NAVROTSKY ET AL.
CHAPTER 4: THERMODYNAMICS OF URANIUM MINERALS AND RELATED MATERIALS
Navrotsky, Alexandra, Tatiana Shvareva and Xiaofeng Guo
Peter A. Rock Thermochemistry Laboratory and NEAT ORU
University of California Davis
Davis CA 95616 USA
e-mail: anavrotsky@ucdavis.edu
influence its stability and affect its formation.
Therefore we first consider thermodynamic effects
of oxidation and doping prior to discussion of
mineral formation and its behaviour in the
environment.
INTRODUCTION
Uranium in natural systems exists in two
oxidation states: tetravalent and hexavalent. The
aqueous chemistry and geologic and environmental
transport of U is dominated by U6+ because of its
much greater aqueous solubility as the uranyl
(UO22+) ion. However, its solid state chemistry is
rich in compounds of both U4+ and U6+, as well as
some materials containing U5+. Thorium, geologically more abundant than U, is associated with it in
U4+ minerals and also forms some specific structures
on its own. Thorium cannot be oxidized to the 5+ or
6+ state and thus lacks the aqueous mobility of U
and does not substitute significantly into uranyl
solid phases. Because of radioactive decay as well
as the general multi-component nature of
geochemical systems, U minerals accommodate
many impurities, including fission products
dominated by the rare earth elements. Interest in the
thermodynamics of U minerals and associated
compounds is motivated by four main concerns: the
mining and processing of U ore (the front end of the
nuclear fuel cycle), the operation of nuclear
reactors, the disposal and environmental transport of
nuclear materials (the back end of the nuclear fuel
cycle), and the use of U and Th as geochemical
indicators, particularly in age dating. The purpose of
this review is to summarize available thermochemical data for U minerals and related materials,
to identify systematic crystal chemical trends in the
data, and to point out areas where further research is
needed.
Formation of uranium oxides
The phase diagram for the UO2–UO3 system
has been developed by Hoekstra et al. (1961) (Fig.
4-1) and shows UO2, U4O9, U3O8 and UO3 as major
stable phases at room and elevated temperatures,
with several phases occurring in more than one
polymorphic form.
The thermodynamic properties of these oxides
have been studied extensively and are summarized
in Table 4-1. For UO2, U3O8 and –UO3 the
preferred data are recommended by CODATA
(1989) and the data for several U oxide phases are
most recently reviewed by Guillaumont et al.
(2008).
UO2 with cubic fluorite structure, space group
Fm3m, a = 5.470 Å, where U is in the 4+ oxidation
state, is stable at room temperature in air. Several
datasets obtained by different methods have been
reported with the CODATA accepted value
determined by combustion calorimetry of UO2 to
U3O8 under O atmosphere performed by Huber et al.
(1969). Later Johnson et al. (1981) confirmed the
recommended value by combustion calorimetry in
F.
Upon increasing temperature above 300 ºC
under higher O partial pressure, the oxidation of U
occurs. At lower O/U ratios it is accompanied by the
incorporation of the additional O atoms into the
fluorite crystal lattice. Extra O occupies interstitial
sites and displaces the lattice anions from their
crystallographic positions, forming the anionic
cuboctahedral sublattice (Desgranges et al. 2009).
Both UO2.25 and UO2.33 phases are characterized by
the presence of such stabilizing cuboctahedra, and
are more exothermic in enthalpy of formation
compared to UO2. When the concentration of the
excess O atoms exceeds a critical level, cubocta-
URANINITE AND RELATED MATERIALS
Uranium dioxide, UO2, by far is the most
environmentally abundant U oxide, however it is not
found in nature in its pure form; it is partially
oxidized and contains additional elements
(impurities and fission products). With composition
varying in individual geological deposits, especially
as a function of age, UO2 forms the mineral
uraninite, approximated by the formula
M2+aRE3+bUO2+x. Both oxidation and doping
Mineralogical Association of Canada Short Course 43, Winnipeg MB, May 2013, p. xx-xx
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NAVROTSKY ET AL.
+
FIGURE 4-1. Phase diagram for the UO2–UO3 system (Hoekstra 1961).
TABLE 4-1. THERMODYNAMIC DATA, PER MOLE OF METAL, FOR U AND TH OXIDES AT 298 K.
Oxide
UO2
UO2.25 (U4O9)
βUO2.33(U3O7)
UO2.67 (U3O8)
αUO3
βUO3
γUO3
δUO3
εUO3
amUO3
βUO2(OH)2
ThO2
ΔHf, el (kJ/mol)
–1085.0 ± 1.0
–1128.0 ± 1.7
–1141.0 ± 2.0
–1191.6 ± 0.8
–1212.4 ±1.5
–1220.3 ±1.3
–1223.8 ±1.2
–1213.7 ±1.4
–1217.2 ±1.3
–1207.9 ±1.4
–1533.8 ±1.3
–1226.4 ± 3.5
ΔSf, el (J/mol.K)
181.2 ± 1.4
197.7 ± 2.4
202.7 ± 2.4
228.5 ± 1.1
235.2 ± 3.3
268.1 ± 1.8
262.1 ± 1.7
453.4 ± 2.2
191.9 ± 4.9
ΔSo(J/mol.K)
77.0 ± 0.2
83.5 ± 0.2
83.5 ± 0.2
94.2 ± 0.2
99.4 ± 1.0
96.3 ± 0.4
96.1 ± 0.4
–
–
–
138.0 ± 4.0
65.2 ± 0.2
ΔGf, el (kJ/mol)
–1031.8 ± 1.0
–1069.1 ± 1.7
–1080.6 ± 1.4
–1123.2 ± 0.8
–1135.3 ± 1.5
–1142.3 ± 1.3
–1145.7 ± 1.2
–
–
–
–1398.7 ±1.8
–1169.2 ±3.5
spectroscopic studies (Allen & Holmes 1993). U3O8
is known as a final product of UO2 oxidation (Huber
& Holley 1969), as well as a reduction product of
UO3 (Bessonov 1970). In accord with the phase
diagram in Fig. 4-1, UO2 completely converts into
U3O8 above 700ºC. Thus neither pure anhydrous
UO2 nor anhydrous UO3 are found under common
environmental or geological conditions.
hedra rearrange into the more energetically
favourable octahedral U3O8 structure. Enthalpies of
formation of UO2.25,UO2.33,UO2.67 and UO3 were
determined by solution calorimetry in a Ce(SO4)2–
H2SO4 mixture (Fitzgibbon et al. 1967).
UO2.67 (U3O8) is a mixed valence oxide, where
U is present in both 4+ and 6+ oxidation states, in
1:2 ratio, as documented by multiple structural and
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NAVROTSKY ET AL.
UO3 is the polymorph most exothermic in
formation enthalpy. It has U in two different
octahedral environments, one with two short uranyl
distances and the other without (Engmann 1963).
The structure is nominally considered a “uranyl
uranate” and consists of uranyl chains interconnected by regular U octahedra. In an aqueous
alteration sequence, the first formed and the most
abundant product is the uranyl oxide hydrate
schoepite, UO3·H2O, with a uranyl cation-based
layered structure (Finch & Ewing 1992). The
complete oxidation of U4+ to U6+ and the hydration
give this mineral a much more negative enthalpy of
formation as shown in Table 4-1. A summary plot of
enthalpies of formation as a function of O content is
shown in Figure 4-2.
In contrast to U, Th forms only one stable
oxide, ThO2 (Fig. 4-3). Adopting the fluorite
structure, it is isostructural with UO2, PuO2, CeO2,
and the high temperature forms of ZrO2 and HfO2.
ThO2 is very refractory and melts above 3390ºC
(Benz 1969). Enthalpy and Gibbs energies of
formation of UO2 and ThO2 from the elements are
similar (Table 4-1).
Effects of doping
The choice of the dopant cations in uraninite
structure strongly depends on environmental
conditions and age of the deposit (rare earth fission
products build up with age). The most common
impurities are Th4+ and Ce4+, trivalent rare-earth
cations RE3+, Pb2+ and Ca2+. Phase diagrams for
these systems have shown that there are extensive
compositional ranges where solid solutions with
cubic fluorite structure can be formed. However
experimental thermodynamic data are scarce, mostly
due to the complex synthesis pathways and
refractory nature of the materials and difficulty in
controlling and determining the exact O
stoichiometry.
The thermodynamic properties of doped urania
were studied mainly through their O potentials, as
reviewed by Fujino et al. (1988) and have strong
dependence on the O/M ratio. Recently high
temperature oxide melt solution calorimetry has
been applied to the investigation of mixing
energetics for stoichiometric ceria-doped urania and
thoria with results supported by DFT calculations
(Hanken et al. 2012, Shvareva et al. 2011).UO2–
CeO2 forms an essentially ideal solid solution in the
entire compositional range with nearly zero mixing
enthalpies (Hanken et al. 2012), due to very small
difference in U4+ and Ce4+ ionic radii. The data also
argue against significant charge transfer to form U5+
(or U6+) and Ce3+ in this system, at least at room
temperature. For the ThO2–CeO2 system, mixing
enthalpies are slightly positive with an interaction
parameter of 15.1 ± 2.2 kJ/mol. and a calculated
critical temperature of 428 K for demixing
(Shvareva et al. 2011). Consistently, for the solid
solution of Ce4+ and Zr4+ oxides with even larger
ionic radius difference, the mixing enthalpies are
more positive resulting in interaction parameter of
51.0 ± 8.0 kJ/mol in the cubic fluorite structure (Lee
et al. 2008).
Fluorite-structured oxides doped with trivalent
ions contain O vacancies to balance the lower cation
charge. These vacancies lend mobility to the O
sublattice, and materials such as yttria-doped
T, oC
FIGURE 4-2. Enthalpies of formation for various U oxides
as a function of O/U ratio.
2
2
FIGURE 4-3. Phase diagram for Th–ThO2 system (Benz
1969).
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NAVROTSKY ET AL.
zirconia and gadolinia-doped ceria, to name a few,
have received extensive attention as ionic
conductors in applications ranging from O sensors
in automobile exhausts (Yao & Yu 1984) to solid
electrolytes for solid oxide fuel cells (SOFC) (Steele
et al. 1994) to gas separation membranes (Dyer et
al. 2000). RE-doped urania and thoria are relevant
to the nuclear fuel cycle and Ce4+ is considered a
useful analog or surrogate for Pu4+ because of
similar ionic radii. Motivated by these materials
science applications, enthalpies of formation of an
extensive series of doped ZrO2, HfO2, CeO2, and
ThO2 materials have been measured. The main
results are discussed in detail in recent reviews
(Navrotsky et al., Navrotsky 2010) and are
summarized together with newly obtained data in
Table 4-2.
The doping of ZrO2, HfO2, CeO2 and ThO2
with aliovalent rare-earth cations creates O
vacancies. In the zirconia and hafnia solid solutions
these vacancies tend to associate with smaller Zr4+
and Hf4+ cation and stabilize 7-fold coordination,
characteristic for monoclinic zirconia and hafnia.
The binary C-type rare earth oxides REO1.5 (RE2O3)
have the bixbyite structure and contain an ordered
array of O vacancies that are located nearest
neighbour to the trivalent cations. The change in
location of vacancies on formation of the doped
fluorite phase is correlated with strongly exothermic
formation enthalpies for all zirconia and hafnia solid
solutions with the largest heat effect associated with
the largest doping cations. An example of
thermodynamic data for ZrO2, HfO2 and CeO2
doped with YO1.5 is shown in Figure 4-4 and
discussed in detail by Navrotsky et al. (2007) and
Simoncic & Navrotsky (2007).
In the case of 8-coordinated cations in ceria and
thoria, vacancies form, due to Coulomb interactions,
near the dopant. Overall, formation enthalpies for
CeO2 and ThO2 systems are positive and reflect heat
effects of two independent phenomena: the regular
solid solution behaviour at lower doping content and
formation of energetically favorable vacancy–
dopant associates at higher dopant concentration
(Fig. 4-5). From enthalpies of formation and mixing,
the enthalpies of vacancy clustering have been
experimentally evaluated (Buyukkilic et al. 2012,
Aizenshtein et al. 2010). There are strong correlations between the composition of maximum ionic
conductivity and least stable enthalpy for RE-doped
thoria and ceria, linking the energetics, defect
equilibria, and transport properties.
Mazeina et al. (2008) have shown for the UO2–
CaO and UO2–YO1.5 systems that partial oxidation
of U and forming UO2–UO3–CaO and UO2–UO3–
YO1.5 solid solutions leads to negative enthalpies of
formation which significantly stabilize the structure.
This is consistent with the observation of Janeczek
& Ewing (1992) that instead of slow post-formation
partial oxidation of U in uraninite, it is likely that
uraninite initially crystallizes as a complex U+4U+6
oxide, even under reducing conditions. In such a
TABLE 4-2. INTERACTION PARAMETERS FOR SOLID SOLUTIONS WITH FLUORITE STRUCTURE.
System
ZrO2–SmO1.5
ZrO2–GdO1.5
ZrO2–DyO1.5
ZrO2–YO1.5
ZrO2–YbO1.5
HfO2–SmO1.5
HfO2–GdO1.5
ZrO2–DyO1.5
ZrO2–YO1.5
ZrO2–YbO1.5
ZrO2–CeO2
Interaction parameter,
kJ/mol
–107.5 ± 8.4 a,h
–104.2 ± 5.7 a,h
–94.0 ± 4.9 a,h
–90.3 ± 6.9 a,h
–89.3 ± 9.7 a,h
–138.1 ± 16.0 a,h
–115.0 ± 11.7 a,h
–119.8 ± 11.4 a,h
–111.8 ± 13.6 a,h
–93.7 ± 14.7 a,h
51.0 ± 8.0 b,h
System
CeO2–LaO1.5
CeO2–GdO1.5
CeO2–SmO1.5
CeO2–NdO1.5
CeO2–SmO1.5–NdO1.5
ThO2–LaO1.5
ThO2–YO1.5
ThO2–CeO2
UO2–CeO2
BiO1.5–DyO1.5
a
Interaction parameter,
kJ/mol
106.3 ± 24.3 i
27.0 ± 10.9 i
179.2 ± 2.8 c,i
166.9 ± 4.6 c,i
61.8 ± 1.7 c,i
27.8 ± 4.1 d,i
119.3 ± 11.4 d
15.1± 2.2 e
0f
–73.0 ± 1.4 g
Simoncic & Navrotsky (2007); b Lee et al. (2008); c Buyukkilic et al. (2012); d Aizenshtein et al. (2010);
Shvareva et al. (2011); f Hanken et al. (2012); g Tran & Navrotsky (2012); h. These negative values reflect strong
ordering of cations and vacancies into clusters; i. Positive values of the interaction parameter for low dopant
concentrations reflect size mismatch, and the association of defects, with an exothermic effect, at higher dopant
concentrations (see text for details).
e
4
NAVROTSKY ET AL.
progress in our laboratory. Though the calorimetry
is straight forward, the determination of exact O
stoichiometry in the samples is critical and
methodology is being fine-tuned to accomplish this
routinely and accurately. It is anticipated that
thermochemical data for materials relevant to
natural complex uraninite will become available
over the next several years.
Zircon-related silicates: USiO4, ThSiO4, CeSiO4,
ZrSiO4, HfSiO4
Uranium(IV) silicate (coffinite) is a secondary
important alteration reduced uranium mineral from
an economic point of view (Finch & Murakami
1999). Thorite (ThSiO4) and coffinite are isostructural with zircon (ZrSiO4) and Th and U have
extensive solubility in zircon (Ioudintsev et al.
1998). Properties of the higher tetravalent actinide
silicates has been predicted from the thermodynamic
systematics as discussed by Mazeina et al. (2005).
With positive enthalpies of formation from the
oxides, thorite and coffinite are metastable at room
temperature but may persist from high temperature
conditions. Coffinite is commonly found in reducing
low temperature environments rich in organic
matter, suggesting its formation by reduction from
U(VI) phases. Such coffinite is commonly very fine
grained (nanophase) and may be hydrated (Frondel
1958, Speer 1980, Smits 1989, Janeczek & Ewing
1992). Thorite needs much higher temperature to
form (Hazen et al. 2009).
Mazeina et al. (2005) suggested that all actinide
orthosilicates show a linear dependence of molar
volume on ionic radius of tetravalent cations in
eightfold coordination, as shown in Figure 4-6 and
Table 4-3. Based on the enthalpies of formation and
ionic radius for USiO4, ThSiO4, ZrSiO4, HfSiO4,
they were able to predict the enthalpies of formation
of AmSiO4, PuSiO4, NpSiO4, PaSiO4.
Some computational work has been done on the
thermodynamic properties of zircon-type actinide
silicates. The total energy of ASiO4 (A = Hf, Th, U,
Pu, Ce) and their solid solutions have been
calculated via density functional theory (DFT) with
local density approximation (LDA) (Ferriss et al.
2010). Total energy of CeSiO4 and PuSiO4 at 0 K
are –5259.13 kJ/mol and –5914.60 kJ/mol, respectively (Ferriss et al. 2010). Although experimental
thermodynamic data for CeSiO4 and PuSiO4 are not
available, Hof,ox can be estimated from the total
energy by looking into the relationship between the
total energies of other ASiO4 with their known
enthalpies
of
formation,
which
gives
FIGURE 4-4. Thermodynamic data for ZrO2, HfO2 and
CeO2 doped with YO1.5. Enthalpies of monoclinic–
cubic transitions Htrs for ZrO2 and HfO2, as well as
enthalpies of transition from C or A-type to cubic
structure for CeO1.5,Htrs(CeO1.5), are reflected in the
plot Navrotsky et al. (2007)
scenario, the doping of U oxide with lower valence
cations enables the formation of O vacancies in the
fluorite structure, which in turn accommodate
excess O atoms associated with U oxidation,
resulting in the most stable arrangement of REdoped partially oxidized U oxide, in the form of
uraninite, abundantly occurring in nature.
Detailed oxide melt solution calorimetric
studies of synthetic RE-doped U oxides are in
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NAVROTSKY ET AL.
FIGURE 4-5. Regular solution parameters (Ω) and experimentally determined enthalpies of
vacancies
clustering (ΔHassociation) for selected RE doped ThO2 and CeO2 systems (Aizenshtein et al. 2010).
TABLE 4-3. ENTHALPY OF FORMATION OF ZIRCON-TYPE ORTHOSILICATES
Hf, ox
(kJ/mol)
Sf, ox (kJ/mol
K)
Hf, el
(kJ/mol)
Gf, el
(kJ/mol)
Sf, el
(kJ/mol K)
19.6 ± 2.0 a
148.9 ± 10.4 e
–2117.5 ± 4.2 a
–2050.3 ± 3.9 a
–225.4 ± 10.5 e
4.3 ± 5.6 a
–0.4 ± 12.8 e
–1991.3 ± 5.4 a
–1883.6 ± 4.0 a
–361.3 ± 12.3 e
Zircon
ZrSiO4
–24.2 ± 2.8 b,c
–12.4 ± 8.5 e
–2034.2 ± 3.1 a
–1919.7 ± 3.1 a
–384.0 ± 8.0 e
Hafnon
HfSiO4
–22.3 ± 4.7 a,d
––
–2050.6 ± 5.1 a,b
––
Phase
Thorite
ThSiO4
Coffinite
USiO4
a
Simoncic & Navrotsky (2007); b Robie & Hemingway (1995); c Molodetsky et al. (1998); d Lee & Navrotsky (2004);
e
calculated values from formation of enthalpy and free energy.
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NAVROTSKY ET AL.
o
H f, ox ,kJ/mol
molar volume, cm
3
huttonite
2
Figure 4-6. Measured and predicted enthalpies of formation of zircon-type orthosilicates MSiO4 from oxides as a function of
ionic radius of cation (Mazeina et al. 2005).
Hof,ox(CeSiO4) = 23.79 kJ/mol, Hof,ox(PuSiO4) =
78.00 kJ/mol. The latter value is more endothermic
than that predicted by Mazeina et al. (2005) (–15
kJ/mol) on the basis of systematic trends relating
enthalpy and ionic radius. Clearly the enthalpy of
formation of CeSiO4 should be measured in the
future.
Alpha decay from U and Th causes radiation
damage (recoil nuclei and alpha particles) in the
zircon structure, leading to amorphization or
"metamictization" radiation-induced transition
(Murakami et al. 1991, Woodhead et al. 1991).
Metamictization is used to age date geologic
materials (Holland & Kulp 1950), and it increases
aqueous solubility (Tole et al. 1985) and leaching
rate (Ewing et al. 1982).
The mechanisms of radiation damage have
been studied extensively (Murakami et al. 1991,
Woodhead et al. 1991, Weber et al. 1994) and the
energetics of radiation damage in natural zircon
have been determined by calorimetry with the
enthalpy of total amorphization being 59 ± 3 kJ/mol
(Ellsworth et al. 1994). This result suggests the
metamict phase is not only kinetically more
chemically reactive (Tole et al. 1985), but also is
significantly destabilized thermodynamically, which
may compromise the retention of actinides in
zircon-containing nuclear waste forms and in natural
samples used for radiometric age dating (Ellsworth
et al. 1994).
is nearly always partly oxidized (Finch & Murakami
1999). The measured enthalpies of formation of Ce,
Th, and U brannerites are shown in Table 4-4.
Enthalpies of formation for Ce- and Thbrannerite are positive, which suggests those two
compounds are metastable at room temperature.
Even though there are no calorimetric entropy data
for these brannerite minerals, Helean et al. (2003)
estimated entropies of formation by assuming the
change of free energy for the formation of each
compounds is zero at its formation temperature.
They suggested that configurational entropy significantly stabilizes brannerite, because of its common
non-stoichiometry and cation disorder. They also
explained phenomenologically the differences
among the enthalpy data. Small Ce cations shorten
the anatase-type sheets, and large Th cations
lengthen the distance between layers. The ideal
distance separating layers is approached when U
cations are dominant, indicating the U brannerite is
significantly more stable than either the Ce or Th
form (Helean et al. 2003).
Enthalpies of formation for Ce- and Thbrannerite are positive, which suggests those two
compounds are metastable at room temperature.
TABLE 4-4. ENTHALPY OF FORMATION OF U-, TH-,
CE-BRANNERITE
Formula
CeTi2O6
ThTi2O6
UTi2O6
Brannerite: UTi2O6, ThTi2O6, CeTi2O6
Brannerite is a common accessory mineral
found in uraninite and coffinite U(IV) deposits. Its
structure has layers of edge-sharing Ti octahedra
and layers of distorted U octahedra, in which the U
a
Hf, ox
(kJ/mol)
29.6 ± 3.6 a
19.4 ± 1.6 a
–6.9 ± 2.3 a, b
Hf, el (kJ/mol)
–2948.6 ± 4.3 a
–3096.5 ± 4.3 a
–2977.9 ± 3.5 a
, Simoncic & Navrotsky( 2007); b, Donaldson et al.
(2005).
7
NAVROTSKY ET AL.
Even though there are no calorimetric entropy data
for these brannerite compounds, Helean et al.
(2003) estimated entropies of formation by
assuming the change of free energy for the formation of each compound is zero at its formation
temperature. They suggested that configurational
entropy significantly stabilizes brannerite, because
of its common non-stoichiometry and cation
disorder. They also explained phenomenologically
the differences among the enthalpy data. Small Ce
cations shorten the anatase-type sheets, and large Th
cations lengthen the distance between layers. The
ideal distance separating layers is approached when
U cations are dominant, indicating the U brannerite
is significantly more stable than either the Ce or Th
form (Helean et al. 2003).
Furthermore, these authors connected the
energetics to irradiation studies by comparing their
critical amorphization doses Dc: Dc(CeTi2O6) <
Dc(ThTi2O6) < Dc(UTi2O6) (Lumpkin et al. 2001,
Lian et al. 2001), which suggests the resistance of a
phase to radiation damage is related to its thermodynamic stability (Helean et al. 2003). Such
relations between radiation resistance and thermodynamic stability have been studied extensively in
pyrochlore structures not containing U (Lang et al.
2010).
Ce-brannerite can be used to estimate the
properties of Pu-brannerite due to similarity in ionic
radii of Ce4+ and Pu4+. Thus Helean et al. (2003)
predicted that PuTi2O6 will be energetically metastable with respect to its binary oxides, and can be
stabilized only at high temperature (> 1500oC),
which is consistent with the difficulties in its
synthesis (Vance et al. 2001)
the full thermodynamic data set, based on cryogenic
heat capacity measurements and oxide melt solution
calorimetric determination of enthalpy of formation,
for cheralite CaTh(PO4)2 (Popa et al. 2008) see
Table 4-5.
These data reflect the enthalpy and free energy
of formation from binary oxides from cheralite are
~55 ± 10 kJ/mol less exothermic than those for
monazite LaPO4 (Ushakov et al. 2001, Ushakov et
al. 2004). Popa et al. (2008) explained this more
positive enthalpy value as originating from
structural disorder of Ca2+ and Th4+. After
considering several solid state reactions related to
cheralite formation and the strong acid–base
interaction between CaO and P2O5, Popa et al.
(2008) concluded that appropriate mineralogical
conditions (with other phases present) have great
impact on incorporation of actinides into monazite
for nuclear waste disposal.
Uranium in garnet
In a recent discovery in Northern Caucasus,
Russia, Galuskina et al. (2010) have shown that
natural garnet can accommodate up to 27 wt.% U.
Also as high as 18 wt.% of U has been reported in
synthetic Ca–Zr–Fe-based garnet (Yudintsev et al.
2002). The latter study suggested that U substitutes
more easily in garnet with larger lattice parameters.
These findings suggest that a specifically tailored
garnet composition may be a good host for U in
nuclear waste.
Study of U substitution in a simple garnet
system may shed light on the substitution
mechanisms and their energetics. Charge balance
for Y3+–M4+ substitution can be maintained by two
mechanisms. One is that charges are balanced by
introducing other divalent cations: Y3+ + Fe3+ = M4+
+ N2+. In the second mechanism proposed by Rak
et al. (2011) from computational studies, which is in
a sense a special case of the first, the divalent cation
is Fe2+, produced by reduction of the Fe3+ already in
the garnet structure. Such reduction is of course
coupled with oxidation, presumably of species
elsewhere in the environment. A series of Ce and of
Substituted monazite: synthetic cheralite
The mineral monazite, LnPO4 (Ln3+ = Ce3+,
3+
La , Nd3+, Sm3+, or Pr3+) has remarkable capacity
of retaining tetravalent actinides, especially U4+ and
Th4+ (McCarthy et al. 1978, Beall et al. 1981, Sales
et al. 1983, Meldrum et al. 1996). Among the many
Ln3+–Ac4+substitutional ceramics, cheralite is one of
the synthetic monazite-based compounds that has
been extensively studied. Popa et al. (2008) reported
TABLE 4-5. THERMODYNAMIC FUNCTIONS FOR CHERALITE (BASED ON ONE PHOSPHATE UNIT)
Formula
Ca0.5Th0.5PO4
Hf, ox (kJ/mol)
–253.2 ± 4.8
Hf, el (kJ/mol)
–1936.4 ± 4.9
Gf, el (kJ/mol)
–1817.7 ± 5.1*
Sf, ox (kJ/mol K)
Sf = –13.7 ± 0.8
Sf,total = –8.0 ± 0.8**
* Calculated values from enthalpy and entropy of formation; **Sf,total includes the configurational
entropy while Sf does not.
8
NAVROTSKY ET AL.
According to the diagram, metaschoepite is
replaced by soddyite and uranophane, uranyl silicate
minerals, if enough dissolved silica is present in
groundwater. Becquerelite, in turn, is stable in more
acidic conditions and also can coexist with uranyl
silicates under certain groundwater compositions. In
addition the trends of formation energetics of uranyl
minerals were considered in terms of acid–base
interactions of corresponding oxides (Shvareva et
al. 2012). The energetics of formation of uranyl
minerals reflects the interaction between uranyl
cations, interlayer cations, and acidic anions, with
energetics depending on the differences in acidity
(or basicity) of interacting oxides and on the crystal
structure formed. Due to the different structural
types, formation enthalpies of all oxide, carbonate,
phosphate and silicate phases cannot be directly
compared; however within each class of materials
the energetic trends for minerals with the same
structure can be derived. Using such information the
stability of isostructural members that are
unavailable for experimental measurements can be
predicted, as for example is shown in Fig. 4-8.
We suggest that a similar approach can be
expanded to other structural types of U materials
and describe it below for the case of alkali and
alkaline earth uranates.
Th-substituted YIG garnets has been synthesized,
and thermodynamic properties obtained for the Cesubstituted garnet suggest this substitution is
energetically favourable.
Uranyl Minerals
Most minerals containing U6+ incorporate the
hexavalent U as the uranyl ion UO22+.
Thermodynamic properties of uranyl-based
hydroxide, carbonate, phosphate and silicate
minerals have been summarized recently (Shvareva
et al. 2012) as have their solubilities (GormanLewis et al. 2008). Most of the data were derived by
simultaneously measuring the enthalpies of
formation of minerals by high temperature oxide
melt solution calorimetry and measuring the Gibbs
free energies of formation by solubility experiments.
Knowing H and G of formation enabled
calculation of S of formation. Table 4-6
summarizes the thermochemical data.
The complete data set allowed the calculation
of stability fields of selected minerals (see Figure
4-7 for an example), which are in reasonable
agreement with previous studies (Finch & Ewing
1992, Chen et al. 1999) and with the composition of
minerals observed in nature.
Figure 4-7. Log ([Ca2+]/[H+]2) vs. Log [H4SiO4] diagram for the most abundant uranyl minerals. Grey lines reflect stability
fields from experimental results. Black and dotted margins outline the stability relationships predicted earlier (Finch &
Ewing 1992, Chen et al. 1999), diamonds and squares describe the water composition discussed in (Chen et al. 1999).
9
Hf, ox, kJ/mol
Hf, el, kJ/mol
Pb0.38(UO2) O (OH)0.76(H2O)0.3
Curite Pb3(UO2)8O8(OH)6·2H2O
10
–2725.2 ± 2.6 h
–3099.3 ± 5.6 h
–2947.2± 4.0 h
–3399.7± 4.0 h
–215.9 ± 6.5
–150.0  4.3
Ca0.5(UO2) (SiO3OH0.5)(H2O)2.5
–2758.6 ± 3.5
h
–1826.1 ±2.1 g
–2046.3 ±12.2
–1007.6±12.0h
–352.5 ±7.2 h
–27.5 ±7.3
h
–635.4 ±10.9g
–964.3 ±6.2
f
–1302.3 ±21.2f
–––––
336.8± 12.6
681.9± 7.2
1020.2± 7.2
53.2± 5.9
196.3± 6.8
169.6± 21.2
283.0± 8.8
276.6± 43.9
203.9± 9.1
291.8± 14.4
S°, J/mol·K*
*data on S were recalculated from Shareva et al. (2012), and calculated using data from a, Kubatko et al, (2006); b, Gorman-Lewis et al. (2008); c,
Kubatko et al, (2003); d, Kubatko et al, (2005); e, Gorman-Lewis et al. (2009); f, Shareva et al. (2012); g, Gorman-Lewis et al. (2007); h, Shareva et
al. (2011).
0
h
h
–2768.1 ± 6.5
–2022.7± 2.5 g
–117.8 ± 4.3 g
–2333.7 ± 4.6
e
Na(UO2)(SiO3OH)(H2O)
(UO2)(SiO4)1/2(H2O)
Soddyite (UO2)2(SiO4) ·2H2O
K-boltwoodite
K(UO2)(SiO3OH)·1.3H2O
Na-boltwoodite
Na(UO2)(SiO3OH)·H2O
Uranophane
Ca(UO2)2(SiO3OH)·5H2O
Uranyl silicates
–227.2 ± 2.3
e
e
–3072.3 ±4.8 e
–238.5 ± 6.0
UO2(PO4)2/3(H2O)4/3
(UO2)3(PO4)2 ·4H2O
–3223.2 ± 4.0 e
–241.0 ± 3.9 e
–497.9±5.0
–––––
–300.5± 23.9
–1674.3 ±4.1
–––
–605.8±5.0
–532.5±8.1
Sf, el, J/mol·K
–1635.1±23.4
–1717.6 ±4.4
–1632.2 ±7.4
Gf, el, kJ/mol
K(UO2)(SiO3OH)(H2O)
UO2HPO4(H2O)3
UO2HPO4 ·3H2O
–4431.6 ±15.3d
–989.3±14.0d
Uranyl phosphates
–5593.6 ± 9.1d
–710.4±9.1d
Na2Ca[(UO2)(CO3)3](H2O)5
K3NaUO2(CO3)3(H2O)
–1716.4 ± 4.2d
–99.1±4.2d
Uranyl carbonates
–75.7±4.1c
–2344.7 ± 4.0 c
–1645.4 ±4.3 a
–161.5±4.3 a
22.3±3.9c
–1822.7 ± 2.4 a
–1724.7 ± 5.1
–53.5±2.4 a
–150.6±4.9
UO2CO3
Grimselite K3NaUO2(CO3)3·H2O
Rutherfordine UO2CO3
Andersonite
Na2Ca[(UO2)(CO3)3]·5H2O
Studtite (UO2)O2·4H2O,
oxide + H2O2
(UO2)O2(H2O)4
Na0..34(UO2)O0.67(OH)(H2O)1.2
Na-Compreignacite
Na2[(UO2)3O2(OH)3]2·7H2O
Studtite (UO2)O2·4H2O,
oxide + H2O
Na(UO2)O(OH)
a
a
Clarkeite Na(UO2)O(OH)
–1898.2 ± 2.3 a
–44.6±2.2 a
Ca0.17(UO2) O0.67(OH) (H2O)1.3
–1536.2 ± 2.8
–26.6±2.8
-UO2(OH)2
a
-UO2(OH)2
Bequerelite
Ca[(UO2)3O2(OH)3]2·8H2O
a
–1791.0 ± 3.2 a
UO3(H2O)2
4.4±3.1a
Uranyl oxides hydrates and peroxides
Formula per one uranyl cation
MetaschoepiteUO3·2H2O
Phase, formula
TABLE 4-6. THERMODYNAMIC FUNCTIONS FOR FORMATION FROM OXIDES AND ELEMENTS OF URANYL MINERALS, UNDER STANDARD TEMPERATURE AND PRESSURE.
NAVROTSKY ET AL.
NAVROTSKY ET AL.
uranate structures.
The three basic types of alkali metal uranates,
categorized by metal to uranium ratio of 4, 2, and 1,
are M4UO5, M2UO4, and M2U2O7, respectively.
Table 4-7 gives Hof,ox, the enthalpies of formation
from oxides, for different types of alkali uranates at
room temperature. These data have been normalized
to per one uranyl unit per formula, analogous to the
approach by Shvareva et al. (2012) in order to be
able to compare the energetic trends among
different stoichiometries.
Contributions of acid–base interactions to
energetics of formation must be considered to
compare formation enthalpies within similar
structural types and among various classes of
uranates. These acid–base interactions are between
alkali and alkaline earth cations and "uranyl"
groups. Thus, the reported data are plotted as
functions of acidity of the alkali oxide, which can be
indexed in the Smith scale (Smith 1987). These
linear relationships shown in Figure 4-9 suggesting
that formation enthalpies from oxides directly
correlate with acidity of oxides, similar to data
hf, ox. kJ/mol
Alkali and Alkaline Earth Uranates
Though these are not minerals, uranates are
potentially of interest to the nuclear fuel cycle,
reactor accidents, and nuclear waste disposal. For
example, alkaline earth uranates are important
phases for incorporation of Ba and Sr as major
fission products into the U matrix (Sali et al. 2000).
Furthermore, uranates show clear thermodynamic
trends of interest from a fundamental chemical point
of view and that may guide predictions for more
complex mineral phases.
The alkali and alkaline earth uranates have
various stable compounds, reflecting different
stoichiometries and, in some cases, U oxidation
states. Griffiths & Volkovich (1999) have reviewed
the structures of known alkali metal uranates as
containing a modified uranyl group, [(UO2)O2]2– in
the case of monouranates, and [(UO2)O1.5]2– or
[(UO1.5)O2]2–in case of diuranates. In each of these
modifications, U is surrounded by two strongly
bonded axial "uranyl" O atoms and four equatorial
O atoms, less strongly bonded to U. Such
fundamental building blocks can be connected in
multiple ways to form planar layers or chains in
Figure 4-8. Observed linear experimental trends (uranyl silicates and hydroxide oxides) and predicted
from linearity data for uranyl hydroxide oxide as a function of cation oxide acidity in Smith scale
(Shvareva et al. 2012).
11
NAVROTSKY ET AL.
TABLE 4-7. ENTHALPY OF FORMATION OF ALKALI URANATES
Formula
Li2U2O7
Na2U2O7
K2U2O7
Li2UO4
Na2UO4
K2UO4
Rb2UO4
Cs2UO4
Li4UO5
Na4UO5
a
ΔHf, ox (kJ/mol)
a
–84.05±2.96
–170.70±2.16 a
–219.85±2.61 a
–146.50±2.60 a
–259.10±3.60 a
–333.70±3.14 a
–347.58±7.98 a
–358.55±2.62 a, b
–219.80±4.60 a
–403.20±1.97 a
ΔHf, el (kJ/mol)
–1606.80 ± 2.65
–1601.90 ± 2.00 a
–1625.25 ± 2.25 a
–1968.20 ± 1.30 a
–1897.70 ± 3.50 a
–1920.70 ± 2.20 a
–1922.70 ± 2.20 a
–1920.78 ± 2.24 a, b
–2639.40 ± 1.70 a
–2456.60 ± 1.70 a
ΔSf, ox (kJ/mol K)
–1505.73 ± 2.01 a
4.3 ± 5.8 d
–1853.19 ± 2.22 a
–1779.30 ± 3.51 a
–1798.50 ± 3.25 a
–1800.14 ± 3.25 a
–1790.07 ± 2.71 a, c
–0.8 ± 7.7 d
–5.2 ± 9.5 d
–10.2 ± 10.0 d
–26.4 ± 7.5 c
–25.9 ± 7.9 d
Simoncic & Navrotsky (2007); b Cordfunke et al. (1986); c O'Hare & Hoekstra (1974); d calculated values from
formation of enthalpy and free energy.
reported in Shvareva et al. (2012) and Le &
Navrotsky (2008). A different alkali to U ratio in
each type of uranate results in different slopes of the
energetic trends in Figure 4-9. The data listed in
Table 4-7 suggest that the higher alkali/U ratio leads
to more exothermic formation enthalpies from
oxides, Hof,ox, apparently due to the stronger
contribution of acid–base interactions.
Though MUO3 uranates(V) have been
categorized as perovskite-type structure with
specified crystallographic position for U in +5
oxidation state (Chippindale et al. 1989, Dickens &
Powell 1991), there were controversial results from
XANES (Allen et al. 1974, Van et al. 2002), in
which a doublet of U4f peaks is present indicating
the existence of mixed valence states. Lately several
attempts have been made to determine an oxidation
state by X-ray absorption analysis (Soldatov et al.
2007, Liu et al. 2009). Results suggest that only one
valence state (5+) presents in MUO3 species, while
the formation of doublet structure may be due to the
susceptible oxidation of U5+and can be removed by
carefully controlled etching process (Soldatov et al.
2007).
Formation of uranates (V) can be achieved by
reduction of M2UO4 or M2U2O7 at 450 to about
600oC (Griffiths & Volkovich 1999, Galkin et al.
1961, Hinatsu et al. 1992). Therefore, the enthalpies
of formation from binary oxides for
uranates(V)MUO3 are expected to be less
exothermic compared to those for mono- and
diuranates, as shown in Table 4-8 (Cordfunke &
Ouweltjes 1981). Thermodynamic data for the
alkaline earth uranates have been collected and
plotted in a similar manner, and the energetics
follow the trends found for alkali uranates, shown in
Table 4-9 and Figure 4-10.
Uranyl Peroxide Materials
Simple uranyl peroxide complexes contain a
single uranyl ion coordinated by as many as three
peroxide groups that, each being bidentate, define
the equatorial edges of hexagonal bipyramidal
coordination polyhedra (Alcock 1968). When
peroxide bridges uranyl ions, the configuration is
bent (Miro et al. 2010, Sigmon et al. 2009,
Vlaisavljevich et al. 2010) and nanoscale cage
clusters containing as many as 60 uranyl ions selfassemble in aqueous systems (Burns 2011). These
soluble clusters carry negative charges, are
associated with counterions in solution, and can be
crystallized. Under acidic conditions in deionized
water, the combination of uranyl and peroxide
causes the precipitation of studtite, [(UO2)(O2)
(H2O)2](H2O)2 (Burns & Hughes 2003). Thus there
3
7
4
hf, ox kJ/mol
ΔGf, el (kJ/mol)
a
5
Figure 4-9. Enthalpies of formation of selected alkali
uranates as a function of oxides acidity.
12
NAVROTSKY ET AL.
TABLE 4-8. ENTHALPY FORMATION OF MUO3 URANATES(V)
Hf, el (kJ/mol)
Gf, el (kJ/mol)
Hf, ox (kJ/mol)
LiUO3
–69.00 ± 2.18
–1522.3 ± 1.8
NaUO3
–133.15 ± 1.73
–1494.9 ± 1.6
–1412.50 ± 1.61
KUO3
–186.95 ± 2.10
–1522.9 ± 1.7
RbUO3
–190.89 ± 4.26
–1520.9 ± 1.8
* Calculated values from formation of enthalpy and free energy
Formula
Sf, ox (kJ/mol K)
–8.8 ± 4.6*
TABLE 4-9. ENTHALPY OF FORMATION OF ALKALINE EARTH URINATES AT 298 K
Formula
Hf, ox (kJ/mol)
Hf, el (kJ/mol)
Gf, el (kJ/mol)
a
a
a
Sf, ox (kJ/mol K)
MgUO4
CaUO4
SrUO4
BaUO4
–31.90 ± 1.73
–143.10 ± 2.67 b
–175.70 ± 3.08 a
–221.90 ± 3.99 a
–1857.3 ± 1.5
–2002.00 ± 2.53 b
–1990.8 ± 2.8 a
–1993.8 ± 3.3 a
–1749.6 ± 1.5
–1888.71 ± 2.42 a
–1881.36 ± 2.80 a
–1883.3 ± 3.4 a
8.9 ± 4.8 d
–11.8 ± 7.1 d
1.4 ± 8.1 d
–14.2 ± 10.6 d
Sr3UO6
Ba3UO6
–254.70 ± 3.75 c
–342.30 ± 10.21 a
–3252.4 ± 2.1 c
–3210.4 ± 8.0 a
–3044.95 ± 9.20 a
–14.4 ± 27.8 d
Simoncic & Navrotsky (2007); Kubatko et al. (2006); Cordfunke et al. (1999); d calculated values from
formation of enthalpy and free energy.
a
b
c
thermodynamic stability and requires the continued
presence of peroxide in solution to stabilize it
(Kubatko 2003) alkali uranyl peroxide formation
from the oxides features highly exothermic ΔHof,ox
values (Armstrong et al. 2012) and these phases
could remain thermodynamically stable even in the
absence of significant peroxide, perhaps even
forming spontaneously by reaction of uranyl
solutions with the O in air. Thus it is possible that
such alkali U peroxide clusters, either as nanophases
or as individual clusters in solution, can persist for
Figure 4-10. Enthalpies of formation of alkaline earth
uranates .
is a richness of uranyl peroxide cluster chemistry in
both aqueous solution and the solid state, the
thermodynamics of which has just begun to be
explored. Recently measured enthalpies of formation of a few of these uranyl peroxide solids
(normalized to one mole of U) are summarized in
Table 4-10 and shown as a function of chargebalancing cation ionic radius in Figure 4-11.
Insight can be gained by directly comparing
formation reactions under environmentally relevant
conditions, i.e., equilibria involving UO2(c),
aqueous systems, and pertinent secondary mineral
phases. In contrast to studtite, which has limited
FIGURE 4-11. Comparison of enthalpies of formation
from the oxides for studtite (Kubatko 2003), LiUT,
NaUT, KUT and the U60 nanocluster solids
(Armstrong et al. 2012), normalized per mole of U.
13
NAVROTSKY ET AL.
TABLE 4-10. SUMMARY OF THERMODYNAMIC DATA FOR URANYL PEROXIDE COMPOUNDS.
Formula
Li4[UO2(O2)3](H2O)10
Na4[UO2(O2)3](H2O)9
K4[UO2(O2)3](H2O)9
Li40K20[UO2(O2)(OH)]60(H2O)214
Hf, ox (kJ/mol)
–260.9 ± 13.2
–515.5 ± 8.9
–453.4 ± 9.3
–138.1 ± 5.0
Hf, el (kJ/mol)
–5540.2 ± 14.0
–5147.4 ± 9.9
–4975.8 ± 9.3
–2784.3 ± 5.1
Data refer to one mole of U (Burns & Hughes 2003, Armstrong et al. 2012)
long times and be transported away from the
uraninite or nuclear fuel from which they form by
oxidation under conditions of high salinity such as
seawater. This may form an additional pathway for
U transport in both the natural environment and in a
reactor accident. The cluster compounds are
metastable with respect to common uranyl mineral
phases discussed above but may represent longlived intermediates in their formation. More
research in the thermochemical stability of this large
class of uranyl peroxide cluster materials is under
way.
ALCOCK, N.W. (1968): Crystal and molecular
structures of sodium uranyl triperoxide, J. Chem.
Soc., A, 1588-1594.
ALLEN, G.C. & HOLMES, N.R. (1993): Mixed
valency behavior in some uranium oxides studies
by x-ray photoelectron spectroscopy. Can. J.
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ALLEN, G.C., CROFTS, J.A., CURTIS, M.T., TUCKER,
P.M., CHADWICK, D. & HAMPSON, P.J. (1974): Xray photoelectron spectroscopy of uranium oxide
phases. J. Chem. Soc., Dalton Trans.1296-1301.
ARMSTRONG, C.R., NYMAN, M., SHVAREVA, T.,
SIGMON, G.E., BURNS, P.C. & NAVROTSKY, A.
(2012): Uranyl peroxide enhanced nuclear fuel
corrosion in seawater. Proc. Natl. Acad. Sci.
U.S.A. 109, 1874-1877, S1874/1871-S1874/1875.
BEALL, G.W., BOATNER, L.A., MULLICA, D.F. &
MILLIGAN, W.O. (1981): The structure of cerium
orthophosphate, a synthetic analog of monazite. J
Inorg. Nuclear Chem. 43, 101-105.
BENZ, R. (1969): Thorium-thorium dioxide phase
equilibriums. J. Nucl. Mater. 29, 43-49.
BESSONOV, A.F. (1970): High-temperature x-ray
diffraction study of uranium trioxide,
Kristallografiya 15, 62-67.
BURNS, P.C. & HUGHES, K.-A. (2003): Studtite,
[(UO2)(O2)(H2O)2](H2O)2: The first structure of a
peroxide mineral, Amer. Mineral. 88, 1165-1168.
BURNS, P.C. (2011): Nanoscale uranium-based cage
clusters inspired by uranium mineralogy. Mineral.
Mag. 75, 1-25.
BUYUKKILIC, S. SHVAREVA, T. & NAVROTSKY, A.
(2012): Enthalpies of formation and insights into
defect association in ceria singly and doubly
doped with neodymia and samaria. Solid State
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CHEN, F., EWING, R.C. & CLARK, S.B. (1999): The
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Concluding Remarks
This review has attempted to summarize the
current state of thermochemical data for uranium
(and thorium) minerals and related materials, with
an emphasis on the structural and bonding factors
which determine the systematic trends seen in
thermodynamic properties. Although knowledge of
these properties has improved substantially in the
past decade, much work remains to be done,
especially in the study of complex phases and solid
solutions. To apply these thermochemical data for
solids to calculate equilibria with aqueous fluids and
silicate melts, the speciation and thermodynamics of
actinides in the latter phases must be better
constrained, especially at high temperature.
ACKNOWLEDGEMENTS
Much of the work presented here is based upon
work supported as part of Materials Science of
Actinides, an Energy Frontier Research Center
funded by the US Department of Energy, Office of
Science, Office of Basic Energy Sciences under
Award Number DE-SC0001089.
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