Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences
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Solid solution formation in the metatorbernite–metazeunerite system (Cu(UO2)2(PO4)2−x(AsO4)x.nH2O) and their stability under conditions of variable temperature

    Abstract

    Mineral phases which can be thought of as members of a metatorbernite–metazeunerite solid solution (Cu(UO2)2(PO4)2−x(AsO4)x.8H2O have been identified in radioactive samples from spoil heaps at the uranium mine site in South Terras, Cornwall (grid reference SW935523). A complete solid solution (0 < x < 2) was synthesized by precipitation from solution using uranium (VI) nitrate and copper (II) chloride and phosphoric acid/arsenic acid in the appropriate molar proportions. Refined unit cell parameters determined by Pawley fitting of powder X-ray diffraction data showed a linear variation in the a unit cell parameter according to Vegard's Law, allowing the composition of the natural mineral phases found at South Terras to be determined from measurement of their unit cell parameters. High-resolution variable-temperature synchrotron powder X-ray diffraction studies were carried out at the Diamond Light Source on three members of this solid solution (x = 0, 1, 2) and showed different structural behaviour as a function of composition and temperature. Metatorbenite (x = 0) retains its tetragonal symmetry at low temperatures and dehydrates to an amorphous phase at 473 K, whereas metazeunrite (x = 2) transforms to an orthorhombic phase at low temperatures, regains its tetragonal symmetry on heating to 323 K and undergoes a further transition to an, as yet, unidentified phase at 473 K.

    This article is part of the theme issue ‘Fifty years of synchrotron science: achievements and opportunities’.

    1. Introduction

    Metatorbernite (Cu(UO2)2(PO4)2.8H2O) is a common secondary mineral resulting from pitchblende (UO2) alteration and can be considered as a significant host for uranium in both natural and anthropogenically contaminated environments [1]. Uranyl phosphates, such as metatorbernite, are known for their low solubility at circumneutral pH and are considered important for the control of uranium mobility in the environment under oxidizing conditions [2]. It has been suggested that U-contaminated groundwater may be decontaminated through the application of polyphosphates to form sparingly soluble uranyl phosphate phases, such as metatorbernite [3].

    The disused uranium mine in South Terras, Cornwall, has been abandoned for nearly a century, but discarded spoil heaps at the site still retain significant levels of activity [4]. The interest in this site is part of a larger study into the fate of radionuclides in the environment (NERC Long-lived Radionuclides In the Surface Environment project). While previous studies have indicated that uranium in these spoil heaps is relatively immobile [5,6], there are no published studies reporting the exact composition and stability of the uranium solid-state phases present in the spoil heaps. Metatorbernite has been reported to be present in areas of this locality [4,5], along with the arsenate analogue of this mineral phase, metazeunerite (Cu(UO2)2(AsO4)2.8H2O). A 2017 study indicated the possibility of a phosphate-containing metazeunerite phase existing at this locality [7].

    Metatorbernite is a member of the autunite family of minerals, which are some of the most widespread and abundant secondary uranium minerals known [8]. The composition of this family of minerals is variable with the ideal formula A(UO2)2(XO4)2.nH2O, where A is a divalent cation, X is arsenic or phosphorus and n is in the region of 10–16 for the autunite group and 6–8 for the meta-autunite group [9]. The mineral autunite has the ideal composition Ca(UO2)2(XO4)2.10–12H2O and is less stable than the phase meta-autunite, Ca(UO2)2(PO4)2.8H2O [10]. There are other known mineral phases related to autunite with monovalent or, more rarely, trivalent cations. The ideal formula for these phases is slightly altered to M+(UO2)(XO4).nH2O or M3+(UO2)(XO4).nH2O [9,11].

    The structures of autunite and meta-autunite minerals are mostly reported to crystallize in primitive tetragonal space groups. However, there are also reports of phases crystallizing in orthorhombic, monoclinic and triclinic space groups as well [11]. The structure can be described as sheets of uranyl arsenate/phosphate, with the composition [(UO2)2(XO4)2]2-, X = P or As, stacked along the c-axis with the cations and varying numbers of water molecules situated between the sheets (figure 1) [12,13].

    Figure 1.

    Figure 1. ac projection of the structure of metatorbernite: red CuO6 octahedra, green UO6 square bipyramids and yellow PO4 tetrahedra are shown, small and large blue spheres represent O atoms. Structure reproduced after [12] using the ATOMS V6.1 atomic structure display package [13]. (Online version in colour.)

    The published structural models for metatorbernite and metazeunerite are all primitive tetragonal; however, there are inconsistences in the reported unit cell parameters and a number of different space groups have been used to describe their structures. Previous structural studies on metatorbernite and metazeunerite phases have been carried out on mineral samples using X-ray diffraction techniques [12,1421]. The work presented in this paper focuses on refinement of unit cell parameters of synthetic phases only and further work on full structural characterization is underway [22].

    In this paper, we report the synthesis and characterization of a metatorbernite–metazeunerite (Cu(UO2)2(PO4)2−x(AsO4)x.8H2O) solid solution for 0 < x < 2. Powder X-ray diffraction (PXRD) data collected on these synthetic samples are compared with PXRD data collected on spoil heap samples removed from the disused uranium mine in South Terras, Cornwall to determine the composition of the naturally occurring mineral metatorbernite-type phases identified. Variable temperature studies carried out on the high-resolution powder diffraction beamline I11 at the Diamond Light Source [23] on three solid solution compositions, Cu(UO2)2(PO4)2−x(AsO4)x.8H2O (x = 0,1, 2) are also reported and show different behaviour according to the value of x.

    2. Methods

    (a) Sampling

    Sampling of the South Terras site was carried out in July 2015. Two sites of interest were identified using a handheld LB124 Berthold monitor; these were named Mine Spoil (MS) and Ore Processing Floor. About 10 l of soil and/or spoil were removed from the surface of each site using a hand shovel to a depth of about 20–30 cm. The samples were packed and stored in clean, polypropylene boxes before being transported to Loughborough University. All samples were dried in an oven at 80°C before being gently crushed with a pestle and mortar. They were then placed into a laboratory sieve shaker and separated into size fractions: greater than 2 mm and less than 2 mm. The greater than 2 mm fraction from the MS sample was spread out in a tray and monitored using a Geiger–Müller counter. Individual samples of especially high activity (HA; greater than or equal to 100 cps) were separated and labelled as high activity samples for further analysis.

    (b) Synthesis

    Metatorbernite was synthesized by adapting a method described by Cretaz et al. [2]. A total of 2.5 g uranyl nitrate hexahydrate (Aldrich) was dissolved in 25 ml of a 0.5 × 10−2 mol l−1 solution of nitric acid. A total of 0.44 g copper (II) chloride dihydrate (reagent grade Sigma Aldrich) was added and shaken until dissolved. A total of 2.5 ml of a 2 M solution of phosphoric acid (reagent grade Sigma Aldrich) was then added. After approximately 1 hour, pale green crystals were observed to form. The crystals were left on a shaker at room temperature for 1 h before being left to settle for 48 h. The product was then filtered under vacuum using a 0.1 µm cellulose membrane and washed three times with deionized water. The crystals were left to dry in an oven at 40°C until a constant weight was achieved.

    Metazeunerite was prepared using the same method but replacing the phosphoric acid with a 2 M solution of arsenic acid. The arsenic acid was prepared by dissolving 2.5 g arsenic (V) oxide (sigma Aldrich reagent grade) in 10 ml of deionized water.

    Members of the solid solution were prepared using different ratios of phosphoric and arsenic acid in the synthetic mixture to produce the compositions Cu(UO2)2(PO4)2−x(AsO4)x.nH2O, x = 0.2, 0.4, 0.5, 0.6, 0.8, 1.0, 1.2, 1.4, 1.5, 1.6 and 1.8.

    (c) Powder X-ray diffraction

    Powder X-ray diffraction data were collected using a Bruker D8 Discover X-ray diffractometer in transmission geometry using Co Kα1 radiation, selected using a Ge 111 single crystal monochromator and a Braun position sensitive detector. For phase identification of samples, data were collected over the 2θ range 5–60° with a step size of 0.0072334° 2θ and a count time of 0.5 s per step. For refinement of structural parameters, data were collected over the 2θ range 9–80° with a step size of 0.0072334° 2θ and a count time of 5.2 s per step.

    Synchrotron powder diffraction data were collected using an Si-calibrated wavelength of λ = 0.82603 Å on the high-resolution powder X-ray diffraction beamline I11 at the Diamond Light Source, UK [23]. The samples were packed into 0.5 mm borosilicate capillary tubes and secured with Kapton tape. Data were collected in transmission geometry using multi-analysing crystal detectors over an angular range of 1–150° 2θ. The PXRD data were collected at 293 K for 30 min. Samples were then cooled using the Cryostream Plus to 105–110 K and a new dataset was collected for 30 min. Data were collected on heating at 50 K intervals until a temperature of 500 K was reached.

    For the purposes of phase identification, the PXRD data patterns were compared to known end-member phases in the International Centre for Diffraction Data (ICDD) Powder Diffraction File (PDF2) database using the search match software contained in the software suite WinXPow. For the refinement of structural parameters, data were analysed using the software Topas Academic V6 [24].

    (d) Inductively coupled plasma optical emission spectrometry

    In total, 0.2 g of either the solid solution phase or mineral sample was weighed into a 15 ml polypropylene centrifuge tube. It was then dissolved in 2 ml 50% HNO3. A 0.1 ml aliquot of sample was removed and added to a 15 ml centrifuge tube; this was then diluted to 10 ml using more 2% HNO3 in deionized water. Elemental analysis was carried out using a Thermo Scientific 6000 series ICAP-6500 duo inductively coupled plasma optical emission spectrometry (ICP-OES).

    3. Results and discussion

    PXRD data collected on the samples from South Terras which were shown to have HA (greater than or equal to 100 cps) identified the presence of metatorbernite-type phases. Figure 2 shows an example of one of the datasets collected on sample HR 001, which was found to contain quartz as an impurity phase. The powder diffraction data did not match exactly with any of the metatorbernite diffraction patterns contained in the database (ICDD PDF2) as the observed peak positions were found to be shifted relative to the database patterns. We confirmed this was a sample effect and not due to misalignment of the diffractometer or the sample preparation method by adding a silicon internal standard (not shown in this dataset). We propose that the composition of the sample deviates from that of metatorbernite and could be a member of a solid solution between metatorbernite and metazeunerite as the observed peak positions were found to lie between the reflection positions of metatorbernite (ICDD PDF 36–406 [15]) and metazeunerite (ICDD PDF 17–146 [25]). ICP-OES analysis confirmed the presence of As in these samples. The results from the sample HR 001 are presented in electronic supplementary material, figure S1, and show the presence of Cu, U, As and P, which can be related to an As-containing metatorbernite-type phase. Fe, Al, Pb and Si are observed in small quantities due to the presence of impurity phases, present in quantities below the detection limits of PXRD techniques. There are only a few reports in the literature regarding the cross-substitution of phosphorus and arsenic in the metatorbernite-type system in nature [3]. Corkhill et al. reported a phosphate-containing metazeunerite phase at South Terras, present as a thin crust on minerals such as muscovite and jarosite [7]. The only report of similar solid solution behaviour is in the related system H(UO2)(PO4)1−x(AsO4)x.3H2O [26,27].

    Figure 2.

    Figure 2. PXRD pattern of spoil heap sub sample with HA compared with ICDD PDF for metatobernite, (ICDD PDF 36-406 [15]) and metazeunerite (ICDD PDF 17-146 [25]) and quartz (ICDD PDF 46-1045). Data were collected on a laboratory X-ray diffractometer using Co Kα1 radiation. (Online version in colour.)

    To confirm the hypothesis that a solid solution between metatorbernite and metazeunerite exists, a synthetic study was carried out. We prepared compositions of the proposed solid solution Cu(UO2)2(PO4)2−x(AsO4)x.nH2O, 0 < x < 2, in 0.2 increments. PXRD data were collected on the samples and found that a complete solid solution existed with no evidence for the presence of any impurity phases. Synchrotron PXRD data collected on selected samples of the solid solution are presented in electronic supplementary material, figure S2.

    Pawley refinements of the laboratory PXRD data in space group P4/n were carried out and the refined unit cell parameters as a function of composition, x, are presented in figure 3. The refined unit cell parameters are a = 6.9736 (9) Å and c = 17.341 (2) Å for metatorbernite and a = 7.1204 (9) Å and c = 17.442 (3) Å for metazeunerite. Comparison with the reported unit parameters for the end-member phases by Locock & Burns of a = 6.98 (5) Å and c = 17.35 (2) Å for metatorberinte and a = 7.11 (1) Å and c = 17.42 (1) Å for metazeunerite show our findings are consistent with previous studies [12]. It should be noted that the reported unit cell parameters for both metatorbernite and metazeunerite show some variation, with a wider range found for reported c parameters. For metatorbernite, a was found to range between 6.95 Å [19] and 6.98 Å [12,17] and c between 17.28 Å [20] and 17.41 Å [17], whereas for metazeunerite the range of refined a unit cell parameters was between 7.10 Å [17] and 7.13 Å [14] and a wider range for c of between 17.38 Å [17] and 17.70 Å [16].

    Figure 3.

    Figure 3. Comparison of unit cell parameters of the metatorbernite–metazeunerite (Cu(UO2)2(PO4)2−x(AsO4)x.nH2O) solid solution refined from laboratory PXRD data. Circles refined unit cell parameter a (b), triangles refined unit cell parameters c (a). (Online version in colour.)

    The variation in reported values could be due to compositional variations away from the ideal (i.e. Cu(UO2)2(XO4)2.8H2O, X = P or As) as all studies were carried out on mineral samples and there is a possibility that the X site is not pure P or As, and/or Cu may have been replaced with other metal cations (e.g. Ca, Fe) as well as the water content not being exactly eight molecules per formula unit [12,1421]. The refined unit cell parameters of the solid solution members show a (nearly) linear trend as a function of composition, which is more obvious in terms of the a parameter than the c parameter. Owing to the layered nature of the structure (figure 1), a change in the number water molecules per formula unit between the layers would have more impact on c than a. All the synthetic samples were dried at 40°C to ensure the transformation from torbernite-type to metatorbernite-type phases was complete. The torbernite-type samples are more hydrous, with a reported 12 water molecules per formula unit and are less stable than the metatorbernite-type samples (with eight water molecules per formula unit), on which this study focuses. It was found that the compositions containing more arsenate ions required longer times to fully complete the transformation. Further investigations are underway to establish what variability exists in terms of water content.

    Using the linear correlation between the a parameter and the AsO4/PO4 ratio (figure 3), we established equation (3.1) showing the relationship between the refined a parameter and composition in terms of x for the solid solution Cu(UO2)2(PO4)2−x(AsO4)x.nH2O. The formulae of two natural samples taken from South Terras were determined using equation (3.1) (table 1). A comparison of the observed data collected on the natural samples to the observed data on the solid solution member with the same approximate composition showed good agreement. (electronic supplementary material, figures S3 and S4).

    Table 1. Refined unit cell parameters from two natural samples of metatorbernite-type phases and the calculated x-value from equation (3.1).

    samplea (Å)c (Å)X
    HR0017.060 (4)17.400 (1)1.2
    bulk HA sample7.018 (6)17.394 (2)0.6

    Our results show that the abundance of arsenic on the South Terras site is significant and natural samples contain mixtures of the two species where arsenate is significant and possibly dominant. While the stability of the phosphate end member in the natural environment has been documented [2], much less is known about the arsenate material or samples with a mixed composition of the AsO4/PO4 species, which appear to be present naturally. This suggests stability studies on the mixed anion materials will be of major interest in assessing the stability of the MS on a long-term basis.

    y=0.0778x+6.9707.

    The scatter in the refined unit cell c parameter between metatorbernite and metazeunerite could be due to several reasons, variation in the water content, as highlighted above, or differences in the space group between the two systems. We have carried out preliminary structural refinements of the high-resolution synchrotron data collected on metatorbenite, which refined well in space group P4/n and metazeunerite, with data fitting better in space group P4/ncc [22]. However, there are real difficulties with refining these structures by X-ray diffraction due to the dominant scattering of uranium compared to all other species, anion disorder and the complexity of this structure. Powder neutron diffraction data will be required to resolve these issues.

    Variable temperature studies on three solid solution compositions, Cu(UO2)2(PO4)2−x(AsO4)x.8H2O (x = 0,1, 2), over the temperature range 110–500 K were also carried out. There was noticeable variability in the behaviour of different compositions as a function of temperature. The metatorbernite sample remained tetragonal on cooling and decomposed on heating, showing an amorphous signature at temperatures greater than 473 K, which recrystallized on cooling to produce an as yet unidentified phase. The high-resolution synchrotron powder diffraction data are presented in electronic supplementary material, figure S5, and the refined unit cell parameters plotted as a function of temperature are shown in figure 4. The unit cell parameters were refined in space group P4/n over the temperature range 105–443 K. Both unit cell parameters were found to expand linearly, with c expanding more than a. A plot of a/c versus temperature (electronic supplementary material, figure S6) shows the linear expansion up to 373 K and then a deviation from linearity, which could indicate the beginnings of a dehydration process.

    Figure 4.

    Figure 4. Refined lattice parameters from variable temperature study of metatorbernite (Cu(UO2)2(PO4)2.8H2O) and the composition Cu(UO2)2(PO4)(AsO4).8H2O compared with reported unit cell parameters from a variable temperature study on a mineral sample of metatorbernite [27]. Filled circles metatorbernite a, filled triangles metatorbernite c, diamonds Cu(UO2)2(PO4)(AsO4).8H2O a, squares Cu(UO2)2(PO4)(AsO4).8H2O c, open circles reported metatorbernite a, open triangles reported metatorbernite c [28]. Note, error bars on the refined parameters were smaller than the markers used. (Online version in colour.)

    The composition Cu(UO2)2(PO4)(AsO4).8H2O was studied over the temperature range 110–473 K and its behaviour was found to be different to that of the end-member phases. The sample remains tetragonal on cooling, as in the case of metatorbernite but on heating to 423 K, the diffraction pattern shifts to higher 2θ values and can be indexed in space group P4/n, with smaller unit parameters refined as a = 7.08224 (4) Å and c = 16.7862 (2) Å (figure 4). On further heating to 473 K, the diffraction pattern changes completely forming a new structure that is still to be identified. The variable temperature data collected on this composition are given in electronic supplementary material, figure S7, and a plot of the a/c ratio as a function of temperature (electronic supplementary material, figure S8). As with the metatorbernite phase, a linear expansion with temperature is observed up to 323 K and the trend is found to be non-linear above these temperatures, which could indicate the start of the dehydration process.

    Figure 4 presents the refined unit cell parameters from the high-temperature PXRD studies of metatorbernite and the composition Cu(UO2)2(PO4)(AsO4).8H2O compared to published unit cell parameters from a high-temperature PXRD study carried out on a mineral sample [28]. The composition of this mineral sample was determined to be Cu(UO2)2(PO4)1.9(AsO4)0.1.2H2O by electron probe microanalysis (EPMA). The findings show that at 129°C (402 K), the diffraction pattern can be indexed in space group P4/n with a smaller unit cell (a = 6.9551 (3) Å, c = 16.6604 (9) Å) than the parent metatorbernite phase. This trend is similar to our results, whereby the mixed arsenate/phosphate composition shows a decrease in a and c unit cell parameters at 423 K. We suggest the deviation between the two studies is due to the different compositions of the phases analysed.

    The metazeunerite sample was studied over the temperature range 110–473 K and exhibits different behaviour on heating and cooling, whereby on cooling the symmetry of the sample is lowered and the diffraction pattern can be indexed on an orthorhombic unit cell with proposed space group Pccn, a = 7.1138 (2) Å, b = 7.1584 (3) Å, c = 17.5439 (9) Å. On heating back to room temperature, the structure remains orthorhombic until 323 K, with the room temperature dataset refined as a mixture of tetragonal and orthorhombic phases. It may be that a more controlled heating rate would allow the sample to transform to the tetragonal phase at room temperature and this needs further investigation. The sample remains tetragonal until 373 K and above this temperature forms a new phase, with an unknown structure. The variable temperature data collected on metazeunerite are presented in electronic supplementary material, figure S9, and figure 5 shows a plot of the refined lattice parameters as a function of temperature. The reversible tetragonal to orthorhombic phase transition is clearly shown on the refined unit cell parameter versus temperature plot (figure 5). Over the temperature range 110–373 K, a linear expansion in the unit cell volume is observed.

    Figure 5.

    Figure 5. Refined lattice parameters from variable temperature study of metazeunerite (Cu(UO2)2(AsO4)2.8H2O). Circles metazeunerite a in tetragonal space group, triangles metazeunerite c in tetragonal space group, crosses metazenerite a in orthorhombic space group, diamonds metzeunerite b in orthorhombic space group, squares metazeunerite c in orthorhombic space group. Note, error bars on the refined parameters were smaller than the markers used. (Online version in colour.)

    The phase changes that occur in metatorbenite and metazeunerite at high temperature have been studied previously using X-ray diffraction, thermogravimetric analysis and Raman spectroscopy [2833]. Most of the studies have been carried out on mineral samples and there is a lack of consistency in the findings due to the compositional variation in the samples.

    Suzuki et al. carried out HTXRD and thermogravimetric analysis (TGA) on a mineral sample of metatobernite and reported a weight loss at 86°C that equated to four or five molecules of H2O per formula unit [29]. The XRD pattern collected at 100°C showed a shift in two theta positions when compared to the dataset collected at room temperature, with the basal spacing decreasing from 8.61 Å to 8.07 Å. The authors claim that the dehydrated phase at this temperature is likely to have the composition Cu(UO2)2(PO4)2.4H2O, losing water molecules from between the uranyl phosphate sheets, but retaining the layered structure. On further heating the sample showed another weight loss and change in diffraction pattern at approximately 200°C and Suzuki and co-workers stated that the dehydrated phase at this temperature has the likely composition Cu(UO2)2(PO4)2.2H2O, with a basal spacing of 6.58 Å. On heating further to 300°C, there are two reflections with d spacings of 6.52 and 5.60 Å; the authors interpret the appearance of the these reflections as being due to the presence of two different hydration states with likely compositions Cu(UO2)2(PO4)2.H2O and anhydrous Cu(UO2)2(PO4)2. There is no evidence from their HT PXRD study of any amorphous phase forming under these conditions, as we observed with the synthetic metatorbernite phase at temperatures greater than 423 K. It should be noted, that they only reported the d spacings of the basal spacing of the phases formed on heating with no refined unit cell parameters given. Suzuki et al. compare their findings with that of Vochten [30], who carried out TGA studies on metatorbernite and found weight loss steps at 120°C, 150°C and 150–400°C, which they stated were the temperatures where the compositions Cu(UO2)2(PO4)2.4H2O, Cu(UO2)2(PO4)2.2H2O and Cu(UO2)2(PO4)2 formed.

    Stubbs et al. [28] studied a mineral sample, that was shown to have the composition Cu(UO2)2(PO4)1.9(AsO4)0.1.2H2O by EPMA, using HT XRD over the temperature range RT to 315°C. These authors carried out more in depth structural analysis on the phases formed on dehydration than Suzuki and reported preliminary structural models for two of the dehydrated phases. Their findings state that at 129°C, a phase crystallizing in the space group P4/n with a smaller unit cell than the parent metatorbernite phase was formed with a suggested formula of Cu(UO2)2(PO4)2.6H2O, on further heating to 173°C, the sample dehydrates to Cu(UO2)2(PO4)2.3H2O and recrystallizes in space group P21. There is a further phase which formed at higher temperatures (209–315°C), but at the maximum temperature used in this study there was still evidence for the presence of the phase Cu(UO2)2(PO4)2.3H2O; no structural model or suggested composition was given for this phase. Stubbs et al. compared their work to that of Frost et al. [31], who carried out controlled rate thermal analysis on a mineral sample of metatorberntite, whereby the heating rate was varied according to whether the sample was undergoing a dehydration event or not. Frost found three mass losses which they equated to the compositions Cu(UO2)2(PO4)2.6.5H2O (at 138°C), Cu(UO2)2(PO4)2.2H2O (at 155°C) and Cu(UO2)2(PO4)2 (at 291°C).

    Our PXRD study on the synthetic analogue of the pure metatorbernite composition at elevated temperatures does not follow the dehydration profile of either of the reported HT XRD studies on mineral samples of metatorbernite [28,29]. We suggest that the differences are due to several factors. Firstly, the sample environment is different; in this study the sample is in a sealed capillary and therefore its dehydration behaviour is different to that of a sample which is heated in an open system. Secondly, the compositional variations are key. This synthetic sample is a pure copper uranyl phosphate, whereas the mineral samples were shown to be either two phase by PXRD [29] or contain arsenate as well as phosphate through EPMA [28]. As discussed above, the dehydration behaviour of the mixed phosphate arsenate composition (Cu(UO2)2(AsO4)2.8H2O) is more in line with that of Stubbs et al. [28] (figure 5).

    Two studies of the high-temperature behaviour of metazeunerite have been reported [32,33], both studies analyse metazeunerite samples using TGA techniques, but neither carry out any in situ HTXRD studies. The study by Frost is on a mineral sample that is stated to be a mixture of zeunerite and metazeunerite, as determined by PXRD analysis. The results of the TGA study, which were carried out under a N2 atmosphere, are discussed in terms of the sample being single-phase metazeunerite and are linked to a HT Raman study carried out on the same sample. Frost et al. [32] show three weight losses that are claimed to be the loss of water, which would give the following compositions on heating Cu(UO2)2(AsO4)2.6H2O (48°C), Cu(UO2)2(AsO4)2.2H2O (88°C) and Cu(UO2)2(AsO4)2 (125–250°C).

    Vochten & Goeminne [33] carried out a TGA study, under a flow of air, on a synthetic sample of metazeunerite, prepared under different conditions to the synthetic samples in our study. They showed four stages of dehydration which would give the following compositions on heating, Cu(UO2)2(AsO4)2.7H2O (108°C), Cu(UO2)2(AsO4)2.3H2O (130°C), Cu(UO2)2(AsO4)2.2.5H2O (185°C) and Cu(UO2)2(AsO4)2 (230°C). Our study did not use a flow of gas, the samples were sealed in borosillicate capillaries and therefore the dehydration behaviour is shown to be different, with only one change in structure observed at 150°C, to an as yet unidentified phase.

    All these studies highlight the variability in the behaviour of metatorbernite-type phases as a function of temperature. The heating rate and atmosphere will play a role in how theses phases dehydrate, but the composition is key too, with differences between mineral and synthetic samples shown as well as differences between metatorbernite and metazeunerite.

    4. Conclusion

    We have shown the presence of a complete solid solution between the end-member phases metatorbernite and metazeunerite (Cu(UO2)2(PO4)2−x(AsO4)x.8H2O) through a controlled synthetic study. Refinement of the lattice parameters confirmed all the solid solution members crystallized in a primitive tetragonal unit cell. The scatter of the c unit cell parameter as a function of composition was suggested to be due to the variable water content of these samples as well as structural differences between the end-member phases, whereby metatorbernite crystallizes in space group P4/n, but metazeunerite crystallizes in space group P4/ncc. However, we found a linear correlation between the a unit cell parameter and the AsO4/PO4 ratio (x in the solid solution Cu(UO2)2(PO4)2−x(AsO4)x.8H2O).

    We have refined the lattice parameters of mineral samples of metatorbernite-type phases, found to be present at the South Terras site and compared to the synthetic samples to predict their compositions using the relationship established between a and the AsO4/PO4 ratio. Our results on the natural samples obtained from the South Terras site show that they may contain mixtures of arsenate and phosphate where arsenate is significant and possibly dominant. While the stability of the phosphate end member, metatorbernite, in the natural environment has been documented, much less is known about the arsenate material or compositions containing mixtures of AsO4 and PO4 species, which appear to present naturally. This suggests stability studies on the mixed anion materials will be of major interest in assessing the stability of the MS on a long-term basis.

    The stability of the two-end member phases and the composition Cu(UO2)2(PO4)(AsO4).8H2O were studied using high-resolution synchrotron powder diffraction data as a function of temperature. These phases have been found to have different behaviour as a function of temperature. On cooling metatorbernite remains primitive tetragonal and on heating above 373 K, the sample becomes amorphous to X rays. The composition Cu(UO2)2(PO4)(AsO4).8H2O remains tetragonal on cooling and on heating, with a noticeable decrease in unit cell parameters at 423 K before transforming to a new structure type at 473 K. In the case of metazeunerite, on cooling the symmetry lowers and the data were indexed on a primitive orthorhombic unit cell. The sample transforms to the tetragonal phase on heating above RT and on continued heating undergoes a phase transition likely due to water loss to a new phase which is still to be identified.

    This preliminary study shows that natural samples of metatorbernite-type phases can have variable compositions, which may effect their stability in nature. Uranium is an environmental contaminant throughout the world, especially in areas where mining and milling of U were carried out, such as South Terras. Uranyl phosphate phases are key in controlling U-mobility and studies have suggested using the precipitation of metatorbernite-type phases to decontaminate groundwater contaminated by U through addition of polyphosphate. However, as we have shown, their stability is variable, dependent upon the composition. Therefore, understanding any stability changes as a function of composition will be key to their use as remediation materials. Ongoing studies into the structures and stabilities of phases in the metatorbernite–metazeunerite solid solution are underway.

    Data accessibility

    This article has no additional data.

    Competing interests

    We declare we have no competing interests.

    Funding

    This work was supported by the Natural Environment Research Council (NE/L000202/1) through a studentship for J.K., and STFC for funding the Diamond Light Source beamtime (EE15103).

    Acknowledgements

    We acknowledge Joan Sutherland and Dr Oliver Preedy for the assistance in synthesizing samples for data collection at the Diamond Light Source and for their assistance during the beamtime. The authors thank Prof. Chiu Tang, Dr Sarah Day and Dr Claire Murray, beamline scientists on the I11 beamline at The Diamond Light Source, for their assistance in setting up the beamline to carry out these experiments.

    Footnotes

    One contribution of 14 to a theme issue ‘Fifty years of synchrotron science: achievements and opportunities’.

    Published by the Royal Society. All rights reserved.

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