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Frost, Ray L., Palmer, Sara J.,& Reddy, B. Jagannadha (2011) Raman spectroscopic study of the uranyl titanate mineral euxenite (Y,Ca,U,Ce,Th) (Nb,Ta,Ti)2O6. Journal of Raman Spectroscopy, 42(5), pp. 1160-1162.

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Notice: Changes introduced as a result of publishing processes such as copy-editing and formatting may not be reﬂected in this document. For a deﬁnitive version of this work, please refer to the published source: http://dx.doi.org/10.1002/jrs.2822 1 Meatmictization and the Raman spectroscopic study of the uranyl titanate mineral 2 euxenite

3 (Y, Ca, U,Ce,Th)(Nb,Ta,Ti)2O6 4 5 Ray L. Frost,  Sara J. Palmer and B. Jagannadha Reddy 6 7 Chemistry Discipline, Faculty of Science and Technology, Queensland University of 8 Technology, GPO Box 2434, Brisbane Queensland 4001, Australia.

9 10 ABSTRACT 11 12 Raman spectra of the uranyl titanate mineral euxenite were analysed and 13 related to the mineral structure. The mineral is metamict. This results in 14 the expansion of the unit cell and consequent chnages in the UO bond 2+ 15 lengths. Observed bands are attributed to the TiO and (UO2) stretching - 16 and bending vibrations, U-OH bending vibrations, H2O and (OH) 17 stretching, bending and libration modes. The mineral euxenite is metamict 18 as is evidenced by the intensity of the UO stretching and bending modes 19 being of lower wavenumber and intensity than expected and with bands 20 that are significantly broader. 21 22 KEYWORDS: Metamictization, euxenite-(Y), uranyl, titanate, Raman spectroscopy 23 24 25 26 27 28 29 30 31 32

 Author for correspondence ([email protected]) P: +61 7 3138 2407 F: +61 7 3138 1804

33 INTRODUCTION 34 35 In the uranium processing industry, certain minerals are difficult to convert to ‚yellow cake‘ 36 as a result of their formulation. Among these uranium minerals are those which contain

37 titanium. These minerals include absite Ti7U2ThO20·5H2O, betafite (Ca,U)2(Ti,Nb)2O6(OH), 4+ 3+ 38 brannerite (U ,REE,Th,Ca)(Ti,Fe ,Nb)2(O,OH)6, euxenite (Y, Ca, U,Ce,Th)(Nb,TaTi)2O6

39 holfertite CaxU2-xTi(O8-xOH4x)·3H2O. Euxenite-(Y) is a mineral in the pyrochlore group,

40 with the chemical formula (Y, Ca, U,Ce,Th)(Nb,Ta,Ti)2O6. A related mineral is polycrase. 41 The difference between the latter two minerals is simply the amount of Ti in the mineral, 42 polycrase having more Ti and less Nb. Related minerals include barian betafite

43 (U,Ba,Ca)2-x(Ti,Nb)2O6-y(OH)1+y and yttrobetafite (Y,U,Ce)2(Ti,Nb,Ta)2O6OH. Betafite 44 typically occurs as a primary mineral in granite pegmatites, rarely in carbonatites. Other rare

45 earth oxides include fergusonite (REE)NbO4, polycrase (Y,Ca,Ce,U,Th)(Nb,Ta,Ti,)2O6, 3+ 46 aeschynite (REE,Ca,Fe,Th) (Ti,Nb)2(O,OH)6 and samarskite (Y,Ce,U,Fe )3(Nb,Ta,Ti)5O16. 47 Many of these minerals have been termed ‚trash‘ minerals because of their ability to 48 incorporate all sorts of metals into the structure. The mineral euxenite-(Y) is potentially a 49 valuable mineral because rare earth elements such as niobium and tantalum can be extracted. 50 51 52 The knowledge of the crystal structures of natural and synthetic uranyl phases is 53 important for better understanding the genesis of uranium deposits, interaction of uranium 54 mine and mill tailings with the environment, actinide transport in soils and the vadose zones 55 and the performance of geological repositories for nuclear waste 1. Uranyl minerals are also 56 observed as products of alteration (hydration-oxidation weathering) of spent nuclear fuel 2. 57 Uranyl minerals exhibit considerable structural and chemical diversity, and reflects 58 geochemical conditions dominant during their formation 3. Uranyl oxide hydrates, such as the 59 studied uranyl minerals schoepite, becquerelite, billietite, curite and vandendriescheite, may 60 be understood especially as weathering products of uraninite in the oxidized zone of uranium 61 deposits 4-6. 62 63 Raman spectroscopy has proven very useful for the study of minerals 7-20. Indeed 64 Raman spectroscopy has proven most useful for the study of diagentically related minerals as 65 often occurs with minerals containing uranyl groups 7-9, 14, 21. Euxenite and polycrase are 66 common in the uranium deposist in Australia. No Raman spectroscopic studies of the uranyl

67 titanium bearing mineral euxenite have been forthcoming. This paper is a part of systematic 68 studies of vibrational spectroscopy of minerals of secondary origin in the oxide supergene 69 zone and their synthetic analogs. In this work we attribute bands at various wavenumbers to 70 vibrational modes of betafite using Raman spectroscopy and relate the spectra to the 71 structure of the mineral. 72 73 EXPERIMENTAL

74 Mineral

75 The mineral euxenite-(Y) was obtained from The South Australian Museum. A description 76 of uranium deposits in the Flinders ranges has been made by Pierre-Alain Wulser. 22 Uranium 77 minerals have been defined in South Australia. 23, 24 Two mineral samples were obtained 78 firstly from the Billeroo Prospect, South Australia [see http://www.world- 79 nuclear.org/info/Australia_Mines/pmines.html] and The Radium Hill Mine, Mingary, Olary, 80 South Australia [http://www.radiumhill.org.au/wb/index.php]. 81 82 The euxenite mineral originated from 0.25 miles south of Mt Victoria mine, Olary Province, 83 South Australia 84 85 The composition of the mineral has been published. 25 The mineral is also known to occur in 86 beach sands as occur in Western Australia. 87

88 Raman microprobe spectroscopy

89 90 The crystals of euxenite-(Y) were placed and orientated on the stage of an Olympus 91 BHSM microscope, equipped with 10x and 50x objectives and part of a Renishaw 1000 92 Raman microscope system, which also includes a monochromator, a filter system and a 93 Charge Coupled Device (CCD). Raman spectra were excited by a HeNe laser (633 nm) at a 94 resolution of 2 cm-1 in the range between 100 and 4000 cm-1. Repeated acquisition using the 95 highest magnification was accumulated to improve the signal to noise ratio. Spectra were 96 calibrated using the 520.5 cm-1 line of a silicon wafer. Uranyl minerals such as the titanium 97 containing minerals are stable under the incident radiation probably because of their 98 refractory nature. It is noted the mineral euxenite is black-brown in colour.

99 Spectral manipulation such as baseline adjustment, smoothing and normalisation were 100 performed using the Spectracalc software package GRAMS (Galactic Industries Corporation, 101 NH, USA). Band component analysis was undertaken using the Jandel ‘Peakfit’ software 102 package which enabled the type of fitting function to be selected and allows specific 103 parameters to be fixed or varied accordingly. Band fitting was done using a Lorentz-Gauss 104 cross-product function with the minimum number of component bands used for the fitting 105 process. The Lorentz-Gauss ratio was maintained at values greater than 0.7 and fitting was 106 undertaken until reproducible results were obtained with squared correlations of r2 greater 107 than 0.995.

108 109 RESULTS AND DISCUSSION 110 111 The mineral euxenite-(Y) is amophous; this no doubt occurs as a result of the process of 112 metamictisation. Nevertheless, Raman spectra could be successfully obtained. The Raman 113 spectra of three different euxenite samples are displayd in Figure 1. Variation in composition 114 can occur depending upon the origin of the sample and the amount and type of cation in the 115 formula. Three minerals of this mineral type are known namely euxenite-(Y), tanteuxenite- 116 (Y) and yttrocrasite-(Y). As a result of variation in composition, variation in intensity of the 117 Raman bands occurs. Further crystal orientation effects can influence the band intensity. 118 119 In each of the Raman spectra, bands are observed at around 903 to 809 cm-1 -1 2+ 120 and at around 835 to 842 cm . These bands are assigned to the ν1 UO2 symmetric stretching 121 vibration. The band is broad and of low intensity. This provides evidence of the effect of 122 metamictization. Metamictisation is also observed by the amorphous nature of the mineral 2+ 123 with no XRD patetrn able to be obtained. Normally the ν1 UO2 symmetric stretching bands 124 are very intense and sharp in the Raman spectra of uranyl minerals. Raman bands in these 125 positions are found in the Raman spectra of other uranyl titanate minerals. Raman bands are 126 found at 809 and 851 cm-1 for davidite-La from Radium hill. Raman bands for betafite are 127 observed at 810 and 893 cm-1. These bands are associated with UO stretching vibrations. The 2+ 128 linear (UO2) group, point group Dh, has four normal vibrations but only three

129 fundamentals: the Raman active 1 symmetric stretching vibration, the infrared active 2 ()

130 doubly degenerate bending vibration, and the infrared active 3 antisymmetric stretching 26-30 131 vibration . The decrease of uranyl group symmetry may result in the splitting of the 2

2+ 2+ 2+ 132 (UO2) , and in activation of the 1 (UO2) in the infrared spectrum and of the 3 (UO2) in 133 the Raman spectrum. 134 135 Raman bands are observed in the 600 to 700 cm-1 region. For sample (a) Raman bands 136 are observed at 615, 642 and 692 cm-1. For sample (b) two raman bands are observed at 624 137 and 658 cm-1 and for sample (c) at 609 and 657 cm-1. These bands are assigned to TiO 138 stretching vibrations. The complexity of the Raman spectrum for sample (a) may be 139 attributed to the peaks due to NbO and YO stretching bands. The intense sharp band at 641 140 cm-1 of brannerite is assigned to the TiO symmetric stretching vibration. For the mineral 141 betafite, two bands were observed at 601 and 657 cm-1 and these bands were also assigned to 142 TiO stretching bands. A shoulder at 611 cm-1 for brannerite which may correspond to the first 143 band for betafite. 144 145 The Raman spectra of euxenite displays bands at 412 and 460 cm-1 (a); 410 and 493 cm-1 (b) 146 and 408 and 494 cm-1 (c). One possible assignemt is that these bands are due to OTiOTiO 147 chanin vibrations and OTiOTiO bending modes. Another possibility is that these bands are

148 due to UO vibrations. Bands in the range 560 – 100 may be simply assigned to the  (U- -1 2+ -1 149 Oequatorial) (560 – 300 cm ),  ()(UO2) bending vibrations (274 – 238 cm ), and 150 molecular deformation and lattice modes (lower than 220 cm-1). However, according to 151 Dothée, a more detailed attribution is possible 31-33. Bands in the range 560 – 390 (591 – 532) -1 -1 152 cm are assigned to the 3 (U3O) bridge elongation, 370 – 328 cm to the  U3(OH)3 or -1 153 U2O(OH) groups elongation, 321 – 290 cm to the  (U3O) out-of-plane bending vibrations, -1 154 220 – 207 to the  (U3(OH)3 out-of-plane bending vibrations, 200 – 193 cm to the  -1 2+ 155 (U3(OH)3) in-plane bending vibrations, and 171 – 108 cm to the (UO2) translations and 156 rotations. 157 158 Some Raman bands are observed in the 1000 to 1800 cm-1 region. In two of the spectra, the 159 signal to noise ratio is low. Raman bands are observed at 1600 cm-1 (a) and 1624 cm-1 (b). 160 These bands are assigned to adsorbed water. In spectrum (c) the spectrum shows better 161 signal to noise. Raman bands are found at 1035, 1061, 1102 and 1270, 1314, 1342 and 1488 162 cm-1. The band at 1314 cm-1 is also found in spectrum (b). The assignment of these bands is 163 open to question. There are two possibilities: firstly the bands could be attributed to organic 164 impurities adsorbed on the mineral surfaces and secondly the bands might be due to

165 combination or overtone bands. What these spectra show is the ability of Raman 166 spectroscopy to determine adsorbed molecules on the surface of the euxenite-(Y) minerals. 167

168 The effect of Metamictization

169 Metamictization (or metamiction) is a natural process resulting in the gradual and ultimately 170 complete destruction of a mineral's crystal structure, leaving the mineral amorphous. The 171 mineral euxenite-(Y) is a typical metamict phase. It is essentially a process of self 172 destruction. The affected mineral is often described as metamict. Most uranyl titanate 173 minerals are metamict. Certain minerals occasionally contain interstitial radioactive 174 elements and it is the alpha radiation emitted from these compounds that is responsible for 175 degrading the mineral crystal structure through internal bombardment. Effects of 176 metamictization are extensive: other than negating any birefringence previously present, the 177 process also lowers a mineral's refractive index, hardness, and specific gravity. The mineral's 178 colour can also be affected: metamict specimens are usually black or brown. Upon 179 metamictization the unit cell expands. This expansion is not uniform and is random. This 180 expansion results in the lengthening of the UO bonds, thus the peak position of the Raman 181 bands of UO stretching vibrations occur at lower wavenumbers. The mineral euxenite-(Y) is 182 brown to black, providing an indication of metamictization. Further, metamictization 183 diffuses the bands of a mineral's scattering spectrum. It is likely that the process of 184 metamictization has affected the Raman spectrum of euxenite-(Y) in that the bands are 185 broad, of low intensity than would be expected and overlapping. This is particularly 186 noticeable in the UO symmetric stretching region where it might be expected that the bands 187 are sharp and intense.

188 CONCLUSIONS 189 190 The minerals containing the uranyl group and titanium as an oxide are of interest 191 because of their difficulty in mineral processing. These minerals include euxenite-(Y), 192 polycrase, holfertite, davidite, bannerite, betafite. The Raman spectra of brannerite were 193 obtained and the spectra compared with the spectra of the uranyl oxyhydroxide hydrates. 2+ 194 Observed Raman bands are attributed to the (UO2) and TiO stretching and bending 195 vibrations, U-OH and OTiO bending vibrations. Minerals such as euxenite-(Y) undergo a 196 process of metamictization, which is a process of crystal self destruction. Such a process is

197 observed in the Raman spectra through the intensity of bands being lower than expected and 198 a broadening of the typical UO stretching and bending modes. 199 200 Acknowledgements 201 202 The financial and infra-structure support of the Queensland University of Technology 203 Inorganic Materials Research Program of the Faculty of Science and Technology is gratefully 204 acknowledged. The Australian Research Council (ARC) is thanked for funding the 205 instrumentation. Mr Ben McHenry of the South Australian museum is thanked for the loan 206 of the minerals.

207

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259 9

260 List of Figures 261 262 Figure 1 Raman spectra of euxenite-(Y) in the 100 to 1000 cm-1 region 263 264 Figure 2 Raman spectra of euxenite-(Y) in the 1000 to 1800 cm-1 region

265 266

267 Figure 1

268

269

270 271 272

273 Figure 2

274 275