6 octahedra shown in
Figure 11aj, with the corresponding sheet aniontopology shown in Figure 11ai. The sheet consistsof complex chains of edge-sharing uranyl pentagonal
bipyramids that are in turn linked through sharing equatorial edges and vertices of the bipyramids with dimersof edge-sharing MoO6 or WO6 octahedra. Identicalsheets are linked into complex double sheets througha MoO4 tetrahedron that shares two equatorial vertices
with MoO6 or WO6 octahedra in the sheet above as wellas the sheet below. The unusual linkage of molybdate

1868

THE CANADIAN MINERALOGISTand tungstate polyhedra in this structure type results inpentanuclear Mo5O20 and W5O20 clusters.
K2(UO2)2(MoO4)O2 anion-topology: The structureof K2[(UO2)2(MoO4)O2] contains the sheet of uranyl
pentagonal bipyramids and molybdate tetrahedra shownin Figure 11al, which is based upon the sheet aniontopology given in Figure 11ak. The sheet contains
chains of edge-sharing bipyramids two polyhedra widethat are cross-linked into a sheet by sharing equatorial
vertices with molybdate tetrahedra.
Sheet anion-topologies containing triangles,
squares, pentagons and hexagons
Twenty uranyl phases, including 17 minerals,
contain sheets based upon anion topologies withtriangles, squares, pentagons and hexagons (Table 7).
These include the large phosphuranylite group of uranyl
phosphate minerals, for which several new structures
have been described recently.
Phosphuranylite anion-topology: The phosphuranylite anion-topology, shown in Figure 12a, contains
pentagons that share edges, resulting in dimers, which
are in turn linked into chains by sharing edges withhexagons. These chains are linked through chains ofedge-sharing triangles and squares, arranged such thatthe triangles and squares alternate along the chain
length. Several populations of this anion topology are
known, but none involves occupancy of the squaresites. In most cases, both the pentagons and hexagons

are populated by uranyl ions, resulting in chains ofedge-sharing pentagonal and hexagonal bipyramids.
Population of the triangles of the anion topology byphosphate tetrahedra, such that a face of the tetrahedroncorresponds to the triangle of the topology, results in theuranyl phosphate sheets of the phosphuranylite group.
Four graphical isomers of this sheet are known; theydiffer in the orientations of apical (non-sheet) ligandsof the phosphate tetrahedra (Figs. 12b–e). For example,
consider the orientations of the tetrahedra attached to asingle chain of uranyl polyhedra in the phosphuranylite
sheet (Fig. 12b). Tetrahedra attached to the chain on
either side immediately opposite each other are in thesame orientation. Along the side of a chain of uranyl
polyhedra, the tetrahedra alternate up and down. In
the case of the vanmeersscheite sheet (Fig. 12c), alltetrahedra that share edges with a given chain of uranyl
polyhedra are in the same orientation.
The interlayers of the structures of minerals of thephosphuranylite group are complex, and some containmore than one type of cation, as well as H2O groups.
The structures of phosphuranylite and vanmeersscheite
are unusual in that both contain uranyl ions in all
three common coordinations: square, pentagonal andhexagonal bipyramids. The square bipyramids arelocated between the sheets in each structure, with theuranyl ions extending roughly parallel to the sheets. In
phosphuranylite, all four of the equatorial ligands ofthe square bipyramid are apical (non-sheet) verticesof the phosphate tetrahedra of sheets on either side. In

FIG. 12. Sheets of uranyl polyhedra and other polyhedra containing 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

contrast, the uranyl square bipyramid in the vanmeersscheite interlayer shares vertices with only two phosphate tetrahedra, one from each adjacent sheet. Linkageof the uranyl phosphate sheets through interlayer uranyl
polyhedra results in open three-dimensional structuresin phosphuranylite and vanmeersscheite.
The structure of hügelite contains sheets that are
topologically identical to those in vanmeersscheite anddumontite, but in this case, the triangles of the anion
topology are populated by arsenate tetrahedra.
Recent studies of uranyl carbonates have revealed
more structural complexity within this chemical class (Liet al. 2000, Hughes Kubatko & Burns 2003). Althoughmany uranyl carbonates that form under somewhat
alkaline conditions contain the isolated uranyl tricarbonate cluster, minerals formed under neutral to acidic
conditions tend to adopt sheet structures. Fontanite is
the only uranyl carbonate with a sheet based upon thephosphuranylite anion-topology (Fig. 12f). This sheet
contains uranyl pentagonal and hexagonal bipyramids,

1869

and the triangles of the anion topology are populated bycarbonate triangles, such that the CO3 group shares anedge with a hexagonal bipyramid and one vertex witha pentagonal bipyramid of an adjacent chain. Calcium
cations and H2O groups located in the interlayer provide
linkages between the sheets in fontanite.
Four uranyl selenites, of which three are minerals,
contain sheets based upon the phosphuranylite aniontopology. Each sheet has chains of uranyl pentagonal
and hexagonal bipyramids linked by bonds to Se4+O3
pyramids with a base of three O atoms and Se4+ at the
fourth apex of the pyramid (Fig. 12g). The strongly
distorted coordination environment about the Se4+
cation is due to the presence of a stereoactive lone-pairof electrons on the cation that is directed away fromthe coordinating anions. In each structure, low-valence
cations and H2O groups provide linkages between the
sheets.
Johannite and Sr[(UO 2 ) 2 (CrO 4 ) 2 (OH) 2 ](H 2 O) 8
contain sheets based upon the phosphuranylite anion-

1870

THE CANADIAN MINERALOGIST

topology (Fig. 12h). However, in this case the hexagonsof the topology are not occupied; the sheet contains
dimers of edge-sharing uranyl pentagonal bipyramidsthat are linked only by sharing vertices with sulfate or
chromate tetrahedra. Edges of the tetrahedra correspondto the hexagons of the anion topology, thus occupancyof the hexagons by uranyl ions would result in short
U6+–S6+ or U6+–Cr6+ interatomic distances.
Roulbaultite anion-topology: The structure of roulbaultite contains the complex sheet shown in Figure 12j,
which is based upon the anion topology shown in Figure
12i. This structure has notable similarities to that of
fontanite (Fig. 12f). Both contain the same chain of
uranyl pentagonal and hexagonal bipyramids, and CO3
triangles. In fontanite, these chains are linked directly
through the sharing of vertices between the CO3 groupof one chain with uranyl pentagonal bipyramids ofan adjacent chain. In contrast, the roulbaultite sheet
also contains chains formed by the sharing of edges
between Cu6 octahedra, and uranyl carbonate chainsare separated by chains of octahedra on either side.
The uranyl carbonate chains are linked to the chainsof octahedra through the sharing of non-chain verticesof the CO3 triangles, as well as equatorial vertices ofuranyl pentagonal bipyramids. Linkages between the
sheets in roulbaultite occur by H bonds to interlayer
H2O groups only.
Miscellaneous anion-topologies with hexagons
Twenty-one compounds, of which only one is amineral, are grouped as miscellaneous sheets with anion
topologies containing hexagons (Table 8, Fig. 13). Only
six anion topologies are needed to account for all of the
21 structures.
-UO3 anion-topology: The simplest anion-topology
involves a two-dimensional tiling of space using only
hexagons (Fig. 13a), and corresponds to the sheetsof uranyl hexagonal bipyramids (Fig. 13b) that are
known from 10 structures. The structure of -UO3 has
sheets constructed from hexagonal bipyramids, but inthe reported structure, no uranyl ions are present, andadjacent sheets are linked through the sharing of apicalligands between adjacent sheets. In the structure of -HIERARCHY OF U6+ MINERALS AND INORGANIC COMPOUNDS[(UO2)(OH)2], sheets of uranyl hexagonal bipyramids
are connected through H bonds that extend through theinterlayer region. Eight additional structures are based
upon sheets of hexagonal bipyramids, with linkages
provided by low-valence cations in the interlayers(Table 8).
Rutherfordine anion-topology: The rutherfordine
anion-topology is composed of hexagons and triangles(Fig. 13c). It contains chains of edge-sharing hexagons
that are identical to those of the -UO3 anion-topology(Fig. 13a), but these chains are now connected in thesecond dimension only by the sharing of vertices rather
than edges, and adjacent chains are separated by triangles. The sheet in the structure of rutherfordine has all of
the hexagons of the anion topology populated by uranyl
ions, and half of the triangles correspond to CO3 groups,
whereas the remaining triangles are vacant (Fig. 13d).
Note that all of the CO3 triangles are in the same orientation. The structure of [(UO2)(SeO3)] contains sheets thatare topologically identical to those in rutherfordine, but
the triangles of the topology are populated by Se4+O3
polyhedra that are distorted owing to the lone-electron
pairs on the Se4+ cations. Neither rutherfordine or[(UO2)(SeO3)] have interlayer constituents.
K2[(UO2)(W2O8] anion-topology: Four structures
have recently been reported (Table 8) that contain theanion topology shown in Figure 13e. The corresponding
sheet contains chains of edge-sharing uranyl pentagonal
bipyramids. Two equatorial edges of the bipyramids areshared with tungstate octahedra, such that octahedra are
attached to either side of the chain (Fig. 13f). Adjacenturanyl tungstate chains are connected by the sharing ofvertices between the tungstate octahedra. Low-valence
cations provide linkages between the sheets, and eachof these structures is anhydrous.
[(UO2)(BO3)O] anion-topology: The uranyl borate
sheet in the structure of [(UO2)(B2O3)O] (Fig. 13h)
contains uranyl hexagonal bipyramids and borate
triangles. The anion topology of the corresponding sheet(Fig. 13g) is similar to that of rutherfordine (Fig. 13c)in the presence of chains of edge-sharing hexagons thatare separated by triangles, but in this case the chains are
not directly linked, but rather are separated by chainsof edge-sharing triangles that are two triangles wide.
The uranyl borate sheets are linked through van der
Waals forces only.
[(UO2)Nb3O8] anion-topology: Two uranyl niobates
and one uranyl vanadate contain the sheet shown in
Figure 13j (Table 8), which is based upon the anion
topology in Figure 13i. The squares in the anion topology
are populated by NbO6 octahedra in [(UO2)Nb3O8], byboth NbO6 and TiO6 octahedra in [(UO2)TiNb2O8],
and by VO5 square pyramids in CsUV3O11. In all threestructures, the sheets are directly linked, with the OUratoms of one sheet being the apical ligands of the octahedra or square pyramids of the adjacent sheet.
[(UO2)(Sb2O4)] anion-topology: The compound[(UO 2)(Sb 2O 4)] possesses a novel sheet of uranyl

1871

hexagonal bipyramids and SbO4 square pyramids (Fig.
13l), with the sheet anion-topology shown in Figure
13k. The sheet contains chains of edge-sharing bipyramids similar to those in rutherfordine (Fig. 13d) and[(UO2)(B2O3)O] (Fig. 13h). Unlike the case of rutherfordine, these chains are not directly linked, but rather
are connected through zigzag chains of edge-sharing
SbO4 square pyramids. Note that the SbO4 polyhedra
are distorted because of the presence of a stereoactive
lone-electron pair, and are geometrically similar to the
Te4+O4 polyhedra in the structure of schmitterite (Fig.
10l).
Miscellaneous topologies
The compounds listed in Table 9 are based upon avariety of sheets with miscellaneous topologies. Each
sheet is shown in Figure 14. The structure of the uranyl
carbonate bijvoetite is unusual in that it contains uranyl
carbonate sheets (Fig. 14a), whereas the majority ofuranyl carbonates contain isolated uranyl tricarbonate
clusters. The sheet contains both uranyl pentagonal
and hexagonal bipyramids, and there are dimers ofeach that result from the sharing of equatorial edges.
The dimers of hexagonal bipyramids are oriented
perpendicular to the length of chains of uranyl polyhedra, and are connected through the dimers of uranyl
pentagonal bipyramids, which extend along the lengthof the chains. As such, each of the equatorial verticesof the pentagonal bipyramids are shared with otheruranyl polyhedra, but two of the equatorial vertices ofeach hexagonal bipyramid are bonded to only one U6+
cation. The resulting chain of polyhedra is unique inuranyl compounds. Carbonate groups are attached to thesides of the chains by sharing edges with the hexagonal
bipyramids, such that each bipyramid is linked to two
carbonate triangles. The chains are linked into sheets
through irregular coordination-polyhedra about trivalentcations, which correspond to Y and various rare-earth
elements, especially Dy. The sheets are linked throughH bonds to H2O groups located in the interlayer of thestructure.
The structure of haiweeite contains sheets of uranyl
pentagonal bipyramids and silicate tetrahedra (Fig.
14b). The bipyramids share edges, giving a chain that isone bipyramid wide, and silicate tetrahedra share equatorial edges of the bipyramids along both sides of thechain. The resulting uranyl silicate chain has the sameanion-topology as that found in the uranyl silicates of
the uranophane group (Fig. 10). However, the way thesechains are connected into sheets distinguishes haiweeite
from the uranophane group. The chains are directly
linked in the uranophane group, as silicate tetrahedra
bridge between adjacent chains of bipyramids. In thehaiweeite sheet, the uranyl silicate chains are linked
through a complex crankshaft-like chain of silicate
tetrahedra, and two such chains in different orientations
are superimposed in the long-range structure, which may

1872

THE CANADIAN MINERALOGIST

be consistent with a domain structure or true disorder ofthe chain orientations. The resulting complex sheets arelinked by Ca cations in the interlayer, as well as throughH bonds to interlayer H2O groups.
The structure of Ag4[(UO2)4(IO3)2(IO4)2O2] contains
a sheet of uranyl pentagonal and hexagonal bipyramids, as well as both I5+O4 and I5+O3 polyhedra. The

hexagonal bipyramids share an edge, forming a chain
one polyhedron wide that is identical to that found in
rutherfordine and other structures shown in Figure 13.
However, unlike the structures in Figure 13, pentagonal
bipyramids are attached to each side of this chain bysharing two of their equatorial edges with two different
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 otherpolyhedra containing higher-valence cations. The corresponding compounds are listed in Table 9.

amongst uranyl compounds. Distorted I4+O4 pyramids
are attached to both sides of the chains of uranyl polyhedra by sharing three of their vertices with uranyl polyhedra. The I4+O4 pyramids, which were described forthe first time from this structure, are strongly distortedby the presence of a stereoactive lone-electron pair. The
complex chains are linked through I5+O3 pyramids that
also are distorted because of a stereoactive lone-electron
pair, and these pyramids share one vertex with the chain
on either side. The sheets are linked through interlayer
Ag cations.
The structure of Ag6[(UO2)3O(MoO4)5] contains
unusual sheets of uranyl pentagonal bipyramids andmolybdate tetrahedra (Fig. 14d). Trimers of uranyl
pentagonal bipyramids result from the sharing of edges,
and these polyhedra share one common O atom. The
trimers are connected by sharing vertices with molybdate tetrahedra that share one ligand with each of two
such trimers. There is also a molybdate tetrahedronthat bridges between two uranyl polyhedra of the same
trimer. This sheet is unusual in that it has the only occurrence of an isolated trimer of uranyl polyhedra. Similar
trimers exist in other sheets, but in all other cases they
are linked directly by the sharing of vertices and edges
between the uranyl polyhedra. The sheets are connected
through bonds to interlayer Ag cations.
The structure of K[UO2Te2O5(OH)] contains the
sheets of uranyl square bipyramids and distorted Te4+O4

pyramids shown in Figure 14e. The Te4+O4 pyramids,
which are distorted because of a stereoactive lone-electron pair, share vertices to form a chain. These chainsare linked by the sharing of equatorial vertices of the
bipyramids with the Te4+O4 pyramids. The sheets arelinked through bonds to the K cations, which are locatedwithin the voids in the sheets.
The structure of Cs4[(UO2)2(V2O7)O2] contains the
sheets of uranyl square bipyramids and vanadate tetrahedra shown in Figure 14f. The uranyl square bipyramids share equatorial vertices, resulting in a chain one
polyhedron wide that is identical to those found isolated
as chains in several anhydrous compounds (Fig. 5a).
The chains are linked into sheets by sharing equatorial vertices with vanadate tetrahedra of pyrovanadategroups. Sheets are linked through Cs cations in theinterlayer.
The structure of Tl3{(UO2)2[Te2O5(OH)](Te2O6)}•
2H2O has the sheets of uranyl pentagonal bipyramids
and Te 4+ O 4 pyramids shown in Figure 14g. The
distorted Te4+O4 pyramids share vertices, resulting in
chains similar to those in K[UO2Te2O5(OH)] (Fig. 14e).
The chains of Te4+O4 pyramids are linked by sharing
equatorial vertices of the uranyl pentagonal bipyramids, and sheets are linked through the Tl cations in
the interlayer.
The structure of Cs6[(UO2)4(W5O21)(OH)2(H2O)2]contains unusual sheets of uranyl pentagonal bipyra-

1874

THE CANADIAN MINERALOGIST

mids, tungstate octahedra, and tungstate squarepyramids (Fig. 14h). Four pentagonal bipyramids
share vertices, forming a ring, with a tungstate squarepyramid located at the center of the ring, such that its
equatorial edges are also equatorial edges of the uranyl
polyhedra. The resulting clusters are linked to four
identical clusters through sharing edges and vertices
with dimers of tungstate octahedra that are formed bysharing an edge.

STRUCTURES CONTAINING INFINITE
FRAMEWORKS OF POLYHEDRA
Burns et al. (1996) included only 26 structures intheir hierarchy that contained frameworks with uranyl
polyhedra. Through the detailed work of several groups
in synthesizing new structures, this structural class has
now ballooned to 56 (Table 10).
Frameworks of uranium polyhedra
Seven framework structures are based solely on
uranium polyhedra. Four of these are polymorphs of
UO3 (Figs. 15a–d). These structures involve a rangeof U6+ coordination polyhedra, and two (-UO3 and-UO3) do not contain uranyl ions. The structure of(UO2)Cl2 does contain a uranyl ion, but four of theequatorial vertices of the pentagonal bipyramids in the
structure are Cl atoms. The bipyramids are linked by
the sharing of edges, resulting in chains that are one

polyhedron wide (Fig. 15e). Identical chains are known
from the structures of moctezumite and (UO2)Cl2(H2O).
Linkage of the chains is through a cation–cation interaction in which some of the O atoms of the uranyl
ion also are equatorial ligands of the bipyramids ofadjacent chains.
The structure of (NH4)3(H2O)2{[(UO2)10O10(OH)][(UO4)(H2O)2]} is an infinite framework of uranium
polyhedra (Fig. 15f), five of which are uranyl squareor pentagonal bipyramids, whereas the sixth is better
described as a distorted octahedron. Uranyl squareand pentagonal bipyramids are linked by sharing
vertices, resulting in a sheet based upon the -U3O8
anion-topology that is topologically identical to thosein ianthinite and spriggite (Fig. 11d). The square sitesof the -U3O8 anion-topology contain both the uranylsquare bipyramids and the distorted U6+6 octahedra.
Two additional uranyl pentagonal bipyramids share an
equatorial edge, giving a dimer. The sheets are stacked
along [001], and are connected into a framework bylinkages to the dimer of bipyramids located between
the sheets. The NH 4 and H 2O groups are locatedwithin channels that extend parallel to the sheets ofpolyhedra.
The compound Pb 2 (H 2 O)[(UO 2 ) 10 UO 12 (OH) 6(H2O)6] is the first Pb uranyl oxide hydrate reported topossess a framework structure (Fig. 15g). It has bothuranyl square and pentagonal bipyramids, as well as
one U6+ site that is in somewhat distorted octahedral
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 MINERALOGISTof vertex- and edge-sharing uranium polyhedra thatare closely related to the chains of polyhedra that makeup the curite sheet (Fig. 11l). The Pb2+ cations arelocated in channels extending through the frameworkof polyhedra.
Frameworks of uranium polyhedra
and silicate tetrahedra
The structures of six framework uranyl silicates, twoof which are minerals, and one uranyl germanate, havebeen described. In each structure, uranyl polyhedra sharevertices or edges with silicate (or germanate) tetrahedra,and the frameworks are composed entirely of uranyl
polyhedra and silicate (or germanate) tetrahedra.
The compound KNa3[(UO2)2(Si4O10)2](H2O)4 was
first found as an alteration phase of actinide-bearing
borosilicate glass under consideration as a containment material for nuclear waste (Burns et al. 2000).
The structure contains sheets of vertex-sharing silicate
tetrahedra involving both four- and eight-memberedrings of tetrahedra (Fig. 15h). Adjacent silicate sheets
are connected through uranyl square bipyramids, suchthat apical ligands of the silicate tetrahedra correspondto all four of the equatorial vertices of the bipyramid(Fig. 15h). Na 4[(UO 2) 2(Si 4O 10) 2](H 2O) 4 was later
synthesized, and has an identical uranyl silicate framework (Li & Burns 2001a).
Soddyite and the isostructural compound [(UO2)2(GeO4)(H2O)2] contain uranyl pentagonal bipyramidsthat form chains by sharing equatorial edges (Fig.
15i). Each bipyramid also shares an edge with a tetrahedron, and the single unshared equatorial vertex of
the bipyramid is occupied by H2O. Adjacent uranyl
silicate (germanate) chains are directly linked as eachtetrahedron shares two edges with bipyramids fromtwo different chains. Additional linkages arise throughH bonds.
The structure of weeksite (Fig. 15j) contains sheetsof uranyl pentagonal bipyramids and silicate tetrahedrathat are topologically identically to those found inhaiweeite. In the case of haiweeite, adjacent sheets areconnected through bonds to Ca cations located in theinterlayer. In weeksite, the sheets are connected into aframework through the apical O atoms of the silicate
tetrahedra, which provides for channels extending
parallel to the sheets that contain lower-valence cationsand H2O.
The structure of Na2[(UO2)(SiO4)] contains U6+ indistorted octahedral coordination, with four short (~2Å) and two longer (~2.3 Å) U6+–O bonds. Each U6+O6
octahedron shares four of its vertices with silicate
tetrahedra, and Na cations are located in channels in
the framework (Fig. 15k).
The compound RbNa[(UO2)(Si2O6)](H2O) contains
rings of four silicate tetrahedra formed by the sharingof tetrahedral vertices (Fig. 15l). Identical rings oftetrahedra are linked into sheets that are parallel to(001) by sharing vertices with uranyl square bipyramids,
such that each of the equatorial ligands of the squarebipyramid is shared with a tetrahedron from a different
ring. The sheets are linked into a framework throughadditional uranyl square bipyramids, such that the four
vertices of the silicate tetrahedra are shared with two
tetrahedra within the ring, and two bipyramids. Notethat similar four-membered rings of silicate tetrahedra
occur in KNa3[(UO2)2(Si4O10)2](H2O)4, in which they
are linked directly to form silicate sheets (Fig. 15h).
Frameworks of uranyl pentagonal bipyramids
and phosphate, arsenate, or vanadate tetrahedra
The compounds [(UO 2 ) 3 (PO 4 ) 2 ](H 2 O) 4 , (UO 2 )[(UO2)(AsO4)]2(H2O)4, (UO2)[(UO2)(AsO4)]2(H2O)5,and (UO2)[(UO2)(VO4)]2(H2O)5 each contain topologically identical sheets of uranyl pentagonal bipyramids
and tetrahedra, as shown in Figure 15m. The sheet isbased upon the uranophane anion-topology (Fig. 10a),but is a novel graphical isomer. Note that all of thetetrahedra that share edges with bipyramids of any given
chain of bipyramids are oriented in the same direction,in contrast to the uranyl silicate sheets shown in Figure
10. In all four structures, adjacent sheets are connected
through uranyl pentagonal bipyramids located between
the sheets, resulting in frameworks. Each of the bipyramids between the sheets shares one of its equatorial
vertices with a tetrahedron in the sheet on either side,and the remaining three equatorial vertices are H2O. In
the cases of [(UO2)3(PO4)2](H2O)4 (Fig. 15m), (UO2)[(UO2)(VO4)]2(H2O)5, and (UO2)[(UO2)(AsO4)]2(H2O)4(Fig. 15n), the sheets of polyhedra are relatively flat. In
the case of (UO2)[(UO2)(AsO4)]2(H2O)5, the sheets arestrongly corrugated (Fig. 15o).
The compounds Cs2(UO2)[(UO2)(PO4)]4(H2O)2,
Rb 2 (UO 2 )[(UO 2 )(PO 4 )] 4 (H 2 O) 2 , K 2 (UO 2 )[(UO 2 )(PO4)]4(H2O)2, Cs2(UO2)[(UO2)(AsO4)]4(H2O)2, andRb2(UO2)[(UO2)(AsO4)]4(H2O)4.5 possess homeotypic
framework structures (Fig. 15p). Each has sheets ofuranyl pentagonal bipyramids and tetrahedra that are
based upon the underlying uranophane anion-topology(Fig. 10a). In each structure, the sheet is the samegraphical isomer as found in -uranophane. Adjacent
sheets are connected into a framework by sharing
vertices with uranyl pentagonal bipyramids locatedbetween the sheets. Each interlayer bipyramid sharesfour of its equatorial vertices with tetrahedra, two fromeach of the adjacent sheets. The remaining equatorial
vertex of the bipyramid is H2O. The low-valence cations
and additional H2O are located in channels extending
through the framework.
The structure of [(UO2)2V2O7] (Fig. 15q) is a framework of uranyl pentagonal bipyramids and vanadate
tetrahedra. It contains sheets that are based upon theuranophane anion-topology, with the same graphicalisomer as found for [(UO 2) 3(PO 4) 2](H 2O) 4, (UO 2)[(UO2)(AsO4)]2(H2O)4, (UO2)[(UO2)(AsO4)]2(H2O)5,HIERARCHY OF U6+ MINERALS AND INORGANIC COMPOUNDS

and (UO2)[(UO2)(VO4)]2(H2O)5 (Fig. 15m). However,in the structure of [(UO2)2V2O7], there are no uranyl
ions located between the sheets; rather, direct sharing
of apical ligands of the vanadate tetrahedra connects the
sheets into a framework.
The structure of Na[(UO2)4(VO4)3] is an unusuallycomplex framework composed of uranyl pentagonal
bipyramids and vanadate tetrahedra (Fig. 15r). The
bipyramids are linked by sharing edges into chainsthat are one bipyramid wide. Vanadate tetrahedra shareequatorial edges of the bipyramids on either side of thechain, in the same fashion as occurs in sheets based
upon the uranophane anion-topology. However, theuranyl vanadate chains in Na[(UO2)4(VO4)3] are notaligned to form sheets, but rather occur in the structurein two mutually perpendicular orientations, and arelinked into a framework with channels that accommodate the Na cations.
Frameworks of uranyl polyhedra
and molybdate tetrahedra
Twelve uranyl molybdate compounds, corresponding to seven distinct crystal structures, are frameworks of uranyl pentagonal bipyramids and molybdate
tetrahedra. The considerable diversity of this chemicalgroup is related to the flexibility of the linkage between
the bipyramids and tetrahedra, which show a rangeof U–O–Mo bond angles (e.g., Krivovichev & Burns
2003b).
The compounds Cs 2 [(UO 2 ) 6 (MoO 4 ) 7 (H 2 O) 2 ],(NH 4 ) 2 [(UO 2 ) 6 (MoO 4 ) 7 (H 2 O) 2 ], and Rb 2 [(UO 2 ) 6(MoO4)7(H2O)2] are isostructural (Fig. 15s). There are
two types of uranyl pentagonal bipyramids, those thatshare each of their equatorial vertices with molybdate
tetrahedra, and those that share only four of their equatorial vertices with molybdate tetrahedra, with the fifth
vertex corresponding to H2O. All of the molybdate
tetrahedra link to four different bipyramids. The lowervalence cations are located in irregular voids within theframework of polyhedra.
The compounds -Cs2[(UO2)2(MoO4)3], Rb2[(UO2)2(MoO4)3] and Tl2[(UO2)2(MoO4)3] are isostructural(Fig. 15t). Each bipyramid shares all five of its equatorial vertices with molybdate tetrahedra, and the tetrahedra are either linked to three or four bipyramids.
The structure contains unusually large channels that
extend along [001] and [100], with dimensions 3.5 
10.5 Å and 4.8  4.8 Å, respectively. The Cs and Rb
cations are located within the channels through theframework.
The structures of Ba(UO 2 ) 3 (MoO 4 ) 4 (H 2 O) 4
(Fig. 15u), -(UO2)(MoO4)(H2O)2 (Fig. 15v), Sr(UO2)6(MoO4)7(H2O)19 (Fig. 15w), Mg(UO2)3(MoO4)4 (H2O)8(Fig. 15x) and Ca[(UO 2 ) 6 (MoO 4 ) 7 (H 2 O) 2 ](H 2 O) n
(Fig. 15w) further illustrate the flexibility of the linkage
between uranyl pentagonal bipyramids and molybdate
tetrahedra. Each involves only vertex-sharing between

1879

these two types of polyhedra, yet the structural connectivities are remarkably diverse. In Ba(UO2)3(MoO4)4(H2O)4 and Mg(UO2)3(MoO4)4(H2O)8, only four of theequatorial vertices of the bipyramids are shared withtetrahedra, with the fifth position occupied by H2O. In
-(UO2)(MoO4)(H2O)2 and Sr(UO2)6(MoO4)7(H2O)19,
all of the equatorial vertices of the bipyramids areshared with tetrahedra. The ratios of bipyramids totetrahedra range considerably in these four structures;in those with more tetrahedra relative to bipyramids,
fewer of the vertices of the tetrahedra are linked
to bipyramids. In Mg(UO 2 ) 3 (MoO 4 ) 4 (H 2 O) 8 andBa(UO2)3(MoO4)4(H2O)4, channels through the framework are bounded by O atoms of the uranyl ions, as well
as by some apical (unshared) ligands of the tetrahedra.
In Sr(UO2)6(MoO4)7(H2O)19 and Ca[(UO2)6(MoO4)7(H2O)2](H2O)n, channels in the framework are delineated by apical O atoms of tetrahedra.
The structure of (NH4)2[(UO2)5(MoO4)7](H2O)5
warrants special attention owing to the presence of large
channels through the framework of uranyl pentagonal
bipyramids and molybdate tetrahedra (Fig. 15y). Eachof the five symmetrically distinct uranyl pentagonal
bipyramids share all five of their equatorial vertices
with tetrahedra, whereas four of the seven distinct
tetrahedra share each of their vertices with bipyramids, and the remaining are each connected to threebipyramids. The considerable range of U–O–Mo bondangles in the structure, from 123.7 to 166.7°, permits
the unprecedented connectivity observed in this structure (Krivovichev et al. 2003). The structure contains
strips of bipyramids and tetrahedra that spiral about the
tubular openings along [001], with a primitive repeatdistance of 70.6 Å. The resulting framework contains
several channels, the largest of which extends along[001] with a cross-section of 7.5 by 7.5 Å. The (NH4)cations and H2O groups are located within the various
channels.
Miscellaneous frameworks
Pyrophosphate P2O7 groups link uranyl pentagonal
bipyramids into a framework in the structure ofNa2[(UO2)(P2O7)] (Fig. 15z). One phosphate tetrahedron of the pyrophosphate group shares an equatorialedge of a bipyramid, a vertex with another adjacentbipyramid, and one vertex with the other tetrahedronof the pyrophosphate group. The other phosphate
tetrahedron shares a vertex with one bipyramid, andthe remaining two vertices are unshared within theframework. The Na cations are located within voids inthe framework.
The compounds -(UO2)(SeO4), -(UO2)(SO4),
and (UO 2 )(MoO 4 ) have unusual frameworks that
include cation–cation interactions (Fig. 15aa). Uranyl
pentagonal bipyramids are linked into single-width
chains extending along [010]. Each linkage between the
bipyramids occurs through a cation–cation interaction,

1880

THE CANADIAN MINERALOGIST

meaning that one of the uranyl ion O atoms of eachbipyramid is also an equatorial vertex of an adjacentbipyramid within the chain. The chains of bipyramids
are linked through tetrahedra, and each of the tetrahedra
shares all four of its vertices with bipyramids of thechains. In the case of the bipyramids, one of the equatorial positions corresponds to the cation–cation interaction, and each of the others is linked to tetrahedra.
The structure of [(UO2)3(PO4)O(OH)(H2O)2](H2O)
has structural elements in common with the phosphuranylite group of minerals (Fig. 15ab). The
framework contains uranyl pentagonal and hexagonal
bipyramids, as well as phosphate tetrahedra. Pentagonal
bipyramids share an equatorial edge, forming a dimer.
The dimers are linked into chains by sharing edges withhexagonal bipyramids, such that four of the equatorial
edges of the hexagonal bipyramids are shared withpentagonal bipyramids, two on either side. The two
equatorial edges of the hexagonal bipyramids that are
not shared with pentagonal bipyramids are shared withphosphate tetrahedra, resulting in uranyl phosphatechains that are topologically identical to those found asparts of sheets in the phosphuranylite group of minerals.
The H2O groups are located at the non-chain equatorial vertices of the uranyl pentagonal bipyramids. The
uranyl phosphate chains are in two mutually perpendicular orientations, and are linked directly through thephosphate tetrahedra, as each tetrahedron shares oneedge with each of two different chains (Fig. 15ab). The
resulting framework has channels along [001] that aredelineated by uranyl phosphate chains on either side.
The framework is electroneutral, with only H2O groups
located in the channels.
The compounds Pb2[(UO2)(TeO3)3] (Fig. 15ad),
Na8[(UO2)6(TeO3)10] (Fig. 15ae) and cliffordite (Fig.
15ad) all contain Te4+ cations that have stereoactive
lone-electron pairs. Typically, the Te4+ cation is strongly
bonded to three atoms of O with bond lengths of ~1.9
Å, as well as some additional anions located about 2.5
to 3.0 Å from the cation. The three short bonds about
the cation are on one side, and the coordination polyhedron is a pyramid that is capped by the cation. The
structures of Pb2(UO2)(TeO3)3 and Na8[(UO2)6(TeO3)10]
each contain uranyl pentagonal bipyramids that shareequatorial vertices and edges with TeO3 pyramids toform the framework. Cliffordite is unusual in that it is
the only framework structure for which all of the U isin hexagonal bipyramidal coordination.
The structure of K2[(UO2)2(VO)2(IO6)2O](H2O) is arather open framework of uranyl square bipyramids anddistorted VO6 and IO6 octahedra (Fig. 15af). The octahedra share edges, forming a chain that extends along[001]. The chains are linked into ribbons by sharing
polyhedron vertices, and these ribbons are linked into
the framework through the equatorial vertices of theuranyl square bipyramids. The K cations and H2Ogroups are located in channels that extend through theframework (Fig. 15af).

The lithium uranyl tungstates Li(UO 2 )(WO 4 ) 2
and Li2(UO2)4(WO4)4O both contain distorted WO6octahedra and uranyl pentagonal bipyramids, andLi2(UO2)4(WO4)4O also has uranyl square bipyramids.
The structure of Li(UO2)(WO4)2 contains sheets ofvertex-sharing WO6 octahedra that are linked throughvertex-sharing with the uranyl polyhedra (Fig. 15ag).
Note that uranyl polyhedra are linked into chains byvertex-sharing. These chains are identical to thosefound in sheets based upon the uranophane aniontopology (Fig. 10). As noted by Obbade et al. (2004c),
insertion of additional chains of uranyl pentagonal
bipyramids in the voids occupied by Li cations would
result in uranyl tungstate sheets with the uranophane
anion-topology. The structure of Li2(UO2)4(WO4)4O
is composed of slabs of uranyl pentagonal bipyramids
and WO6 octahedra that are based upon the uranophane
anion-topology (Fig. 15ah). These slabs are linked intoa framework by sharing vertices with uranyl squarebipyramids.
The perovskite-like structures of Ba2MgUO6 andK9BiU6O24 (Figs. 15ai, aj) can be described as derivatives of the -UO3 structure-type (Fig. 15d). In bothcases, the U6+ cations are in holosymmetric octahedral
coordination, with no uranyl ion present. These structures are derived from that of -UO3 by doubling theunit-cell edges, replacing some of the UO6 octahedrawith either MgO6, BiO6 or KO6 octahedra, and withBa or K cations inserted into the 12-coordinated sitesof the channels.
The structures of -Cd(UO2)O2, Cu(UO2)O2, andMn(UO2)O2 (Fig. 15ak) are closely related, although
they are not strictly isostructural, as they crystallize
in different space-groups. Each contains uranyl squarebipyramids and octahedra that are occupied by the divalent cations. The OUr atoms are part of the octahedral
coordination of the divalent cations, and as a result the
U6+–O bond lengths are longer than usual.
The structure of UCr2O6 involves a framework ofuranyl square bipyramids and chromate octahedra (Fig.
15al). The chromate octahedra share edges, resulting in
brucite-like sheets, but one-third of the octahedral sites
are vacant. The uranyl square bipyramids are locatedbetween the chromate sheets, in correspondence withthe vacant sites of the sheets of octahedra.
The compound U4+(UO2)(PO4)2 contains both U4+
coordinated by seven O atoms, and U6+ present as the
typical uranyl pentagonal bipyramid. The uranium polyhedra share edges, giving chains that are cross-linked
into a framework by sharing edges and corners with
PO4 tetrahedra (Fig. 15am).
The compound Cs[(UO2)3(HIO6)(OH)(O)(H2O)](H2O)15, as well as the isostructural Li, Na, K and Rb
compounds, have the unusual framework of uranyl
pentagonal bipyramids and periodate octahedra shownin Figure 15an. The most unusual aspect of this relatively open framework is that some of the linkages
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 theU(3) cation, which is involved in the cation–cation interaction, by the HIO64– ion results in transfer of significant
electron-density from the HIO64– ion to the uranyl ion,thus stabilizing the cation–cation interaction.

NANOSTRUCTURE MATERIALS
Nanostructure materials exhibit important properties
with promise for many potential applications. Suchmaterials also provide unique opportunities to study
fundamental structure–property relationships, and theproperties 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 awide variety of polyoxometalates built on WO, MoO,
and NbO framework structures (Müller et al. 1998).
Following the discovery of carbon nanotubes, considerable attention has also been devoted to synthesizing a
range of oxidic nanotubular materials, which also have

FIG. 17.

FIG. 16. U–32 uranyl peroxide nanosphere formed by thelinkage of 32 chemically identical uranyl peroxide polyhedra. From Burns et al. (2005).

Nanoscale tubules in a uranyl selenate. From Krivovichev et al. (2005a).

1882

THE CANADIAN MINERALOGIST

tremendous potential in materials applications. Prior to
2005, no materials have been reported with nanoscale
aggregates or tubules of higher-valence actinide ions.
Burns et al. (2005) described an unprecedented
family of actinyl peroxide nanoclusters. These chemically and structurally elegant spherical clusters wereobtained from alkaline solutions containing peroxide
under ambient conditions. The clusters are assembled
from topologically identical uranyl peroxide polyhedra,and contain 24, 28 or 32 uranyl peroxide polyhedra
within the clusters (e.g., Fig. 16). Burns et al. (2005)
provided evidence for the self-assembly and persistenceof the some of these clusters in alkaline solutions,prior to crystallization in complex molecular crystals.
The assembly and persistence of these nanoclusters insolution suggest that they may influence the mobilityof actinides in the environment, such as where highly
alkaline nuclear wastes leak into the vadose zone. The
complexity of the clusters, which constitute a newclass of polyoxometalates, is in sharp contrast to otherclusters of cation polyhedra of higher bond-valence inuranyl compounds that are illustrated in Figure 4.
Krivovichev et al. (2005a, b) reported the synthesis
and structures of two remarkable new uranyl selenate
compounds that contain nanoscale tubules (Fig. 17). In
each case, uranyl pentagonal bipyramids share vertices
with selenate tetrahedra, resulting in open sheets that arerolled into single-walled tubes, with external diametersof 17 and 25 Å, and inner crystallographically free
diameters of 4.7 and 12.6 Å. This discovery presents
the fascinating possibility that many of the myriad ofsheets containing uranyl polyhedra may be able to form
nanoscale tubules. The range of sheet topologies andcompositions suggests many potential applications ofsuch materials.

SUMMARY AND FUTURE OUTLOOKThe convergence of greatly enhanced technologies
for detection of X-rays in single-crystal experimentswith a renewed interest in the crystal chemistry of U6+
has resulted in an explosion of new structures containing
U6+. During the past decade, known structures of uranyl
compounds has more than doubled. These studies havefurther revealed the tremendous structural elegance,
beauty, and complexity of uranyl compounds, which arereflected in the many Figures of this manuscript.
Of the 368 structures covered in this structural hierarchy, 204 (55.4%) are based upon sheets of polyhedra
containing cations of higher valence, and of the 89
minerals, 69 (77.5%) contain sheets. Structures based
upon infinite chains and frameworks of polyhedra areabout equally abundant, and correspond to 57 (15.5%)
and 56 structures, respectively. Ten uranyl minerals areclassed as infinite-chain structures, whereas only three
have frameworks of polyhedra. There are 43 uranyl
compounds with structures based upon isolated clustersof polyhedra, of which seven are minerals. Eight struc-tures contain isolated uranyl polyhedra. In 1996, Burns
et al. placed 180 structures in their hierarchy, of which
56 are minerals, 106 contain sheets, 19 contain chains,
22 contain clusters, 26 are frameworks, and sevencontain isolated uranyl polyhedra. Structures discovered since 1996 have revealed many novel topologicalarrangements of uranyl polyhedra and other polyhedra
containing higher-valence cations. Many of the new
structures also correspond to topological arrangementsthat were known in 1996, and they expand the range ofcompositions known to adopt a given topology.
Tremendous advances have been made in understanding the crystal chemistry of uranyl compounds,
including the structures of uranyl minerals. The structures of virtually all of the common uranyl minerals
have been resolved, but the structures are still either
unknown or unrefined for about half of the known uranyl
minerals, many of which are extremely rare. There has
been extensive exploration of many of the chemicalclasses of inorganic uranyl compounds, and much of
the recent work has been focused on uranyl compounds
containing tetrahedrally coordinated hexavalent cations
such as Mo, S, and Cr, as well as cations that containstereoactive lone-electron pairs such as Te4+ and Se4+.
Despite the considerable effort expended on structural
studies of such chemical classes of uranyl compounds,
new structural units and topologies continue to be foundon a regular basis, which suggests that many more novel
structural connectivities await discovery.
In developing the current structural hierarchy, I
have attempted to provide a cohesive framework forthe discussion and understanding of the structures ofinorganic uranyl compounds. I consider this step tobe essential toward application of knowledge of thesestructures to mineralogical, geological, environmental
and technological issues. Just one example of the importance of understanding the structures and chemicalcomposition of uranyl minerals and compounds lies in
the area of nuclear waste disposal. Alteration of nuclear
waste forms such as spent nuclear fuel and actinidebearing waste glasses in a geological repository undermoist oxidizing conditions will obviously result incomplex suites of uranyl compounds. Some of thesealteration phases will incorporate radionuclides into
their structures, which is important for the long-term
performance of the repository. Theoretical predictionsof such incorporation mechanisms, as well as experimental verification of incorporation require a detailed
understanding of the crystal structures. Determination of the thermodynamic stabilities and solubilities
of uranyl compounds relies heavily on the structural
formulae derived from crystallographic studies, as thecompositions of many of these phases were incorrectly
assigned in the absence of knowledge of their structures.
Synthesis and structural analysis have permitted thediscovery of many new uranyl compounds that may be
relevant to repository performance. The ultimate prediction of thermodynamic properties and occurrences

HIERARCHY OF U6+ MINERALS AND INORGANIC COMPOUNDS

of uranyl compounds on the basis of their structuralconnectivities is a tantalizing prospect, but this goal
requires a detailed understanding of the crystal chemistry of uranium.
Reports of both nanoscale clusters containing uranyl
polyhedra and nanotubular uranyl compounds appearedin the spring of 2005, and represent a new directionin the crystal chemistry of uranium and actinides ingeneral. Discovery of these novel materials occurred
after many years of research concerning uranyl crystal
chemistry, at a time when the casual observer might
have considered studies of uranium crystal chemistry
to be a relatively mature field. Now, there are many
research opportunities concerning nanoscale uraniumbased materials, including their importance for understanding chemical processes involving actinides, thepotential for novel materials applications, and theimpact of such materials on transport of actinides inthe environment.
Unlike U6+, relatively few crystal structures havebeen analyzed that contain neptunium or plutonium
in higher oxidation states (5+ or 6+). The degree towhich the considerable knowledge concerning U6+
crystal chemistry may be transferable to Np and Pu
compounds is unclear. The few structures of Np5+, Np6+,
Pu5+ and Pu6+ compounds that are known do containthe same types of coordination polyhedra as U6+, actinyl
ions in square, pentagonal and hexagonal bipyramidal
polyhedra. As such, one might expect that Np and Pu
structures will in many ways be similar to those alreadyknown from uranyl compounds. This may largely be thecase for Np6+ and Pu6+, although their slightly longer
bond-lengths, as compared with U6+, may lead to somenovel structural connectivities in structures where thesharing of edges between actinyl polyhedra and polyhedra containing higher-valence cations is important.
In the cases of Np5+ and Pu5+, the actinyl ion has aformal valence of 1+, and the bond strengths within
the actinyl ions are likely to be somewhat lower thanin the case where a hexavalent cation is present. As
such, the O atoms of the actinyl ions containing Np5+
and Pu5+ will be more readily shared with other actinyl
polyhedra, thus cation–cation interactions are likely
to be much more important than in uranyl structures.
The few structures of Np5+ compounds that are in theliterature, as well as about a dozen structures my group
and I have recently determined, commonly do containcation–cation interactions, which results in dramaticallydifferent structures as compared to uranyl compoundswith similar chemical compositions.

ACKNOWLEDGEMENTS
I acknowledge the Mineralogical Association of
Canada for support and encouragement throughout
my ongoing education and career. I am grateful toProf. Rodney Ewing and Dr. Mark Miller for their1883

tremendous contributions in the development of
our original hierarchy of uranyl structures that was
published in 1996. My research group at the University of Notre Dame has contributed in many ways to
my understanding of uranyl crystal chemistry; I am
grateful to all members of my group, past and present:
post-doctoral fellows Sergey Krivovichev, Christopher
Cahill, Robert Shuvalov, Francis Hill, Philip Almond,
Oksana Kurmakova, graduate students Yaping Li,
Andrew Locock, Karrie-Ann Hughes, Jennifer Jackson,
Tori Ziemann, Paul Giesting, Amanda Klingensmith,Cara Alexopoulos, Bridget McCollam, undergraduates
Erin Keppel, Alex Garza, Rebecca Glatz, Kara Tholen,
Andrew Irvine, Aaron Martin, Kathryn Deely, Leslie
Hayden, Jessica Callahan, Theodore Flynn, Lauren
Sturm, Peter Hotchkiss, Virginia Kelly, Nathan Porter,
Patrick Horan, Karen Carlaccini, Ginger Sigmon, Maha
Jain, Jinalee Almes, and Erin Diminick. Leslie Hayden
prepared many preliminary versions of the figures ofuranyl compounds used in this manuscript.
I thank Prof. Thomas Albrecht-Schmitt and Prof.
Sergey Krivovichev for their reviews of this manuscript,
and Prof. Krivovichev for providing preprints of severalof his papers. Thanks also to Prof. Frank C. Hawthorne
and Prof. Robert F. Martin for editing the manuscript.
I acknowledge financial support for several yearsof study of uranyl crystal chemistry from the Environmental Management Sciences Program of the UnitedStates Department of Energy (DE–FG07–97ER14820),
the National Science Foundation for my studies of Pb
uranyl oxide hydrate minerals (EAR98–04723), andthe National Science Foundation for the Environmental
Molecular Science Institute at the University of Notre
Dame (EAR02–21966). I also acknowledge current
funding from the Office of Science, Technology andInternational of the United States Department ofEnergy that will permit the continuation of this lineof research.

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1887________ (1987c): Synthèse et structure du niobouranate decésium: CsNbUO6. Acta Crystallogr. C43, 404-406.

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1888

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Synthesis and crystal structure of rubidium uranyl dimolybdate Rb2UO2(MoO4)2•(H2O). Zh. Neorg. Khim. 45,
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________ & ________ (2001b): Crystal chemistry of uranyl
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KOBATKO, K.-A. & BURNS, P.C. (2006): A novel arrangementof silicate tetrahedra in the uranyl silicate sheet of oursinite, (Co0.8Mg0.2)[(UO2)(SiO3OH)]2(H2O)6. Am. Mineral.
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________ & ________ (2002a): Crystal chemistry of
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________ & ________ (2002b): Crystal chemistry of
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HIERARCHY OF U6+ MINERALS AND INORGANIC COMPOUNDS________ & ________ (2003i): A novel rigid uranyl tungstate
sheet in the structures of Na2[(UO2)W2O8] and - and Ag2[(UO2)W2O8]. Solid State Sci. 5, 373-381.
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________ & ________ (2004d): Synthesis and crystal structure of Cu2[(UO2)3((S,Cr)O4)5](H2O)17. Radiochem. 46,
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1890

THE CANADIAN MINERALOGIST________, CAHILL, C.L. & BURNS, P.C. (2001b): Synthesis,
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1892

THE CANADIAN MINERALOGISTPADEL, L., POIX, P. & MICHEL, A. (1972): Préparation et étude
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________ (1977): Preparation, étude structurale et vibrationnelle des complexes M2U2O5Cl4.2H2O (M = Rb, Cs):mise en évidence d’un anion tétranucléaire [(UO2)4O2
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________, ________ & PIRET-MEUNIER, J. (1988): La françoisite-(Nd), nouveau phosphate d’uranyle et de terres
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________, ________, ________ & GERMAIN, G. (1983): La
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________, ________ & DECLERCQ, J.-P. (1979): Structure ofphuralumite. Acta Crystallogr. B35, 1880-1882.
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________ & ________ (1977a): Kasolite, Pb(UO2)(SiO4)•
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________ & ________ (1964): Studies of the torbernite
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RYAN, R.R. & ROSENZWEIG, A. (1977): Sklodowskite, MgO•
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SAADI, M., DION, C. & ABRAHAM, F. (2000): Synthesis andcrystal structure of the pentahydrated uranyl orthovanadate(UO2)3(VO4)2•5H2O, precursor for the new (UO2)3(VO4)2
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SADIKOV, G.G., KRASOVSKAYA, T.I., POLYAKOV, Y.A. & NIKOLAEV, V.P. (1988): Structural and spectral studies on potassium dimolybdatouranylate. Inorg. Mater. 24, 91-96.
SAINE, M.-C. (1989): Synthèse et structure de KU2O7 monoclinique. J. Less-Common Metals 154, 361-365.
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HIERARCHY OF U6+ MINERALS AND INORGANIC COMPOUNDS1893

SEREZHKIN, V.N., BOIKO, N.V. & MAKAREVICH, L.G. (1980a):
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SWIHART, G.H., GUPTA, P.K.S., SCHLEMPER, E.O., BACK, M.E.
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________, ________ & T RUNOV , V.K. (1982): Crystal
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S YKORA , R.E. & A LBRECHT -S CHMITT , T.E. (2003): Selfassembly of a polar open-framework uranyl vanadyl
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________, EFREMOV, V.A. & TRUNOV, V.K. (1980b): Crystal
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________, SOLDATKINA, M.A. & BOIKO, N.V. (1983): Refinement of the crystal-structure (NH4)4[UO2(CO3)3]. J. Struct.
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________, _______ & E FREMOV , V.A. (1981a): Crystalstructure of the uranyl selenate tetrahydrate. J. Struct.
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________, ________ & ________ (1981b): Crystal-structureof MgUO2(SO4)2•11H2O. J. Struct. Chem. 22, 454-457.
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SHUVALOV, R.R. & BURNS, P.C. (2003): A monoclinic polymorph of uranyl dinitrate trihydrate, [(UO2)(NO3)2(H2O)2]•(H2O). Acta Crystallogr. C59, i71-i73.
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________ & ________ (2004): Hydrothermal synthesis andcrystal structure of Cs4[(UO2)4(W5O21)(OH)2(H2O)2]:a new polar uranyl tungstate. J. Solid State Chem. 177,
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________, KING, J.E., ILLIES, A.J. & ALBRECHT-SCHMITT, T.E.
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________, MCDANIEL, S.M., WELLS, D.M. & ALBRECHTSCHMITT, T.E. (2002a): Mixed-metal uranium(VI) iodates:hydrothermal syntheses, structures, and reactivity ofRb[UO2(CrO4)(IO3)(H2O)], A2[UO2(CrO4)(IO3)2] (A =K, Rb, Cs), and K2[UO2(MoO4)(IO3)2]. Inorg. Chem. 41,
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________, KOVBA, L.M. & SEREZHKIN, V.N. (1979): Crystal
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________ & ________ (1968): The crystal structure of copper
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________, ________ & ________ (1983): The crystal structure of molybdatouranylates of magnesium and zinc ofcomposition M(UO 2) 3(MoO 4) 4(H 2O) 8 (M = Mg,Zn).
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________, ________ & GEBERT, E. (1972b): The structure of
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________, _______ & ________ (1984b): Crystal structure ofMg(UO2)6(MoO4)7•18H2O and Sr(UO2)6(MoO4)7•15H2O.
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________, VISTE, A., HOEKSTRA, H.R. & TANI, B.S. (1972a):
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________, SEREZHKIN, V.I., SEREZHKINA, L.B. & KOVBA, L.M.
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STERNS, M., PARISE, J.B. & HOWARD, C.J. (1986): Refinementof the structure of trilead(II) uranate(VI) from neutron
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TANCRET, N., OBBADE, S. & ABRAHAM, F. (1995): Ab initio
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1894

THE CANADIAN MINERALOGISTTAYLOR, J.C. (1971): The structure of the  form of uranylhydroxide. Acta Crystallogr. B27, 1088-1091.

WELLER, M.T., DICKENS, P.G. & PENNY, D.J. (1988): The
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________ & BANNISTER, M.J. (1972): A neutron diffraction
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________, LIGHT, M.E. & GELBRICH, T. (2000): Structureof uranium(VI) oxide dihydrate, UO3•2H2O; synthetic
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________ & MUELLER, M.H. (1965): A neutron diffraction
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________ & ________ (1986): Neues über Oxouranate (VI):Na4UO5 und K4UO5. Rev. Chim. Minérale 23, 828-848.
________ & ________ (1987): Ein neues Oxouranat (VI):
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Received January 24, 2005, revised manuscript accepted
June 7, 2005.