American Mineralogist, Volume 90, pages 1308–1314, 2005

Dehydration processes in the meta-autunite group minerals meta-autunite, metasaléeite,
and metatorbernite
YOHEY SUZUKI,1 TSUTOMU SATO,2 HIROSHI ISOBE,3 TOSHIHIRO KOGURE,4
AND TAKASHI MURAKAMI4,*
1

Frontier Research System for Extremophiles, Japan Marine Science and Technology Center, 2-15, Natsushima-cho, Yokosuka 237-006, Japan
2
Institute of Nature and Environmental Technology, Kanazawa University, Kanazawa, Ishikawa 920-1192, Japan3
Department of Earth Science, Kumamoto University, Kumamoto, Kumamoto 860-8555, Japan
4
Department of Earth and Planetary Science, the University of Tokyo, Hongo, Tokyo 113-0033, Japan

ABSTRACT
We investigated dehydration processes in uranyl phosphate minerals of the meta-autunite groupthat consist of uranyl phosphate sheets and interlayer cations, and water molecules. Meta-autunite[Ca(UO2PO4)2⋅6H2O], metasaléeite [Mg(UO2PO4)2⋅8H2O], and metatorbernite [Cu(UO2PO4)2⋅8H2O]
were selected for our study. The change in basal spacing between two adjacent uranyl phosphate sheetswith temperature was examined by temperature-controlled X-ray diffraction (XRD) analysis from room
temperature (RT) to 300 °C to determine structurally distinct, dehydrated phases. Thermogravimetricand differential thermal analyses (TG/DTA) were also performed under conditions similar to thoseused for the XRD analysis to clarify the hydration states of the dehydrated phases. Retention of thestructure of the uranyl phosphate sheets under a high vacuum, equivalent to 300 °C, was conÞrmed bytransmission electron microscopy. Meta-autunite, metasaléeite, and metatorbernite decreased their basal
spacings by losing water molecules. Comparison of the TG/DTA and XRD results indicates that thechanges in basal spacings of the dehydrated phases with temperature are as follows: 8.32 Å (6 H2O
per unit formula) at RT, 7.31 (? H2O) and 6.68 Å (? H2O) at 75 °C, 6.34 Å (2 H2O) at 120 °C, and 5.81
Å (1 H2O) at 300 °C for meta-autunite; 8.29 (8 H2O) and 7.73 Å (? H2O) at RT, 6.62 Å (probably, 2H2O) at 40 °C, 6.54 Å (2 H2O) at 160 °C, and 5.52 Å (1 H2O) at 300 °C for metasaléeite; and 8.61 Å(8 H2O) at RT, 8.07 Å (4 H2O) at 100 °C, 6.58 Å (2 H2O) at 200 °C, and 5.60 Å (1 H2O) at 300 °C for
metatorbernite. The dehydration processes revealed by XRD and TG/DTA under similar experimental
conditions are slightly different from those obtained by previous studies. Our results clearly demonstrate
the presence of previously unknown dehydrated phases of the meta-autunite group minerals with basal
spacings less than 6 Å that may have distinct thermodynamic properties.

INTRODUCTION
Uranium mobility in near-surface settings is mainly controlled
by redox transformations between soluble hexavalent uranium[U(VI)] and insoluble tetravalent uranium [U(IV)] (Langmuir
1978; Klinkhammer and Palmer 1991; Suzuki et al. 2002, 2003,
2004a, 2004b). Reduction of U(VI) to U(IV) that occurs under
reducing conditions leads to precipitation of U(IV)-bearing minerals such as uraninite, UO2+x. Under oxidizing conditions, oxidative dissolution of U(IV)-bearing minerals results in mobilizationof uranium due to formation of uranyl carbonate complexes(Langmuir 1978). In a wide range of uranium deposits, uranyl
ions released from U(IV)-bearing minerals in primary orebodies
are immobilized in insoluble uranyl phosphate minerals that persist for geological periods (Leo 1960; Snelling 1980). Nanocrystalline uranyl phosphate minerals that form during crystallizationof iron oxides have been suggested to be ubiquitous (Murakami
et al. 1997; Sato et al. 1997). Thus, uranyl phosphate minerals
play important roles in long-term immobilization of uranium.
To remediate contaminated groundwaters, sediments, and soils,* E-mail: murakami@eps.s.u-tokyo.ac.jp0003-004X/05/0809–1308$05.00/DOI: 10.2138/am.2005.1568

technologies that enhance formation of uranyl phosphate minerals have recently drawn attention. For example, reactive barriers
that use apatite [Ca5(PO4)3(F,OH)] to promote uranyl phosphate
precipitation have been studied to suppress uranium transport ingroundwaters (Fuller et al. 2002). In addition, micro-organisms
capable of precipitating uranyl phosphate minerals (Macaskie
et al. 2000; Suzuki and BanÞeld 1999, 2004) have potential to
be used for immobilization of uranium.
Uranyl phosphate minerals are diverse and occur in Þve major
structural divisions, of which the autunite, meta-autunite, and thephosphuranylite groups are the most important (Finch and Murakami 1999). The autunite and meta-autunite group minerals are
more important than the phosphuranylite group minerals in termsof natural abundances because of their low solubilities (Grenthe etal. 1992). The autunite and meta-autunite group minerals possessuranyl phosphate or arsenate sheets that are connected by interlayer
cations and water molecules. Minerals in the autunite and metaautunite groups are easily dehydrated by heating, desiccation, andevacuation. Thermal analysis of these minerals has been extensively
conducted (Vochten et al. 1979; Vochten and Deliens 1980; Vochten
and Van Doorselaer 1984; Weigel and Hoffman 1976). In contrast,
few studies on structural changes associated with dehydration have

1308

SUZUKI ET AL.: DEHYDRATION OF URANYL PHOSPHATE MINERALS

been performed. Butt and Graham (1981) reported dehydrated phases that are structurally distinct from previously identiÞed phases
by X-ray diffraction (XRD) analysis. Sverjensky (1992) indicatedthat the thermodynamic properties of the autunite and meta-autunitegroup minerals vary among the different dehydrated phases, andimplied that the determination of hydration states is important. In
the present study, the structural changes during dehydration caused
by increasing temperature were monitored by a powder XRD device
equipped with a temperature-controlling system to reveal structurally distinct, dehydrated phases. A structural change due to thehigh vacuum of a transmission electron microscope (TEM) was
also analyzed based on selected area electron diffraction (SAED)
patterns. Thermal analysis was conducted under conditions as comparable to those used for the XRD analysis as possible to clarify
the hydration states of the dehydrated phases.

CRYSTAL CHEMISTRY OF META-AUTUNITE,METASALÉEITE, AND METATORBERNITE
The members of the autunite and meta-autunite groups have ageneral composition of A(UO2PO4)2⋅nH2O or A(UO2AsO4)2⋅nH2O,
where A represents a divalent cation, two monovalent cations, orsome other cation(s) (A = Ca, Ba, Mg, Sr, Cu, Pb, Fe, Mn, Co, Ni,
1/2HAl, 2Na, 2K, 2NH4, or 2H) and n refers to the hydration number(Beintema 1938). For uranyl phosphate minerals of the autunite andmeta-autunite groups, a uranyl phosphate sheet consists of corner6–
shared tetrahedra of PO3–
4 and square bipyramids of UO6 (Burns
et al. 1997) (Fig. 1a). Three members of the meta-autunite group,
meta-autunite, metasaléeite, and metatorbernite were chosen for thisstudy because they are often found at uranium ore deposits (Leo1960; Snelling 1980). Dehydrated phases have been proposed forthe three minerals. For example, four hydrated phases have been
identiÞed for the calcium-bearing member of the autunite andmeta-autunite groups including autunite, a fully hydrated phase,and meta-autunite I, Ia, and II (“meta” means partially hydrated)(Frondel 1958; Sowder et al. 2000; Takano 1961). Unless otherwise
stated, we refer to meta-autunite I as meta-autunite. Meta-autunite
Ia and II have been revealed by XRD analysis at room temperature(RT) after heating to 22–50 °C and 65–150 °C, respectively.
Although the structure of autunite has recently been reÞned
(Locock and Burns 2003a), that of meta-autunite has not yet been
determined satisfactorily. Vochten and Deliens (1980) showed thatmeta-autunite has a tetragonal subcell with Ca(UO2PO4)2⋅7H2O anda = 6.981 and c = 8.448 Å while Makarov and Ivanov (1960) indicated Ca(UO2PO4)2⋅6H2O for the chemical composition with similarcell dimensions. Takano (1961) suggested that meta-autunite II isorthorhombic. Meta-autunite Ia has a c dimension of ∼9 Å (Sowder
et al. 2000). Saléeite, Mg(UO2PO4)2⋅10H2O, is monoclinic. P21/c,with β = 135.17°, a = 6.951, b = 19.947, and c = 9.896 Å, and Z =2 (Miller and Taylor 1986). The unit cell of saléeite can be recast
into a pseudotetragonal setting. Metasaléeite has not been widely
accepted as a separate mineral species. The name metasaléeite
was proposed for the lower hydrate of saléeite by Mrose (1950)
who described material from the Democratic Republic of Congo
with chemical formula Mg[(UO2)(PO4)]2⋅8H2O. Powder diffraction data for metasaléeite from Brazil were given by Cassedanne
et al. (1986), PDF 41-1389, and metasaléeite has been found incentral Portugal (Pinto et al. 2001). The structure of metasaléeite is currently unknown. Metatorbernite, Cu(UO2PO4)2⋅8H2O, is1309

tetragonal, P4/n, with a = b = 6.975 Å, c = 17.416 Å, and Z = 2(Locock and Burns 2003b). The structural model of metatorbernite
according to Locock and Burns (2003b) is shown in Figures 1aand b. The interlayer Cu2+ of metatorbernite is coordinated by twouranyl O atoms and by four water molecules, forming a squarebipyramid as shown in Figure 1b.

EXPERIMENTAL PROCEDURE
The meta-autunite, metatorbernite, and metasaléeite samples were from Cunha
Baxia, Portugal, Shinkolobwe, Congo, and Koongarra, Australia, respectively.
The chemical states of the water molecules in meta-autunite, metatorbernite, andmetasaléeite at RT were analyzed by infrared spectroscopy (IR) for each sample in a
KBr pellet under ambient conditions covering the range from 4000 to 400 cm–1.
Differential thermal analysis (DTA) and thermogravimetric analysis (TG)were carried out with a constant N2 ßow of 100 mL/min. To begin with, a rate ofheating of 5 °C/min for the temperature range from RT to 800 °C was applied todetermine the number of water molecules originally contained in the three samples.
Calcium sulfate dihydrate (CaSO4⋅2H2O) and magnesium sulfate heptahydrate(MgSO4⋅7H2O) were used for TG standards, and showed that the measurement
errors were within ±5%. A heating rate of 2.5 °C/min to 500 °C was then used withanother set of three unheated samples to reveal details of the dehydration processes.
For temperature-controlled XRD analysis, another set of three unheated samples
was used. The samples were mounted evenly on copper slides. The samples were
exposed to a constant N2 ßow of 100 mL/min for 10 min. Then, under the same N2

FIGURE 1. Crystal structure of metatorbernite according to Locock
and Burns (2003b).The dashed line indicates a unit cell. A uranyl
phosphate sheet consisting of corner-shared tetrahedra of PO3–
4 (light
gray in color) and square bipyramids of UO6–
6 (black) is shown. The
metatorbernite structure is viewed along [001] (a) and along [100] (b).
Cu2+ ion is coordinated by two uranyl O atoms and by four water molecules,
forming a square bipyramid (light gray in Fig. 1b). Water molecules notcoordinating to Cu2+ ion are shown as gray circles.

1310

SUZUKI ET AL.: DEHYDRATION OF URANYL PHOSPHATE MINERALSßow, the temperature was increased to 300 °C at a rate of 2.5 °C/min. XRD patterns
were obtained using a scan rate of 6 º/min at a Þxed temperature with monochromatized CuKα radiation at 40 kV and 30 mA. The experimental errors of the d-spacings
determined by XRD were at most 0.04 Å.
To examine changes in the cell parameters under high vacuum, meta-autunite wasobserved and analyzed using a TEM equipped with energy-dispersive spectroscopy(EDS). The TEM specimens were prepared as follows: platy crystals of meta-autunite
were embedded with epoxy resin and squeezed between two glass slides to orient c*
normal to the glass slides. The glass slides were sliced parallel to c*, and polishedmechanically, and some of the slices were thinned to electron transparency by Ar ion
milling and were then carbon-coated. The TEM was operated at 200 kV. SAED pattern analysis was conducted for the specimens. Chemical compositions of the threeminerals were checked by EDS analysis (data deposited).

RESULTS AND DISCUSSION
Chemical states of interlayer water molecules in metaautunite, metasaléeite, and metatorbernite
The IR spectra of metatorbernite, meta-autunite, andmetasaléeite in the region of 4000 to 2500 cm–1 in which theabsorption bands are attributed to the O-H stretching vibration
are shown in Figures 2a, b, and c, respectively. There are two
distinct bands at around 3350 and 2900 cm–1 in the spectrum of
metatorbernite, while there is a single broad band at around 3400
cm–1 in the spectra of meta-autunite and metasaléeite. The O-H
stretching frequency depends on the strength of the hydrogenbond; an increase in the strength of the hydrogen bond results in
an increase in the O-H stretching frequency. In the structure ofmetatorbernite there are two kinds of water molecules: one coordinates to Cu2+ and the other does not (Locock and Burns 2003b).
Because the bond strength between Cu2+ and the oxygen atom ofa water molecule is strong due to its covalent bonding character,the strength of the hydrogen bond becomes weak. Therefore
the O-H stretching band of the water molecules coordinating toCu2+ is shifted toward a lower frequency, resulting in two broad
bands in the region of 3700 to 2700 cm–1. On the contrary, there
is one broad band for metasaléeite and meta-autunite, which
may indicate that the bond strength of the interlayer Cu2+ to theoxygen atom of a water molecule is stronger than those of Ca2+and Mg2+. Differences in the IR spectra other than in the region

FIGURE 2. Infrared spectra of (a) metatorbernite, (b) meta-autunite, and(c) metasaléeite in the region of 4000 to 2500 cm–1 in which absorption
bands are attributed to the O-H stretching vibrationof 4000 to 2500 cm–1 have been described to be minor (Suzuki
et al. 1998).
Changes in hydration states during dehydration
TG and DTA curves for meta-autunite, metasaléeite, andmetatorbernite in the temperature range from RT to 500 °C areshown in Figures 3a, b, and c, respectively. TG analysis up to 800°C revealed that meta-autunite, metasaléeite, and metatorbernite
contain 5.7 ± 0.3, 7.5 ± 0.4, and 8.0 ± 0.4 moles of water per unit formula, respectively. Unless otherwise stated, the hydration numberof n water molecules with an error of 0.m water molecules per unit
formula is hereafter referred to as n(m) H2O, e.g., 5.7(3) H2O.
The DTA curve of meta-autunite shows three endothermic
peaks at 29, 61, and 108 °C (shown by arrows in Fig. 3a). In the
TG curve, the hydration state of meta-autunite changes to 5.0(3),3.2(2), and 2.0(1) H2O according to the three water-loss events,
respectively. After the third water-loss event at 108 °C, dehydration proceeds in a continuous manner. The DTA and TG curves ofmetasaléeite show two endothermic peaks at 30 and 67 °C (arrows
in Fig. 3b) indicating the presence of hydration states with 5.4(3)and 3.2(2) H2O, respectively. Above 67 °C, dehydration proceeds
continuously into an anhydrous state. The DTA and TG curvesof metatorbernite show two hydrate states of 6.2(3) and 4.4(2)H2O corresponding to the two endothermic peaks at 70 and 86 °C,
respectively (Fig. 3c).
Changes in structures during dehydration
To obtain more information about the dehydration processes,
temperature-controlled XRD analysis was performed under condi-

FIGURE 3. TG and DTA curves of (a) meta-autunite, (b) metasaléeite,and (c) metatorbernite measured from room temperature to 500 °C witha constant N2 ßow of 100 mL/min. The heating rate was 2.5 °C/min.
Arrows show endothermic peaks.

SUZUKI ET AL.: DEHYDRATION OF URANYL PHOSPHATE MINERALS

tions similar to those for the thermal analysis. Representative XRD
patterns of meta-autunite, metasaléeite, and metatorbernite in thetemperature range RT to 300 °C are shown in Figures 4a, b, and c,
respectively. Only the XRD patterns between 7.5 and 17.5û 2θ appear in Figure 4 because we only observed signiÞcant peaks in the2θ-angle range. Because the three minerals have layered structures
whose sheets are inferred to be parallel {001}, the 00l reßections
are enhanced in powder diffraction patterns. The spacing between
two adjacent uranyl phosphate sheets is referred to as the basal
spacing hereafter for convenience.
The XRD pattern of meta-autunite at RT shows a reßection at
8.32(3) Å. Because Vochten and Deliens (1980) and Sowder et al.
(2000) have reported that a reßection at ∼8.4 Å corresponds to thebasal spacing of meta-autunite, we infer that the 8.32 Å spacingis the basal spacing. While two major reßections at 7.31(2) and6.68(2) Å were observed in the XRD pattern at 75 °C, there wasonly one reßection at 6.34(2) Å at 120 °C. Butt and Graham (1981)
assumed that the meta-autunite reßections at 6.50 and 7.27–7.50
Å observed at 90–120 °C correspond to the basal spacings. At 300°C, a reßection was obtained at 5.81(2) Å.
A high vacuum induces dehydration of the autunite and

FIGURE 4. Temperature-controlled XRD patterns of (a) meta-autunite,(b) metasaléeite, and (c) metatorbernite observed in the temperature range
from room temperature to 300 °C with a constant N2 ßow of 100 mL/
min. The temperature was increased to 300 °C at a rate of 2.5 °C/min.
The XRD patterns are only shown between 7.5 and 17.5° 2θ becausesigniÞcant peaks were observed only in this range. Experimental errors
were less than 0.04 Å.

1311

meta-autunite group minerals (Butt and Graham 1981). TEM
specimens in which the c* axis of meta-autunite was oriented
approximately normal to an electron beam were characterized
to examine changes in the cell parameters under a high vacuumof TEM. A typical SAED pattern for meta-autunite is shown in
Figure 5. Although the reßections at ∼6 Å along c* make one
diffraction spot (between the center and the 3.2 Å spot in Fig.
5), the higher-order reßections are resolved at 2.9 and 3.2 Å.
The higher-order reßections at 2.9 and 3.2 Å clearly indicatethat there are dehydrated phases with basal spacings of 5.8 (=2.9 × 2) and 6.4 (= 3.2 × 2) Å under high vacuum. The 5.8 and6.4 Å phases correspond to those with basal spacings of 5.81 Å
at 300 °C and of 6.34 Å at 120 °C observed by XRD (Fig. 4a).
Because the reßection at 2.9 Å is much stronger than that at 3.2
Å, meta-autunite mainly transformed into the phase with a basal
spacing of 5.8 Å rather than that with a basal spacing of 6.4 Å.
Thus, the reßections obtained by XRD analysis were all assigned
to the basal spacings of meta-autunite.
The SAED pattern also shows that the periodicity along[110] is 4.9 Å, indicating that the a- and b-dimension of ∼6.9 Åis maintained under high vacuum. That is, the structure of theuranyl phosphate sheets remains unchanged in the dehydrated
phase with a basal spacing of 5.81 Å at 300 °C, which is consistent
with the suggestion that the destruction of the structure of metaautunite occurs at 600 °C (Cejka 1999).
At RT, a dominant reßection at 8.29(3) Å and a minor reßection
at 7.73(3) Å were obtained for metasaléeite (Fig. 4b). At 40 °C there
were three reßections at 8.23(3), 7.71(3), and 6.62(2) Å. A high peakat 6.54(2) Å was observed at 160 °C. Two reßections at 6.43(2) and5.52(1) Å are shown in the XRD pattern at 300 °C whereas only
one reßection was obtained for meta-autunite at 300 °C. We inferthat as in the case of meta-autunite, the reßections in Figure 4b are
attributed to the basal spacings of metasaléeite during dehydration
except for one at 5.52 Å that will be discussed below.

–
FIGURE 5. [110] SAED pattern of meta-autunite. The specimen was
prepared to orient the c* axis normal to the electron beam.

1312

SUZUKI ET AL.: DEHYDRATION OF URANYL PHOSPHATE MINERALS

Metatorbernite has a major reßection at 8.61(3) Å and a minor
reßection at 5.43(1) Å at RT (Fig. 4c). The 8.61 and 5.43 Å spacingsare consistent with the 002 and 102 reßections of metatorbernite,
respectively (Locock and Burns 2003b). At 100 and 200 °C, reßection peaks at 8.07(3) and 6.58(2) Å were obtained, respectively.
There were two reßection peaks at 6.52(2) and 5.60(1) Å at 300°C. These reßections, except those at 5.43 Å at RT and 5.60 Å
at 300 °C, are inferred to correspond to the basal spacings ofmetatorbernite.
We will now discuss whether or not the reßections at 5.52
and 5.60 Å for metasaléeite and metatorbernite, respectively, at
300 °C correspond to basal spacings. The reßections with basal
spacings of ∼6.5 Å occurring at 160 and 200 °C for metasaléeite
and metatorbernite, respectively, are slightly shifted and lose intensity at 300 °C when the reßections at ∼5.5 Å appear (Figs.
4b and 4c). This suggests that the dehydrated phases with basal
spacings of ∼6.5 Å lose their domains and other phases that havespacings of ∼5.5 Å grow at 300 °C. Although the 00l reßections
are enhanced, the peaks at ∼5.5 Å show broadening similar to
the meta-autunite peak at 5.81 Å (Fig. 4) that was conÞrmed to
be a basal spacing by TEM (Fig. 5). We may occasionally havehkl reßections even for oriented samples as shown by the 102
reßection of metatorbernite at RT (Fig. 4c). Because the TEM
observations revealed that the structure of the uranyl phosphatesheets is retained, we can roughly calculate the d102-spacing at 300°C. The maximum d102-spacing at 300 °C is calculated to be ∼4.8
Å if 6.6 Å, a basal spacing of metatorbernite at 200 °C (Fig. 4c),
is a maximum basal spacing at 300 °C. The value of ∼4.8 Å issigniÞcantly smaller than the value of ∼5.5 Å appearing at 300°C. The above consideration leads us to the interpretation that themetasaléeite and metatorbernite reßections at 5.52 and 5.60 Å,
respectively, at 300 °C correspond to basal spacings.
Hydration of structurally distinct phases occurring during
dehydration
In this section, we attempt to clarify the hydration states ofstructurally distinct phases occurring during dehydration by combining the results of the XRD and thermal analyses. First, it should
be noted that there is a limit to accurate comparison between the
XRD and TG/DTA results because perfectly identical experimental
conditions were impossible to produce in the different instruments.
In addition, a few stages of dehydration proceed simultaneously,that is, a few dehydrated phases co-exist at a given temperature(Fig. 4) although TG/DTA analysis provides one hydration state at agiven temperature. Therefore, hydration numbers of the structurally
distinct phases were carefully estimated from the results obtained by
TG/DTA. The basal spacings revealed by XRD and the corresponding hydration numbers by TG/DTA are tabulated in Table 1, where
XRD data from some previous studies are also given.
We will compare the hydration numbers of the structurally distinct phases for the present study to those from previous studies
below. However, most previous studies presented either XRD or
TG/DTA data but not both. In addition, because the experimental
conditions are different, changes in basal spacing and hydration statewith temperature for the present study do not precisely correspondto those for the previous studies. Therefore, the comparison will
be carefully discussed.
Meta-autunite. At RT, meta-autunite has a basal spacing of

8.32 Å (Fig. 4a) and a hydration state of 5.7(3) H2O (Fig. 3a), close
to 6 H2O. We can interpret that meta-autunite with a basal spacing of 8.32 Å has 6 H2O. However, the value of 8.32 Å is slightly
smaller than that of ∼8.4 Å given by Vochten and Deliens (1980)
and Sowder et al. (2000), and a hydration number of 6 to 7 H2O
is suggested for meta-autunite (Sowder et al. 2000).
There are two dehydrated phases with basal spacings of 7.31
and 6.68 Å at 75 °C (Fig. 4a). The DTA curve shows a water-loss
event at 61 °C that corresponds to a hydration state of 3.2(2) H2O
by TG (Fig. 3a). Because meta-autunite has two dehydrated phasesthat do not occur at other temperatures and the TG curve is steep
at around 60 °C, we failed to determine the hydration numbers ofthe dehydrated phases at 7.31 and 6.68 Å at 75 °C.
The dehydrated phase with a basal spacing of 6.34 Å at 120°C was observed by XRD analysis (Fig. 4a). The thermal analysis
shows an endothermic peak at 108 °C corresponding to the presence of a hydration number of 2.0(1) H2O (Fig. 3a). Because theendothermic peak ends at ∼120 °C, the dehydrated phase with abasal spacing of 6.34 Å is formed by this water-loss event. Thus,
the hydration state of the dehydrated phase with a basal spacing of
6.34 Å at 120 °C is 2 H2O.
Meta-autunite has a dehydrated phase with a basal spacing of
5.81 Å at 300 °C (Fig. 4a), which was conÞrmed by SAED (Fig.
5). The TG curve of meta-autunite shows 1.0(1) H2O for the hydration number at 300 °C and a continuous decrease from 300 to
500 °C (Fig. 3a). We therefore interpret that the dehydrated phase
contains 1 H2O.
The basal spacings of meta-autunite revealed by XRD for thepresent study are in good agreement with those of Butt and Graham(1981): 7.31 and 6.68 Å at 75 °C correspond to the 7.50 and 6.54
Å at 90 °C of Butt and Graham (1981), and 6.34 Å at 120 °C tothe 6.50 Å at 120 °C of Butt and Graham (1981) as shown in Table
1. This agreement suggests that the ∼7.4 and ∼6.5 Å dehydrated
phases are relatively stable. We did not observe meta-autunite Ia or
II (Takano 1961; Vochten and Deliens 1980; Sowder et al. 2000)
in the present study. The TG data of Sowder et al. (2000) suggest
the presence of dehydrated phases with 2, 1, and 0 H2O at ∼90,
∼160, and ∼240 °C, respectively. In contrast, our results show thatthe dehydrated phases with 2 and 1 H2O occur at 120 and 300 °C,
respectively, and lose water continuously up to 500 °C (Fig. 3a).
Metasaléeite. Metasaléeite has a basal spacing of 8.29 Å at
RT (Fig. 4b), and the thermal analysis shows the hydration stateis 7.5(4) H2O (Fig. 3b) at RT. Because there is another phase witha smaller basal spacing, 7.73 Å, and with much less intensity at
RT (Fig. 4b), the phase with a basal spacing of 8.29 Å probably
has 8 H2O. In contrast, Frost and Weier (2004) suggested 7 H2O
for the hydration state of metasaléeite. We failed to determine thehydration state of the phase with a basal spacing of ∼7.7 Å that
also appears at 40 °C (Fig. 4b).
There are three peaks at 8.23, 7.71, and 6.62 Å at 40 °C (Fig.
4b). We interpret that the dehydrated phase at 8.23 Å is a remnantof the phase at 8.29 Å at RT because of the decrease in intensity
and slight peak-shift with increase in temperature. Similarly, thedehydrated phase at 6.62 Å is possibly a precursor of the phase at
6.54 Å at 160 °C (Fig. 4b). The hydration state of the phase witha basal spacing of ∼7.7 Å is not known.
At 160 °C the dehydrated phase at 6.54 Å has a single andrelatively strong peak (Fig. 4b). The TG curve shows that the cor-

SUZUKI ET AL.: DEHYDRATION OF URANYL PHOSPHATE MINERALSTABLE 1.
T (°C)

Basal spacings and estimated hydration states of dehydrated
phases of meta-autunite, metasaléeite, and metatorbernite
meta-autunite
B. S.*
H. S.† B. S.(R)‡
8.32(3)6
9.02§
8.74||
8.47**
8.41#
8.43§
8.16**
8.10§

metasaléeite
B. S.
H. S.
8.29(3)8
7.73(3)?

metatorbernite
B. S.
H. S.
8.61(3)
8

RT
RT
RT
RT
RT
RT
RT
40
8.23(3)8
7.71(3)?
40
8.49||
40
6.62(2)
2?
60
8.41||
75
7.31(2)?
75
6.68(2)?
90
7.50||
90
6.54||
100
8.07(3)4120
6.34(2)
2
7.27||
120
6.50||
160
6.54(2)2
200
6.58(2)2300
5.81(2)
1
6.43(2)2?
6.52(2)2?
300
5.52(1)1
5.60(1)
1* Basal spacing (angstroms in unit) with error in parentheses as revealed by XRD
analysis in the present study.
† Hydration state of meta-autunite, metasaléeite or metatorbernite during
dehydration. The hydration number of n water molecules with the uncertainty ofm water molecules per unit formula is designated as n(m) H2O.
‡ Basal spacings of dehydrated phases reported in the references below.
§ Data from Sowder et al. (2000). The 9.02 Å phase is meta-autunite Ia at RT
cooled from up to 50 °C, the 8.43 Å phase meta-autunite I at RT from up to 70°C, and the 8.10 Å phase meta-autunite II at RT from up to 150 °C.
|| Data from Butt and Graham (1981).
# Data from Vochten and Deliens (1980).
** Data from Takano et al. (1961). The 8.47 Å phase is meta-autunite I at RT,
dried after being soaked in water, and the 8.16 Å phase meta-autunite II at RT
cooled from 110 °C.

responding hydration state is 2.5(1) H2O (Fig. 3b). Consequently,
the dehydrated phase contains 2 or 3 H2O. As discussed below, some
portion of the dehydrated phase at 6.54 Å at 160 °C persists at 300°C together with a further dehydrated phase at 5.52 Å (Fig. 4b),and the hydration state is ∼1 H2O at 300 °C (Fig. 3b). In addition,
the dehydrated phases Å of meta-autunite and metatorbernite withbasal spacings of 6.3–6.6 have 2 H2O (Table 1). We infer that thehydration state of the dehydrated phase at 6.54 Å is 2 H2O rather
than 3 H2O.
There are two dehydrated phases with basal spacings of 6.43
and 5.52 Å at 300 °C for metasaléeite (Fig. 4b). The dehydrated
phase at 6.43 Å is a remnant of the phase at 6.54 Å at 160 °C
because of the decrease in intensity and slight peak-shift withincrease in temperature. The TG curve shows that the hydration
state is slightly higher than 1 H2O at 300 °C (Fig. 3b) where thedehydrated phases at 6.43 and 5.52 Å occur. Therefore, the dehydrated phase at 5.52 Å has either 1 or 0 H2O. A recent study ofmetasaléeite revealed that a dehydrated phase with 2 H2O loses 1
H2O at 231 °C, and loses 1 H2O at 376 °C to become anhydrous(Frost and Weier 2004). In addition to the results of Frost and Weier(2004), the TG curve shows that metasaléeite loses water continuously from 300 to 500 °C. We interpret that the dehydrated phase
at 5.52 Å contains 1 H2O.
Frost and Weier (2004) indicated by thermal analysis that saléeite (10 H2O) dehydrates into 7, 4, 2 and 1 H2O phases at ∼40, ∼70,
∼90, and ∼230 °C, respectively, and Þnally becomes anhydrousat ∼380 °C. We observed dehydrated phases of metasaléeite with1313

2 H2O at 160 °C and with 1 H2O at 300 °C. The two results are
consistent in terms of the dehydration process, but not in terms ofthe temperatures of formation. The presence of a dehydrated phasewith 2 H2O at 193 °C shown by Vochten and Van Doorselaer (1984)
is consistent with our results.
Metatorbernite. Our metatorbernite has a basal spacing of 8.61
Å and 8.0(4) H2O at RT (Figs. 4c and 3c, respectively), which is ingood agreement with the data of Locock and Burns (2003b).
Metatorbernite has a dehydrated phase with a distinct peak at
8.07 Å at 100 °C (Fig. 4c). The DTA curve shows an endothermic peak at 86 °C and the hydration number revealed by TG is
4.4(2) H2O at 100 °C where the endothermic peak ends (Fig. 3c).
Hence, the dehydrated phase with a basal spacing of 8.07 Å isformed by this water-loss event, and contains 4 or 5 H2O. In the
structure of metatorbernite, water molecules coordinate to Cu2+
more strongly than those to Ca2+ and Mg2+ as discussed above
and the four water molecules coordinating to Cu2+ (Fig. 1b) areconsidered to be retained during dehydration. This inference issupported by the high intensity of the peak at 8.07 Å at 100 °C.
Therefore, the dehydrated phase with a basal spacing of 8.07 Å
most probably contains 4 H2O.
The dehydrated phase of metatorbernite with a basal spacingof 6.58 Å at 200 °C was observed by XRD analysis (Fig. 4c). The
intensity of the 6.6 Å peak is not so strong as those at 8.61 or 8.07
Å, but is signiÞcantly strong. The TG data shows that a dehydrated
phase contains 2.1(1) H2O at 200 ûC (Fig. 3c). We infer that thehydration state of the dehydrated phase is 2 H2O.
There are dehydrated phases with basal spacings of 6.52 and5.60 Å at 300 °C for metatorbernite (Fig. 4c). We interpret that thedehydrated phase at 6.52 Å is a remnant of the phase at 6.58 Å at
200 °C because of the decrease in intensity and slight peak-shift
with increase in temperature. Therefore, the dehydrated phase at∼6.5 Å probably has 2 H2O. At 300 °C, the TG curve shows thatthe hydration state is slightly higher than 1 H2O (Fig. 3c), which
may be due to co-existence of the 6.52 Å phase with the 5.60 Å
one (Fig. 4c). Metatorbernite loses water continuously from 300
to 500 °C. Therefore, we infer that the phase at 5.60 Å at 300 °C
has 1 H2O.
Vochten et al. (1981) showed by TG/DTA that metatorbernite
with 8 H2O dehydrates to 4, 2, and 0 H2O phases at 120, 150, and
150–450 °C, respectively, which is almost consistent with our results except that a 1 H2O phase occurs in the present study.
We have shown the hydration states of structurally distinct
phases during dehydration of meta-autunite, metasaléeite, andmetatorbernite by temperature-controlled XRD and TG/DTA.
The present results are unique because the dehydrated phases arepresented with both basal spacings and hydration numbers. The
presence of previously unknown dehydrated phases with basal
spacings less than 6 Å has been demonstrated for the Þrst time in
the present study.

ACKNOWLEDGMENTS
The authors are indebted to P.L. Airey, P. Duerden, A.A. Snelling, and M.
Deliens for providing the samples. M. Saigo is acknowledged for her assistancewith the XRD analysis. This manuscript was greatly improved by the comments of
P.C. Burns and an anonymous reviewer. The electron microscopy was performed in
the Electron Microbeam Analysis Facility of the Department of Earth and Planetary
Science, the University of Tokyo. Part of the present study was supported by the
Grant-in-Aid program of the Ministry of Education, Culture, Sports, Science andTechnology to T.M.

1314

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