ÓÄÊ 548.3
© A. J. LOCOCK
TRENDS IN ACTINIDE COMPOUNDS
WITH THE AUTUNITE SHEET-ANION TOPOLOGY
Department of Earth and Atmospheric Sciences, University of Alberta,
1—26 Earth Sciences Building, Edmonton, Alberta, T6G 2E3, Canada;
e-mail:[email protected]
The autunite-type sheet consists of actinyl square bipyramids that share their equatorial vertices
with the apices– of phosphate or arsenate tetrahedra, to form layers with general stoichiometry
[(AnO2)(XO4)] , where An = hexavalent actinide, and X = pentavalent P or As. The sheets are separated by interlayer regions that generally contain H2O groups and either monovalent-, divalent- or trivalent-cations. The sheets are linked by hydrogen bonding and through bonds from the interlayer cations to oxygen atoms of the sheets. Changes in hydration state are discrete, and lead to separate, but
related compounds. The autunite-type sheet has been reported from, or can reasonably be inferred to
occur in, 109 inorganic compounds, including the minerals: abernathyite, arsenuranospathite, autunite, bassetite, chernikovite, chistyakovaite, heinrichite, kahlerite, «kirchheimerite», lehnerite, meta-ankoleite, meta-autunite, metaheinrichite, metakahlerite, metakirchheimerite, metalodevite, metanatroautunite, metanováèekite, metasaléeite, metatorbernite, meta-uranocircite I, meta-uranocircite II, meta-uranospinite, metazeunerite, novaáèekite I, nováècekite II, przhevalskite, sabugalite,
saléeite, sodium uranospinite, threadgoldite, torbernite, trögerite, «uramarsite», uramphite, uranocircite, uranospathite, uranospinite, vochtenite, zeunerite. Transuranium-bearing materials constitute 33 of these 109 compounds. This contribution examines the systematics of the phosphate and arsenate compounds that contain the autunite sheet-anion topology, mainly as a function of their interlayer contents.
INTRODUCTION
Current research interest in the ~40 minerals that contain the autunite-type sheet (Finch e. a., 1999) results mainly from their significance to the environment. The
stability of these minerals in the near-surface and their low solubilities (e. g., Vochten e. a., 1984a; Markovic e. a., 1988; ) limit the mobility of uranium in the weathering regime (Sowder e. a., 1996; Murakami e. a., 1997), in soils contaminated
with actinides (Buck e.a, 1996; Roh e. a., 2000), and in the vadose zone (Catalano
e. a., 2006; Arai e. a., 2007). Thus, technologies such as reactive barriers that use
phosphate to halt the transport of uranium in groundwater have been recommended
(Fuller e. a., 2002). Autunite-type compounds can also be precipitated by bacteria,
leading to proposals for the use of biotechnology to remediate actinide-contamination (Basnakova et al. 1998; Macaskie et al. 2000; Beazley e. a., 2007).
The lamellar structure of autunite-type compounds gives rise to their capability
for ion exchange, and makes them excellent hosts for intercalation chemistry (Al115
Fig. 1. The autunite-type sheet found in Li[(UO2)(PO4)](H2O)4 (a) and the sheet found in
Cs2[(UO2)(PO4)]2(H2O)5 (b). Actinyl square bipyramids are shaded, and tetrahedra are stippled.
berti e. a., 1996). In this structure type, actinyl phosphate (or actinyl arsenate) sheets are separated by interlayer regions that generally contain H2O groups and either
monovalent-, divalent- or trivalent-cations. The sheets are linked by hydrogen bonding and through bonds from the interlayer cations to oxygen atoms of the sheets.
The autunite-type sheet consists of actinyl square bipyramids (Burns e. a., 1997)
that share their equatorial vertices with the apices of phosphate or arsenate tetrahedra, to form layers with general stoichiometry [(AnO2)(XO4)]1–, where An = hexavalent actinide, and X = pentavalent cations. As the sheet is made up of vertex-sharing polyhedra, it has a significant degree of in-plane structural flexibility,
and can vary its geometry to accommodate different interlayer contents: the acute
angle defined by the equatorial edge of an actinyl square bipyramid and the edge of
its neighbouring tetrahedron ranges from ~70 to 90° (Fig. 1).
The corrugated autunite-type sheet was first described by Beintema (1938),
and has been reported from the structures of 52 inorganic compounds and can reasonably be inferred (on the basis of chemical compositions and powder diffraction
data) to occur in 57 additional inorganic compounds (Locock, 2007). Of these 109
compounds and minerals, 33 are transuranium-bearing compounds, and from the
few structure refinements performed to date, the transuranium-bearing compounds
are isotypic with their uranium-bearing counterparts (Forbes, Burns, 2007). The
underlying anion topology, in which only the relative positions of the oxygen
atoms that are at least two-connected within the sheet are considered (Burns,
2005), also forms the basis for the «interrupted autunite-type sheet» in which either
half of the actinyl positions are vacant, stoichiometry [(AnO2)(XO4)2]4–, or half of
the tetrahedra are vacant, stoichiometry [(AnO2)2(XO4)]1+. Both types of interrupted
sheets have been found in framework uranyl vanadates (Abraham, Obbade, 2007),
whereas in phosphates and arsenates, only the former has been reported (once, in a
uranyl phosphate, Linde e. a., 1980).
Although the autunite-type sheet exhibits tetragonal symmetry, the crystallographic symmetry adopted by a given compound depends on the nature of the interlayer cation and on the state of hydration (i. e., the number of H2O groups). It is likely that investigation of compounds with the autunite-type sheet has been complicated by their pseudosymmetry and ease of dehydration. In the tables, for
116
compounds whose structures have not been refined, crystal systems rather than
space groups are listed: Q tetragonal, O orthorhombic, and M monoclinic. The cell
dimensions and chemical formulas of such compounds should be treated as provisional. This contribution examines the systematics of the structures of phosphate
and arsenate compounds that contain the autunite sheet-anion topology with a focus on the nature of the interlayer contents and in the order: monovalent-, divalentand trivalent-cations. Hybrid organic-inorganic autunite-type materials are also
briefly reviewed, and a summary of analytical challenges inherent in the diffraction analysis of the general structural group is presented.
MONOVALENT INTERLAYER CATIONS
In the case of monovalent cations and complex ions, the symmetry of a given
autunite-type structure is not only a function of the identity of the interlayer ion, but
also of the temperature of refinement, and the nature of the tetrahedral cation (Table 1, 2). Compounds with Li in the interlayer have tetragonal symmetry and stoichiometry Li[(AnO2)(XO4)](Z2O)4, where Z = H or D (deuterium), and Li is ordered in
fourfold coordination by the O atoms of the interlayer H2O groups (Fig. 2).
In contrast, at room temperature, phosphate structures with either the monovalent cations Na, K, Rb, Ag, Tl, H3O or NH4 in the interlayer adopt tetragonal symmetry and stoichiometry A[(AnO2)(XO4)](Z2O)3, Z = H or D, with the monovalent
cation (A) substituting for the oxygen atoms of the interlayer H2O in the ratio 1:3
(Fig. 3). The corresponding arsenates behave in the same fashion, with the exception of Rb[(UO2)(AsO4)](H2O)3, in which there are two interlayer O sites, one of
which is half-occupied by Rb, whereas the other does not exhibit site occupancy disorder (Fig. 3). The small size of the Na cation results in additional positional disorder in the interlayer of the Na-bearing structures (Locock e. a., 2004a; Forbes,
Burns, 2007). It should be noted that in the oxonium-bearing compounds chernikovite and trogerite (Table 1, 2), the disorder is restricted to the H (or D) atoms (Morosin, 1978; Fitch e. a., 1983).
At low temperatures, the tetragonal disordered structures undergo phase transitions to lower crystallographic symmetries (Fitch e. a., 1982b; Cole e. a., 1993), as
a result of the interlayer sites becoming fully ordered. The exact symmetry obtained is dependent on the composition, e. g., K[(UO2)(PO4)](D2O)3 at low temperature is orthorhombic, whereas D3O[(UO2)(AsO4)](D2O)3 has triclinic symmetry
(Fig. 4; Table 1, 2). Both low-temperature structures are pseudotetragonal (Fitch
e. a., 1982b; Cole e. a., 1993).
Compounds with the large cation Cs do not exhibit site occupancy disorder,
perhaps because Cs is too large to substitute easily for the O of interlayer H2O groups (Fig. 5). Rather compounds such as Cs2[(UO2)(PO4)]2(H2O)5 and Cs(H3O)
[(UO2)(AsO4)]2(H2O)5 adopt monoclinic (pseudotetragonal or pseudo-orthorhombic) symmetry, with ordered interlayer contents (Locock e. a., 2004a).
Inspection of the structural data available for autunite-type compounds with
monovalent interlayer cations generally reveals familiar trends:
1) substitution of As for P in a structure results in expanded cell dimensions,
except when such substitution results in a change in cation ordering, and therefore
cell contraction, as in the example of Rb[(UO2)(AsO4)](H2O)3;
117
Table 1
Autunite-type phosphates with monovalent cations
Formula
Mineral
Li[(UO2)(PO4)](H2O)4
Na[(UO2)(PO4)](H2O)3
K[(UO2)(PO4)](D2O)3
K[(UO2)(PO4)](D2O)3**
Rb[(UO2)(PO4)](H2O)3
Cs2[(UO2)(PO4)]2(H2O)5***
H3O[(UO2)(PO4)](H2O)3
NH4[(UO2)(PO4)](H2O)3
Ag[(UO2)(PO4)](H2O)3
Tl[(UO2)(PO4)](H2O)3
Na[(NpO2)(PO4)](H2O)3
K[(NpO2)(PO4)](H2O)3
Rb[(NpO2)(PO4)](H2O)3
NH4[(NpO2)(PO4)](H2O)3
K[(PuO2)(PO4)](H2O)x
H3O[(PuO2)(PO4)](H2O)x
NH4[(PuO2)(PO4)](H2O)x
K[(AmO2)(PO4)](H2O)x
Rb[(AmO2)(PO4)](H2O)x
Cs[(AmO2)(PO4)](H2O)x
NH4[(AmO2)(PO4)](H2O)3
Status*
S. G.
a (Å)
c (Å)
Ref.
det
det
det
det
det
det
det
det
det
det
det
det
det
det
inf
inf
inf
inf
inf
inf
inf
P4/n
P4/ncc
P4/ncc
P21cn
P4/ncc
P21/n
P4/ncc
P4/ncc
P4/ncc
P4/ncc
P4/ncc
P4/ncc
P4/ncc
P4/ncc
Q
Q
Q
Q
Q
Q
Q
6.96
6.96
6.99
6.99
7.01
9.87
7.00
7.03
6.93
7.02
7.01
6.96
7.03
6.98
6.96
7.02
7.00
6.91
6.94
6.94
6.95
9.14
17.27
17.78
17.61
17.98
17.65
17.49
18.09
16.93
17.98
16.99
17.83
17.87
18.02
8.94
8.85
9.07
9.00
9.02
8.82
9.00
1
1
2
3
1
1
4
5
1
1
6
6
6
6
7
7
7
8
8
8
9
Metanatroautunite
Meta-ankoleite
Chernikovite
Uramphite
* Structure determined (det), or structural affiliation inferred (inf); ** At 10 K; b 6.97 Å; alternative
setting of space group Pna21; deuterated; *** b 9.96 Å: a 2 = 6.98, b 2 = 7.04 Å.
1 — Locock e. a., 2004a; 2 — Fitch, Ñole, 1991; 3 — Cole e. a., 1993; 4 —Morosin, 1978; 5 —Botto e. a.,
1975; 6 — Forbes, Burns, 2007; 7 — Fischer e. a., 1981; 8 — Lawaldt e. a., 1982; 9 — Weigel, Hoffmann,
1976c.
Table 2
Autunite-type arsenates with monovalent cations
Formula
Mineral
Li[(UO2)(AsO4)](D2O)4
Status*
S. G.
a (Å)
c (Å)
Ref.
det
P4/n
7.10
9.19
1
2
Na[(UO2)(AsO4)](H2O)3
Sodium uranospinite
det
P4/ncc
7.15
17.33
K[(UO2)(AsO4)](H2O)3
Abernathyite
det
P4/ncc
7.17
17.87
2
det
P4/ncc
7.17
18.05
3
K(H3O)[(UO2)(AsO4)]2(H2O)6
Rb[(UO2)(AsO4)](H2O)3
det
P4/n
7.19
17.64
2
Cs(H3O)[(UO2)(AsO4)]2(H2O)5**
det
P21/n
14.26
17.22
2
Trögerite (deuterated) det
P4/ncc
7.16
17.64
4
D3O[(UO2)(AsO4)](D2O)3
118
T a b l e 2 (ïðîäîëæåíèå)
Status*
S. G.
a (Å)
c (Å)
Ref.
det
P –1
7.16
17.55
5
det
P4/ncc
7.19
18.19
3
Ag[(UO2)(AsO4)](H2O)3
det
P4/ncc
7.09
17.05
2
Tl[(UO2)(AsO4)](H2O)3
det
P4/ncc
7.19
17.97
2
Na[(NpO2)(AsO4)](H2O)3.5(?)
inf
Q
7.11
8.71
6
K[(NpO2)(AsO4)](H2O)3
inf
Q
7.12
8.96
6
H3O[(NpO2)(AsO4)](H2O)3
inf
Q
7.15
8,87
6
NH4[(NpO2)(AsO4)](H2O)3
inf
Q
7.18
9.00
6
K[(PuO2)(AsO4)](H2O)x
inf
Q
7.12
8.92
7
(H3O)0.6Rb0.4[(PuO2)(AsO4)](H2O)x
inf
Q
7.14
17.83
7
H3O[(PuO2)(AsO4)](H2O)x
inf
Q
7.12
9.06
7
NH4[(PuO2)(AsO4)](H2O)x
inf
Q
7.10
8.77
7
K[(AmO2)(AsO4)](H2O)x
inf
Q
7.10
9.09
8
Rb[(AmO2)(AsO4)](H2O)x
inf
Q
7.15
17.73
8
Cs[(AmO2)(AsO4)](H2O)x
inf
Q
7.09
17.72
8
NH4[(AmO2)(AsO4)](H2O)x
inf
Q
7.11
8.93
8
Formula
Mineral
D3O[(UO2)(AsO4)](D2O)3***
NH4[(UO2)(AsO4)](H2O)3
Uramarsite
* Structure determined (det), or structural affiliation inferred (inf); ** b 7.14 Å; *** At 4 K; b 7.11;
deuterated.
1 — Fitch e. a., 1982a; 2 — Locock e. a., 2004a; 3 — Ross, 1964; 4 —Fitch e. a., 1983; 5 — Fitch e. a.,
1982b; 6 — Weigel, Hoffmann, 1976b; 7 — Fischer e. a., 1981; 8 — Lawaldt e. a., 1982.
2) structures with ordered interlayer cations have lower symmetry than those
that exhibit site occupancy disorder; ordering takes place as temperature decreases;
3) substitution of heavier actinides results
in reduced cell volume, as a result of the «actinide contraction» (the ionic radii decrease in
the order U, Np, Pu, Am);
4) interlayer cations of similar size adopt
identical structures; the substitution of very differently-sized cations results in different, but
related structures.
In many of the transuranium-bearing compounds, unit cell parameters are only available
from powder X-ray diffraction data, and these
have generally been reported to yield tetragonal
Fig. 2. The structure of Li[(UO2)(PO4)](H2O)4 projected
along [100], with Li atoms shown as striped spheres, O
atoms as larger gray spheres, and H atoms as small black
spheres. H—O bonds are shown as short thick rods, H...O
distances as thin rods.
119
Fig. 3. The room-temperature structure of abernathyite, K[(UO2)(AsO4)](H2O)3 (a) projected along
[100]: K atoms and O atoms of H2O groups occupy the same interlayer position (in the ratio 1:3), H
atoms are not shown, but a possible hydrogen-bonding scheme is shown; the room-temperature
structure of Rb[(UO2)(AsO4)](H2O)3 (b) projected along [100], gray spheres correspond to the O
atoms of H2O groups, and the black spheres correspond to positions that are occupied by both Rb
atoms and O atoms.
Fig. 4. The structure of K[(UO2)(PO4)](D2O)3 at 10 K (a), projected along [100]: K atoms shown as
striped spheres, O atoms as larger gray spheres, and H atoms as small black spheres; the structure of
D3O[(UO2)(AsO4)](D2O)3 at 4 K (b), projected along [100]: note the nearly symmetrical environment of the deuterium atom that forms the oxonium group. H—O bonds are shown as short thick
rods, H...O distances as thin rods.
Fig. 5. Cs2[(UO2)(PO4)]2(H2O)5 projected along [110], Cs atoms are shown as black spheres, and O
atoms as gray spheres (a) and the structure of Cs(H3O)[(UO2)(AsO4)]2(H2O)5 projected along [010],
with the O atom of the oxonium group shown as striped (b). Possible networks of hydrogen bonds
have been omitted, except for the oxonium group.
Fig. 6. The «interrupted autunite-type sheet» found in K4(UO2)(PO4)2, projected along [001] (a) and
the structure projected along [100] (b). Potassium atoms are shown as gray spheres, and K—O bonds
have been omitted for clarity.
cells with c-cell dimensions in the vicinity of 8.5—9 Å. In contrast, the compounds
with determined structures have c-cell dimensions in the vicinity of 17—18 Å
(Table 1, 2). It is probable that the small c-cell dimensions reported are in fact subcells, as the larger c-dimension may be difficult to detect by conventional powder
diffraction methods.
The structure of K4[(UO2)(PO4)2] is also based on the autunite sheet-anion topology and contains monovalent cations (K) in its interlayer region (Linde e. a.,
1980). In the «interrupted autunite-type sheet» found in this compound, half of the
uranyl square bipyramids are vacant (Fig. 6). The potassium ions in the interlayer
provide charge balance and connectivity. The existence of this structure makes it
possible to conceive that a vacancy-type solid solution mechanism could operate
between the autunite-type sheet and the «interrupted autunite-type sheet», yielding
the formula: A2+2m[(AnO2)2–m(XO4)2](H2O)n–m, (written for compounds with monovalent interlayer cations). Evidence for this mechanism is lacking in the literature,
with the exception of an investigation of meta-autunite from whose chemical analysis uranyl-ion vacancies can be inferred (Bermanec e. a., 2005), although aspects
of this study have been debated (Locock, 2006; Bermanec e. a., 2006).
DIVALENT INTERLAYER CATIONS
A considerable number of compounds exhibit the autunite-type sheet with divalent cations in their interlayer regions. These can be considered in two broad groups
based on the environment of the interlayer cation: those that have coordination numbers greater than six — Ca, Sr, Ba, Pb (Table 3, 4); and those that adopt sixfold coordination — Mg, Mn, Fe, Co, Ni, Cu, Zn, Cd (Table 5, 6). Unlike compounds with
monovalent interlayer cations, those that contain divalent interlayer cations do not
exhibit site-occupancy disorder. Further, with the exception of the divalent-Cu-bearing compounds, none have tetragonal symmetry, although pseudosymmetry is common. For a given divalent-cation, more than one state of hydration may occur. The
interlayer spacing and relative orientations of the sheets varies with the nature of the
interlayer cation and with the state of hydration, giving rise to separate, but related,
structures. Changes in relative humidity can lead to different hydration states (by dehydration or rehydration), and thus to changes in the structure of the material in question. Such changes in hydration state are sharply defined (Beintema, 1938); the H2O
content in these compounds does not vary as in zeolite minerals, but rather is required to maintain the integrity of the hydrogen-bonding network.
Crystal structure refinements have been reported only for a few of the autunite-type compounds with large divalent cations (Ca, Sr, Pb, Ba), and these structures
all have either orthorhombic or monoclinic symmetry. The large divalent interlayer
cations are coordinated by H2O groups and by O atoms of the sheet to a lesser extent, and have coordination-numbers that range from seven to ten, depending on
the structure. This range in possible coordination number, coupled with the flexibility of the autunite-type sheet, gives rise to more than one structure for the same interlayer cation, albeit with different hydration states and different interlayer spacings (Table 3, 4).
The structure of autunite, Ca[(UO2)(PO4)]2(H2O)11, in which Ca is in ninefold
coordination (Fig. 7, a) is adopted by its Sr-bearing analogues, Sr[(UO2)(PO4)]2
122
Table 3
Autunite-type phosphates with large divalent cations
Formula
Status*
Mineral
Ca[(UO2)(PO4)]2(H2O)2(?)
Ca[(UO2)(PO4)]2(H2O)6.5(?)
Ca[(UO2)(PO4)]2(H2O)11
Sr[(UO2)(PO4)]2(H2O)6
Sr[(UO2)(PO4)]2(H2O)8
Sr[(UO2)(PO4)]2(H2O)11
Pb[(UO2)(PO4)]2(H2O)4
Pb[(UO2)(PO4)]2(H2O)8
Ba[(UO2)(PO4)]2(H2O)6**
Ba[(UO2)(PO4)]2(H2O)7
Ba[(UO2)(PO4)]2(H2O)10
Ca[(NpO2)(PO4)]2(H2O)6
Sr[(NpO2)(PO4)]2(H2O)6
Ba[(NpO2)(PO4)]2(H2O)6
Meta-autunite
Autunite
Przhevalskite
Meta-uranocircite II
Meta-uranocircite I
Uranocircite
inf
inf
det
inf
inf
det
inf
inf
det
det
inf
inf
inf
inf
S. G.
a (Å)
b (Å)
c (Å)
Ref.
O
6.55 7.06 8.16
Q
6.99
8.46
Pnma 14.01 20.71 7.00
Q
6.99
8.53
Q
7.00
18.79
Pnma 14.04 21.01 7.00
O
7.24 7.24 18.22
Q
6.93
17.13
P21/c 9.88 16.87 9.79
P21
6.94 17.63 6.95
M
7.01 6.99 21.20
Q
6.98
8.43
Q
6.98
8.52
Q
6.98
17.31
1
2
3
4
5
6
7
8
6, 9
6
6
10
10
10
2 = 6.99, c 2 = 6.92 Å.
1 — Takano, 1961; 2 — Blanchard, 1987; 3 — Locock, Burns, 2003a; 4 — Hoffmann, 1972; 5 — Weigel,
Hoffmann, 1976a; 6 — Locock e. a., 2005a; 7 — Soboleva, Pudovkina, 1957; 8 — Ross, 1956; 9 —
Khosrawan-Sazedj, 1982a; 10 — Weigel, Hoffmann, 1976b.
* Structure determined (det), or structural affiliation inferred (inf); **a
Table 4
Autunite-type arsenates with large divalent cations
Formula
Mineral
Ca[(UO2)(AsO4)]2(H2O)6.5(?)
Ca[(UO2)(AsO4)]2(H2O)11
Sr[(UO2)(AsO4)]2(H2O)8
Sr[(UO2)(AsO4)]2(H2O)11
Pb[(UO2)(AsO4)]2(H2O)8**
Ba[(UO2)(AsO4)]2(H2O)7
Ba[(UO2)(AsO4)]2(H2O)10
Ca[(NpO2)(AsO4)]2(H2O)6
Ca[(NpO2)(AsO4)]2(H2O)10
Sr[(NpO2)(AsO4)]2(H2O)8
Ba[(NpO2)(AsO4)]2(H2O)7
Ca0.5(H3O)[(PuO2)(AsO4)]2(H2O)x
Sr0.7(H3O)0.6[(PuO2)(AsO4)]2(H2O)x
Meta-uranospinite
Uranospinite
Metaheinrichite
Heinrichite
Status*
S. G.
a (Å)
b (Å)
c (Å)
Ref.
inf
Q
7.14
17.00 1
inf
O
14.35 20.66 7.17 2
det P2/c 7.15 7.10 18.90 2, 3
det Pnma 14.38 20.96 7.17 2
inf
O
9.86 9.99 36.28 4
inf
M
7.08 17.70 7.09 2
det P2/c 7.15 7.13 21.29 2
inf
Q
7.14
17.03 5
inf
Q
7.13
20.42 5
inf
Q
7.12
19.45 5
inf
Q
7.08
17.78 5
inf
Q
7.13
17.06 6
inf
Q
7.14
19.66 6
2 = 6.97, b 2 = 7.06 Å.
1 — Walenta, 1965; 2 — Locock e. a., 2005a; 3 — Pushcharovskii e. a., 2003; 4 — Nawaz, 1967; 5 —
Weigel, Hoffmann, 1976b; 6 —Fischer e. a., 1981
* Structure determined (det), or structural affiliation inferred (inf); **a
123
Table 5
Autunite-type phosphates with divalent transition metals or Mg
Formula
Mg[(UO2)(PO4)]2(H2O)8
Mg[(UO2)(PO4)]2(H2O)10
Mn[(UO2)(PO4)]2(H2O)8
Mn[(UO2)(PO4)]2(H2O)10
Fe2+[(UO2)(PO4)]2(H2O)8
Co[(UO2)(PO4)]2(H2O)10
Ni[(UO2)(PO4)]2(H2O)10
Ni[(UO2)(PO4)]2(H2O)12
Cu[(UO2)(PO4)]2(H2O)8
Cu[(UO2)(PO4)]2(H2O)12
Zn[(UO2)(PO4)]2(H2O)9.5
Cd[(UO2)(PO4)]2(H2O)7
Mineral
Status*
S. G.
a (Å)
inf
det
inf
det
inf
det
det
det
det
det
inf
inf
Q
P21/n
M
I2/m
M
P21/n
P21/n
P-1
P4/n
P4/nnc
Q
Q
7.22
6.95
7.04
6.97
6.98
6.95
6.95
7.00
6.98
7.03
7.01
6.97
Metasaleeite
Saléeite
Lehnerite
Bassetite
Metatorbernite
Torbernite
b (Å)
19.95
17.16
20.38
17.07
19.93
19.82
7.00
c (Å)
Ref.
17.73
6.98
6.95
6.98
7.01
6.96
6.97
11.17
17.35
20.81
20.19
18.08
1
2
3
4
5
4
4
4
6
6
7
7
* Structure determined (det), or structural affiliation inferred (inf).
1 — Cassedanne e. a., 1986; 2 — Miller e. a., 1986; 3 — Vochten, 1990; 4 — Locock e. a., 2004b; 5 —
Vochten e. a., 1984; 6 — Locock, Burns, 2003b; 7 — Pozas-Tormo e. a., 1986.
Table 6
Autunite-type arsenates with divalent transition metals or Mg
Formula
Mg[(UO2)(AsO4)]2(H2O)8
Mg[(UO2)(AsO4)]2(H2O)10
Mg[(UO2)(AsO4)]2(H2O)12
Mn[(UO2)(AsO4)]2(H2O)8
Mn[(UO2)(AsO4)]2(H2O)12
Fe2+[(UO2)(AsO4)]2(H2O)8
Fe2+[(UO2)(AsO4)]2(H2O)12
Co[(UO2)(AsO4)]2(H2O)8
Co[(UO2)(AsO4)]2(H2O)12
Ni[(UO2)(AsO4)]2(H2O)7–8(?)
Ni[(UO2)(AsO4)]2(H2O)12
Cu[(UO2)(AsO4)]2(H2O)8
Cu[(UO2)(AsO4)]2(H2O)12
Zn[(UO2)(AsO4)]2(H2O)8
Mg[(NpO2)(AsO4)]2(H2O)8
Mineral
Metanováèekite
Nováèekite Ii
Nováèekite I
Metakahlerite
Kahlerite
Metakirchheimerite
«Kirchheimerite»**
Metazeunerite
Zeunerite
Metalodevite
Status*
inf
det
det
det
det
det
inf
det
det
inf
det
det
det
inf
inf
S. G.
a (Å)
b (Å)
c (Å)
Ref.
Q
7.16
8.58
P21/n 7.13 20.09 7.16
P-1
7.16 7.16 11.31
P-1
7.22 9.92 13.34
P-1
7.14 7.14 11.36
P-1
7.21 9.82 13.27
Q
14.30
21.97
P-1
7.20 9.77 13.23
P-1
7.16 7.16 11.29
Q
7.18
17.24
P-1
7.15 7.16 11.26
P4/n
7.11
17.42
P4/nnc 7.18
20.86
Q
7.16
17.20
Q
7.09
17.47
1
2
2
2
2
2
1
2
2
3, 4
2
5
5
6
7
* Structure determined (det), or structural affiliation inferred (inf); ** Not an approved mineral species
name.
1 — Walenta, 1964; 2 — Locock e. a., 2004b; 3 — Ondrus e. a., 1997; 4 — Nabar, Iyer, 1977; 5 —
Locock, Burns, 2003b; 6 — Agrinier e. a., 1972; 7 — Weigel, Hoffmann, 1976b.
124
Fig. 7. The structure of autunite, Ca[(UO2)(PO4)]2(H2O)11, projected along [001], with Ca
atoms shown as black spheres and O atoms in the interlayer as gray spheres (a)
and Sr[(UO2)(AsO4)]2(H2O)8, projected along [010] (b).
(H2O)11 and Sr[(UO2)(AsO4)]2(H2O)11; it can therefore also be inferred that its
As-bearing equivalent, uranospinite, is isostructural and has the formula Ca[(UO2)
(AsO4)]2(H2O)11 (Table 4). Autunite can dehydrate rapidly to form meta-autunite,
Ca[(UO2)(PO4)]2(H2O)6—7(?), for which no satisfactory structure has yet been presented; the model of Makarov and Ivanov (1960) yields inappropriate interatomic
distances. Direct synthesis of meta-autunite has not been reported, and its formation by dehydration of autunite likely induces so much mosaic spread (Alberti e. a.,
1996; Clearfield, Costantino, 1996) that single-crystal structure determinations
have not been successful. The structure of Sr[(UO2)(AsO4)]2(H2O)8, is closely related to that of autunite (Fig. 7, b). In this compound, Sr is in eightfold coordination,
and the stacking of the sheets are offset from the arrangement found in autunite,
and the interlayer spacing is reduced (Pushcharovskii e. a., 2003; Locock e. a.,
2005a).
The autunite-type compounds that contain the large Ba cation differ from the
Ca- and Sr-bearing compounds, and exhibit a wider range of structural variation,
although only three structures have been refined: heinrichite, Ba[(UO2)(AsO4)]2$
$(H2O)10, meta-uranocircite I, Ba[(UO2)(PO4)]2(H2O)7, and meta-uranocircite II,
Ba[(UO2)(PO4)]2(H2O)6 (Table 3, 4). From the isotypy observed for Sr[(UO2)$
$(PO4)]2(H2O)11 and Sr[(UO2)(AsO4)]2(H2O)11, it can be inferred that uranocircite,
Ba[(UO2)(PO4)]2(H2O)10, is isostructural with heinrichite, and that metaheinrichite,
Ba[(UO2)(AsO4)]2(H2O)7 corresponds to meta-uranocircite I (Locock e. a., 2005a).
Uranocircite, meta-uranocircite I, and meta-uranocircite II are related via progressive dehydration, the structural consequences of which include: increasing connectivity of the Ba position to the sheets, rather than to interlayer H2O groups; decreases in the interlayer spacing; shifts in the relative positions of the uranyl phosphate
125
Fig. 8. Heinrichite, Ba[(UO2)(AsO4)]2(H2O)10, projected along [100], with Ba atoms shown as
black spheres and O atoms as larger gray spheres (a); meta-uranocircite I, Ba[(UO2)(PO4)]2(H2O)7,
projected along [001] (b); meta-uranocircite II, Ba[(UO2)(PO4)]2(H2O)6, projected along [101] (c).
Fig. 9. Novacekite I, Mg[(UO2)(AsO4)]2(H2O)12, projected along [100], with Mg-centered octahedra
striped, H atoms omitted, O atoms of H2O groups shown as gray spheres, and a possible scheme of
hydrogen bonding shown.
sheets; and shifts in the relative positions of the Ba atoms (Fig. 8). A similar dehydration relationship is expected between heinrichite and metaheinrichite.
In the structures of autunite-type compounds with Mg or divalent transition
metal cations (Table 5, 6), almost all of the interlayer cations occur as near-regular
octahedra. The exception is Cu, whose coordination environment may be described as a tetragonal dipyramid (distorted sixfold-coordination), as a result of a
Jahn-Teller distortion, and whose structures will be discussed separately below.
Three states of hydration are observed among the compounds with divalent cations
in octahedral coordination, and these differ in symmetry: the triclinic dodecahydrates, the monoclinic decahydrates and the triclinic octahydrates (Miller, Taylor,
1986; Locock e. a., 2004b).
In the triclinic dodecahydrates, six H2O groups coordinate the interlayer cation,
and a network of hydrogen bonding connects the interlayer octahedra to the sheets
(Fig. 9). These structures are strongly pseudosymmetric: for example, the matrix
[-1-10/1-10/001] transforms the triclinic cell parameters of Mn[(UO2)(AsO4)]2
(H2O)12 to a pseudomonoclinic C-centered cell with dimensions: a 10.192, b
10.002, c 11.362 Å, a 90.04°, b 101.79°, g 89.94°. However, the ordered nature of
the interlayer contents requires that the true symmetry is triclinic.
Similarly, the monoclinic dodecahydrates are pseudotetragonal (Table 5, 6),
with b ranging from 90.4—91.0°. The exact symmetry adopted appears to be a function of the size of the interlayer cation: Mg, Co and Ni compounds have space gro127
Fig. 10. Novacekite II, Mg[(UO2)(AsO4)]2(H2O)10, projected along [100]; the structure of
Mn[(UO2)(PO4)]2(H2O)10, projected along [100] (a) and interlayer polyhedra are striped, H atoms
omitted, O atoms of H2O groups shown as gray spheres, and possible schemes of hydrogen
bonding are shown (b).
up P21/n (Table 5, 6), whereas Mn[(UO2)(PO4)]2(H2O)10, with the larger Mn cation
in the interlayer, takes on space group I2/m (alternative setting of space group
C2/m) and thereby has a somewhat different configuration of the hydrogen bonding that connects the interlayer octahedra to the sheets (Fig. 10). The similarity of
the two polymorphs of the decahydrate structures may help to explain the presence
of stacking faults in some of crystals investigated (Locock e. a., 2004b).
The interlayer cation in the triclinic structures of the octahydrates is coordinated by five interlayer H2O groups, whereas the sixth vertex is a shared apical oxygen atom of a uranyl square bipyramid (Fig. 11). The octahydrates do not exhibit
strong pseudosymmetry (Locock, Burns, 2004b) The three structure types observed — triclinic dodecahydrates, monoclinic decahydrates, and triclinic octahydrates — can be related by dehydration. Comparison of Figures 9, 10 and 11 reveal
that as dehydration progresses, the interlayer spacing decreases, the sheets shift in
their relative positions, and the positions of the interlayer octahedra shift relative to
the sheets.
The structures of the Cu-bearing compounds differ from those that have other
divalent transition metals in the interlayer; only dodecahydrates (torbernite and zeunerite) and octahydrates (metatorbernite and metazeunerite) are observed (Table 5, 6), and both sets of hydrates have tetragonal symmetry (Locock e. a., 2003b).
The Cu occurs in Jahn-Teller distorted sixfold-coordination (as tetragonal dipyramids), coordinated by four interlayer H2O groups, and two longer distances to the
O atoms of uranyl ions. Upon partial dehydration, the dodecahydrate structure
128
Fig. 11. Metakahlerite, Fe[(UO2)(AsO4)]2(H2O)8, projected along [100]. The legend is the same as
in Fig 10.
Fig. 12. Torbernite, Cu[(UO2)(PO4)]2(H2O)12, projected along [100] (a) and metatorbernite,
Cu[(UO2)(PO4)]2(H2O)8, projected along [100] (b). The legend is the same as in Fig. 10.
Table 7
Autunite-type compounds with trivalent interlayer cations
Formula
Al[(UO2)(PO4)]2(OH)(H2O)8
(HAl)0.5[(UO2)(PO4)]2(H2O)8**
Al0.6790.33[(UO2)(PO4)]2(H2O)15.5
Al1-x9x[(UO2)(PO4)]2(H2O)20+3xF1–3x
Fe3+[(UO2)(PO4)]2(OH)(H2O)7
Fe2+Fe3+[(UO2)(PO4)]4(OH)(H2O)13
La[(UO2)(PO4)]3(H2O)14
Pr[(UO2)(PO4)]3(H2O)14
Nd[(UO2)(PO4)]3(H2O)13
Al[(UO2)(AsO4)]2(F,OH)(H2O)6.5
Al1–x9x[(UO2)(AsO4)]2(H2O)20+3xF1–3x
Mineral
Threadgoldite
Sabugalite
Uranospathite
Vochtenite
Chistyakovaite
Arsenuranospathite
Status*
S. G.
a (Å)
b (Å)
c (Å)
det C2/c 20.17 9.85 19.72
inf
M
20.82 9.84 9.85
det P-1
7.00 13.71 14.02
det Pnn2 30.02 7.01 7.05
inf
M
6.96 6.94 21.05
inf
M
12.60 20.00 10.00
inf
Q
6.96
22.28
inf
Q
6.96
22.11
inf
Q
6.96
22.04
inf
M
19.99 9.79 19.62
inf
O
30.37 7.16 7.16
Ref.
1
2
3
3
4
5
6
6
6
7
3
* Structure determined (det), or structural affiliation inferred (inf); ** Transformed from original setting
by matrix [101/0-10/00-1]. 1 — Khosrawan-Sazedj, 1982b; 2 — Vochten, Pelsmaekers, 1983; 3 — Locock
e. a., 2005b; 4 — Vochten, Goeminne, 1984; 5 — Zwaan e. a., 1989; 6 — Pozas-Tormo e. a., 1987; 7 —
Chukanov e. a., 2006.
transforms to the octahydrate structure: the stacking of the autunite-type sheets
changes (relative to the dodecahydrate, every alternate sheet in the octahydrate is
displaced by a 2 along the [110] direction), and the interlayer spacing decreases
(Fig. 12). The structures of the dodecahydrates are pseudo-I-centered, whereas the
octahydrates exhibit twinning by reticular merohedry (Locock, Burns, 2003b).
The structural data available for autunite-type compounds with divalent interlayer cations generally reinforces the structural themes introduced for the monovalent-containing compounds:
1) substitution of As for P in a structure results in expanded cell dimensions;
2) substitution of heavier actinides results in reduced cell volume, as a result of
the «actinide contraction»;
3) interlayer cations of similar size adopt identical structures; the substitution
of differently-sized cations results in different, but related structures;
4) pseudosymmetry is common; most unit cells can be recast in pseudotetragonal form.
For the transuranium-bearing compounds with divalent interlayer cations, unit
cell parameters are available only from powder X-ray diffraction data, all of which
are reported to yield tetragonal cells. The c-cell dimensions cluster either around
8.5 Å, or in the range 17—20 Å (Table 3, 4, 6). The assignment of tetragonal symmetry is not supported by the refined structures of the analogous uranyl compounds
(except for the Cu-bearing structures). As discussed below in the section on Analytical Challenges, this discrepancy probably results from the influence of preferred
particle orientation and pseudosymmetry on the results and interpretation of the
powder diffraction data.
130
Fig. 13. A portion of the structure of uranospathite, Al1–x9x[(UO2)(PO4)]2(H2O)20+3xF1–3x, projected
along [001] (a); Al0.6690.33[(UO2)(PO4)]2(H2O)15.5, projected along [100] (b); a portion of the structure of threadgoldite, Al[(UO2)(PO4)]2(OH)(H2O)8, projected along [021] (c). The legend is the same
as in Figure 10, but with disordered O atoms shown as striped spheres.
TRIVALENT INTERLAYER CATIONS
Aluminum, ferric iron, and lanthanides comprise the trivalent cations found in
the interlayers of compounds with autunite-type sheets (Table 7). Of these, crystal
structure refinements have been reported only for a few Al-bearing compounds.
However, these Al-bearing compounds exhibit the widest range of hydration states
and interlayer spacings of any of the autunite-type structures (Fig. 13): uranospathite, Al1–x9x[(UO2)(PO4)]2(H2O)20+3xF1–3x, with a basal (interlayer) spacing of
15.0 Å, is the «Dagwood sandwich» of the autunite-type structures, whereas threadgoldite, Al[(UO2)(PO4)]2(OH)(H2O)8, has a basal spacing of 9.4 Å (Khosrawan-Sazedj, 1982b; Locock e. a., 2005b). The Al in these structures invariably occurs in octahedral coordination, and a network of hydrogen bonds connects the interlayer octahedra to the autunite-type sheets. In threadgoldite, interlayer Al is present
as a dimer of edge-sharing octahedra, {Al2(OH)2(H2O)8}, whereas in the other struc131
tures, the Al is present in isolated octahedra (Fig. 13). The structure of the Fand As-bearing analogue of threadgoldite, chistyakovaite, Al[(UO2)(AsO4)]2
(F,OH)(H2O)6.5, has not been determined (Chukanov e. a., 2006). Although the
two minerals can be expected to have similar structures, they are not likely to be isotypic, as the cell volume of chistyakovaite is ~2 % smaller than that of threadgoldite
(both minerals have b = 110.7°) despite the presence of the larger As cation. In addition, chistyakovaite is reported to exhibit monoclinic P symmetry, rather than the
C-centering found in threadgoldite (Khosrawan-Sazedj, 1982b; Chukanov e. a.,
2006).
ORGANIC INTERLAYER CATIONS
In addition to the inorganic materials and minerals reviewed above, hybrid organic-inorganic structures based on the autunite sheet-anion topology have been
synthesized, generally via intercalation reactions (which induce substantial mosaic
spread into the resultant crystals). The guest organic molecules range from alkylammonium cations (Weiss e. a., 1957; Fern*andez-Gonz*alez e. a., 1976), to protonated heterocyclic amines and aromatic amines (Moreno-Real e. a., 1987; Liu e. a.,
1995). The orientation of the intercalated organic cations has typically been interpreted from measurement by powder diffraction of the basal spacings of the resultant materials, with few unit-cell data and no structures published. Hydrothermal
synthesis yielded the first organic-bearing autunite-type structure to be refined: triethylenediammonium diuranyl diarsenate trihydrate (Locock, Burns, 2004). As in
the inorganic structures, the organic cations are located in the interlayer region between the autunite-type sheets (Fig. 14), along with H2O groups that are held in the
structure only by hydrogen bonding.
ANALYTICAL CHALLENGES
The non-tetragonal symmetry (whether orthorhombic, monoclinic or triclinic)
of many autunite-type compounds is in sharp contradiction to the tetragonal symmetry usually attributed to them on the basis of powder X-ray diffraction data, but
is in excellent agreement with the observations of biaxial optical properties for
such compounds. This apparent discrepancy may be explained in part by considering the influence of the autunite-type sheet on investigations carried out by powder X-ray diffraction. The autunite-type sheet, [(AnO2)(XO4)], contains
~65—80 % of the electron density present in an autunite-type compound. The sheet (which has p4 plane group symmetry) therefore dominates the X-ray scattering
intensities, and can cause the compound to exhibit strong pseudosymmetry (usually pseudotetragonal). Further, as the autunite-type sheet is strongly bonded, such
compounds normally develop a platy morphology and perfect basal cleavage,
which can cause considerable preferred orientation in a powdered sample. The
combination of pseudosymmetry and preferred orientation can result in misinterpretation of the powder diffraction data as being consistent with tetragonal symmetry.
132
Fig. 14. The structure of triethylenediammonium diuranyl diarsenate trihydrate, projected along
[001]. The (N2C6H14)2+ cations are represented as striped polyhedra, and the O atoms of H2O groups
shown as gray spheres.
A further difficulty in the study of autunite-type compounds is their possible
change(s) in hydration state during analysis or between testing. Dehydration can
take place under ambient conditions of moderate-low relative humidity, (slightly)
elevated temperature, or be induced by the conditions of electron-microprobe analysis (Locock e. a., 2004b). Conversely, high relative humidity can induce rehydration (particularly in autunite sensu stricto). In addition, some autunite-type compounds will undergo phase transitions at or near room temperature (De Benyacar, De
Abeledo, 1974; Pham-Thi, Colomban, 1985; Pham-Thi e. a., 1985; Locock e. a.,
2004a).
Single-crystal structure refinement is also occasionally a fallible analytical
method. Several erroneous crystal structures of autunite-type compounds have
been reported. In some cases, the structure is simply missing a center-of-symmetry
(Piret e. a., 1979; Pushcharovskii e. a., 2003). Sometimes, an inappropriate solution (Barinova e. a., 2003) or more often, inappropriate space group has been chosen
(Hanic, 1960; Makarov, Ivanov, 1960; Makarov, Tobelko, 1960; Hennig e. a.,
2003), which results in inappropriate interatomic distances, partially occupied atomic positions, incorrect displacement parameters, and ostensible positional disorder. Similar misleading results are obtained if the unit cell under consideration is
incorrect (Calos, Kennard, 1996; Alekseev e. a., 2003). The high linear absorption
coefficient of actinide-bearing compounds makes it essential to apply corrections
for absorption to the intensity data if at all possible. The absorption of the primary
X-ray beam during the collection of intensity data can also be responsible for the
(catastrophic) dehydration of a crystal under investigation, and its ensuing transformation to a highly-oriented powder.
133
CONCLUSIONS
The structural details of a compound that contains the autunite-type sheet depends on its intrinsic chemical composition (actinyl ion, tetrahedral cation, interlayer cation, and hydration state). The hydration state, in turn, depends on the ambient temperature and relative humidity. Autunite-type compounds exhibit several
modes of structural variation: in-plane rotation of the polyhedra that constitute the
sheet; shifts in the stacking of the sheets; changes (expansion or contraction) in the
interlayer spacing. Despite this level of complexity, several common themes are
observed:
1) substitution of As for P in a structure generally results in expanded cell dimensions, except when such substitution results in a change in cation ordering;
2) structures with ordered interlayer cations have lower symmetry than those
that exhibit site occupancy disorder, and ordering takes place as temperature decreases;
3) substitution of heavier hexavalent actinides results in reduced cell volume,
as a result of the «actinide contraction»;
4) interlayer cations of similar size and identical formal charge adopt identical
structures, the substitution of very differently-sized or different-valence cations results in different, but related structures;
5) more than one state of hydration may occur for a given divalent- or trivalent-cation, yielding separate, but related structures;
6) pseudosymmetry is common, most unit cells can be recast in pseudotetragonal form.
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