Raman microscopy of autunite minerals at liquid nitrogen

temperature

Ray L. Frost• and Matt Weier Inorganic Materials Research Program, School of Physical and Chemical Sciences, Queensland University of Technology, GPO Box 2434, Brisbane Queensland 4001, Australia. Published as: Frost, F. L. and Weier, M. Raman microscopy of autunite minerals at liquid nitrogen temperature. Spectrochimica Acta. Part A, Molecular and Biomolecular Spectroscopy, 2004. 60(10): p. 2399-409. Abstract:

Uranyl micas are based upon (UO2 PO4)- units in layered structures with hydrated

counter cations between the interlayers. Uranyl micas also known as the autunite minerals are of general formula M(UO2)2(XO4)2.8-12H2O where M may be Ba, Ca, Cu, Fe2+, Mg, Mn2+ or ½(HAl) and X is As, or P. The structures of these minerals have been studied using Raman microscopy at 298 and 77 K. Six hydroxyl stretching bands are observed of which three are highly polarised. The hydroxyl stretching vibrations are related to the strength of hydrogen bonding of the water OH units. Bands in the Raman spectrum of autunite at 998, 842 and 820 cm-1 are highly polarised. Low intensity band at 915 cm-1 is attributed to the ν3 antisymmetric stretching vibration of (UO2)2+ units. The band at 820 cm-1 is attributed to the ν1 symmetric stretching mode of the (UO2)2+ units. The (UO2)2+ bending modes are found at 295 and 222 cm-1. The presence of phosphate and arsenate anions and their isomorphic substitution are readily determined by Raman spectroscopy. The collection of Raman spectra at 77 K enables excellent band separation. Key words: autunite, metautunite, arsenate, phosphate, vanadate, Raman

spectroscopy Copyright 2004 Elsevier Introduction There are more than 200 uranium minerals. [1] The chemistry of uranium is important for the solution of environmental problems; for example the remediation of contaminated sites and the restoration of soils. Uranyl phosphates and arsenates form one of the largest and most widespread group of uranium minerals. [1] The minerals are known as autunites and are also known as uranyl micas. Many of these minerals may be found in Australia. [2] The autunite group of minerals are tetragonal uranyl arsenates, phosphates and vanadates. The minerals have a general formula M(UO2)2(XO4)2.8-12H2O where M may be Ba, Ca, Cu, Fe2+, Mg, Mn2+ or ½(HAl)

• Author to whom correspondence should be addressed (r.frost@qut.edu.au)

and X is As, or P or V. Autunites are common minerals, yet have been rarely studied in terms of Raman spectroscopy. The minerals have a layer-like structure. [3],[4, 5] A characteristic feature of the minerals is their layer structure in which uranium is bound in uranyl-phosphate layers. The cations and water are located in the interlayer space. The mineral autunite has the formula Ca[(UO2)2(PO4)]2.12(H2O) . Autunite is amongst the most abundant and widely distributed of the uranyl phosphate minerals, yet because of its pseudo-tetragonal symmetry and rapid dehydration in air, the details of its symmetry and structure are uncertain. The structure contains the well-known autunite type sheet with composition [(UO2)(PO4)], resulting from the sharing of equatorial vertices of the uranyl square bipyramids with the phosphate tetrahedra. [6] The calcium atom in the interlayer is coordinated by seven H2O groups and two longer distances to uranyl apical O atoms. Two symmetric independent H2O groups are held in the structure only by hydrogen bonding. [3] Most uranyl minerals are hydrated and as such both water and hydroxyls play a significant role in the structures. It is common for water to play a major role in the degree of polymerisation because of the asymmetric nature of hydrogen bonding systems. Water may bond to the interstitial cation or may simply be held in the structure through hydrogen bonding. Water groups play an important role in satisfying bond-valence requirements. The role of water and the number of water units in the empirical formula determines structural arrangements in the uranyl mica interlayer. [1] For example: Ca(UO2)2(PO4)2.11H2O (autunite) →Ca(UO2)2(PO4)2.8H2O (metautunite)

Ca(UO2)2(PO4)2.8H2O (metautunite) →Ca(UO2)2(PO4)2.xH2O (partially dehydrated metautunite) Burns proposed that uranyl mineral structures be based upon a topological arrangement of anions within each sheet as a convenient basis for the classification of these sheets. [7] Cejka et al. has reported the infrared spectroscopy of many uranyl minerals. [5, 8-10] The free uranyl ion (UO2)2+ with point symmetry D∞h should exhibit three fundamental modes symmetric stretching vibration ν1, bending vibration ν2 and the antisymmetric stretching vibration ν3. The bending mode is doubly degenerate since it can occur in two mutually independent planes. [8] Hence the linear uranyl group has four normal vibrations but only three fundamentals. In a linear symmetric uranyl ion belonging to the D∞h point group the ν1 band is found in the 900 to 750 cm-1 region and is Raman active but only appears in the infrared spectrum in the case of substantial symmetry lowering. The antisymmetric stretching vibration is active in the infrared and inactive in the Raman. Lowering of the symmetry results in the activation of all fundamentals. Farmer reported the infrared spectral results of some autunite minerals. [11] The ν1 mode of PO4 was given as 920 cm-1, ν2 as 472 and 435 cm-1, ν3 as 1123 and 1023 cm-1 and ν4 as 615 and 545 cm-1. [11] The values for torbernite were listed as ν1 mode at 915 cm-1, ν2 as 465 cm-1, ν3 as 1115 and 1023 cm-1 and ν4 as 615 and 550 cm-1. Farmer gave the position of the (UO2)2+ bands as ν1 at 805 cm-1 for torbernite and ν3 as 915 cm-1. The interpretation of this assignment is open to question. Cejka et al. reported the infrared spectrum of sabugalite and suggested that the weak absorption band at 810 cm-1 was attributable to the symmetric stretching mode of the (UO2)2+ unit and that the band at 915 cm-1 was attributable to

the antisymmetric stretching vibration of the (UO2)2+ unit. [10] The ν2 bands of the (UO2)2+ units were found at 298 and 254 cm-1. Herein lies the difficulty in that both the ν1 bands of PO4 and (UO2)2+ is found at the same spectral positions making interpretation by infrared spectroscopy difficult. In this paper we report the Raman spectra at liquid nitrogen temperature of selected autunites and meta-autunites and relate the spectra to their structures.

Experimental

Minerals: The following minerals were obtained from the Museum Victoria and have

been characterized. The minerals were checked for phase composition using X-ray diffraction and for chemical composition using the electron probe.

Autunites:

M27677 Autunite, Star Brite Mine, Mt. Spokane, Washington M46683 Autunite, Cunha Baixa mine, Beira Alta Province, Portugal M42016 Autunite, Arkaroola Station, Mt. Painter, Flinders Ranges, South

Australia M29775 Nováčekite, Aldama Chihuahua, Mexico M42797 Sabugalite, El Loba Mine, La Hoba, Don Benito District, Badajoz,

Spain M45673 Saléeite, ERA Ltd Ranger Uranium Deposit No. 3 Pit, Jabiru,

Northern Territory, Australia M38558, Saléeite, South Alligator River, Northern Territory, Australia

M27808 Torbernite, Mount Painter, 9km N of Arkaroola, South Australia M39006 Torbernite, Elsharana, Northern Territory, Australia M34931 Heinrichite, White King Mine, Lakeview, Oregon, USA

Metautunite M27680 Meta-autunite, Autun, France M47219 Meta-torbernite, El Sharana mine, South Alligator River, Northern

Territory, Australia M20948 Metazeunerite, Gilgai, New England, NSW M34627 Metazeunerite, Wheal Edward Bottalock, Cornwell, England M36686 Metakahlerite (yellow), Near Dalbeattie, Kircudbrightshire, Scotland,

Zeunerite (green)

Raman microscopy

The crystals of the autunite minerals were placed and oriented on a polished metal surface on the stage of an Olympus BHSM microscope, which is equipped with 10x and 50x objectives. The microscope is part of a Renishaw 1000 Raman microscope system, which also includes a monochromator, a notch filter system and a thermo-electrically cooled Charge Coupled Device (CCD) detector. Raman spectra were excited by a Spectra-Physics model 127 He-Ne laser (633 nm) and acquired at a nominal resolution of 2 cm-1 in the range between 100 and 4000 cm-1. The crystals were oriented to provide maximum intensity. All crystal orientations were used to obtain the spectra. Power at the sample was measured as 1 mW. The incident radiation was scrambled to avoid polarisation effects. Raman spectra at liquid nitrogen

temperature were obtained using a Linkam thermal stage (Scientific Instruments Ltd, Waterfield, Surrey, England).

Spectracalc software package GRAMS. Band component analysis was undertaken using the Jandel ‘Peakfit’ software package, which enabled the type of fitting function to be selected and allows specific parameters to be fixed or varied accordingly. Band fitting was done using a Gauss-Lorentz cross-product function with the minimum number of component bands used for the fitting process. The Gauss-Lorentz ratio was maintained at values greater than 0.7 and fitting was undertaken until reproducible results were obtained with squared regression coefficient of R2 greater than 0.995.

Results and discussion Polarised spectra The polarised spectra of the hydroxyl stretching region of autunite Ca[(UO2)2(PO4)]2.12(H2O)) from Arkaroola Station, South Australia at 77 K are shown in Figure 1. The results of the band component analyses are reported in Table 1. Hydroxyl stretching bands are observed in the 77 K spectra at 3523, 3489, 3432, 3400, 3343, 3262 and 3077 cm-1. The number of bands and their positions may be compared with the 298 K Raman spectrum in which three bands were observed at 3511, 3470 and 3268 cm-1. The widths of the bands in the polarised spectrum are 29.2, 17.6, 69.1, 19.2, 29.6, 35.2 and 26.9 cm-1 respectively. The bands are considerably broader in the depolarised spectrum. The relative intensity of the band at 3489 cm-1 is 10.9 % in the polarised spectrum and 7.8 % in the depolarised spectrum; the relative intensity of the band at 3262 cm-1 decreases from 22.2 % to 8.1 % from the polarised to the depolarised spectrum. The band at 3343 cm-1 also shows some polarisation. The significance of these results is that the identification of the bands which show polarisation shows that these are the OH symmetric stretching vibrations. The spectra in the OH stretching region shows six band of which three are polarised and three depolarised. This gives rise to the concept of three water molecules in the autunite structure with three different hydrogen bond strengths. The Raman spectra clearly show that the water in autunites is in a highly ordered structure through bonding to the uranyl phosphate surfaces or in water of hydration of the calcium cation.

The polarised and depolarised Raman spectra of the (UO2)2+ and PO4 stretching region are shown in Figure 2. The results of the band component analysis are given in Table 1. Bands are observed at 1007, 998 and 990 cm-1 in the polarised spectrum. The intensity of the 998 cm-1 band relative to the 990 cm-1 band changes significantly in the polarisation/depolarisation experiment. A band is also observed at 842 cm-1. This band is also highly polarised. The mineral autunite is a uranyl phosphate and the normal position of a PO4 symmetric stretching vibration would be in the 930 to 950 cm-1 range. No band is observed in the spectra in this position. One possible assignment is that the highly polarised band at 842 cm-1 is due to the PO4 symmetric stretching vibration. It is proposed that the vibrational unit is (UO2PO4)- rather than individual UO2 or PO4 units. It is possible that coupling occurs between

the UO2 and PO4 symmetric stretching vibrations. The low intensity band at 820 cm-1 is attributed to the symmetric stretching mode of the UO2 unit.

Hydroxyl stretching region at 77 K. The Raman spectrum at 298 K of the hydroxyl stretching region of autunites shows three bands at 3511, 3470 and 3268 cm-1. The Raman spectrum of meta-autunite at 298 K displays bands at 3358, 3198 and 3033 cm-1. The observation of three bands in the Raman in the 298 K spectrum suggests at least three different types of water molecules which may be described in terms of strength of hydrogen bonding. The bands at 3560 cm-1 are ascribed to weak hydrogen bonding, whilst the bands at 3033 cm-1 to stronger hydrogen bonding. The Raman spectrum of autunite at 77 K is given in Figure 1 and the description is given above. A comparison of the Raman spectra at 298 and 77 K is shown in Figure 3. The Raman spectrum of torbernite at 298 K shows three curve resolved components at 3359, 3197 and 3032 cm-1 (Figure 4). In the 77 K spectrum of the hydroxyl stretching region seven bands are observed at 3396, 3338, 3296, 3166, 3070, 2992 and 2919 cm-1. The bandwidths of these bands are 49.8, 21.3, 82.7, 60.5, 220.0, 116.0 and 41.9 cm-1 respectively. The Raman spectrum of metatorbernite shows three bands at 3362, 3200 and 3020 cm-1 (Figure 5). The infrared spectrum of metatorbernite shows complexity with bands observed at 3412, 3339, 3263 and 2919 cm-1. Infrared bands at 3570, 3430 and 2940 cm-1 were reported by Farmer. [11] Cejka et al. also reported infrared bands at 3360 and 2930 cm-1 [4] The observation of bands at low wavenumbers suggests that there are short hydrogen bonds between the water and PO4 units.

Studies have shown a strong correlation between OH stretching frequencies and both O…O bond distances and H…O hydrogen bond distances [12-15]. Libowitzky (1999) based upon the hydroxyl stretching frequencies as determined by infrared spectroscopy, showed that a regression function can be employed relating the above correlations with regression coefficients better than 0.96 [16].

The function is ν1 = 3592-304x109exp(-d(O-O)/0.1321) cm-1. If we use the Raman wavenumbers and using this type of equation calculations of hydrogen bond distances can be made. The stretching wavenumbers for torbernite with bands at 3396, 3338, 3296, 3166, 3070, 2992 and 2919 cm-1 give hydrogen bond distances of 2.795, 2.761, 2.741, 2.692, 2.666, 2.647 and 2.632 Å. Thus there is a range of hydrogen bond distances from 2.632 to 2.795 Å. One means of looking at these results is that the hydrogen bond distances represent energy levels in which the hydrogen bonds can be found. At 77 K, there are some seven energy levels. The implication is that the water is in a highly ordered state between the torbernite uranyl phosphate layers. In contrast the spectrum at 298 K shows three overlapping bands indicating a quantum continuum of states of hydrogen bonding for metatorbernite. The Raman spectrum of the hydroxyl stretching region of saléeite at 298 and 77 K are shown in Figure 6. In the Raman spectrum at 298 K three bands are observed at 3512, 3488 and 3296 cm-1. In the Raman spectrum at 77 K six bands are observed at 3522, 3489, 3408, 3340, 3264 and 2979 cm-1. What is most interesting is the increase in intensity of the band at 2979 cm-1. The effect of decreasing the

temperature to 77 K, resulted in the formation of new energy levels of hydrogen bonding. The Raman spectrum of the second saléeite sample at 77 K gave bands at 3521, 3488, 3429, 3397, 3341 and 3263 cm-1. Such a difference in spectra in the hydroxyl stretching region is significant as this means that the molecular structure of the two saléeite samples in the interlayer is different. The Raman spectrum of metazeunerite is shown in Figure 8. Three bands are observed in the 298 K spectrum at 3371, 3238 and 3136 cm-1. In the 77 K spectrum five bands are observed at 3350, 3287, 3191, 3128 and 2952 cm-1. The Raman spectrum of a second zeunerite sample shows more complexity (Figure 9) with hydroxyl stretching bands observed at 3434, 3373, 3378, 3288, 3167, 3055, 2993 and 2923 cm-1. The Raman spectroscopic study of the hydroxyl stretching region of each of the autunites shows multiple bands at different wavenumbers. Each of the bands is relate to a hydrogen bond with different bond strengths. These bands show that the water in the interlayer of autunites is highly ordered. Different autunites with the same composition show different spectral patterns in the hydroxyl stretching region. This fundamentally means that each autunite is different according to the arrangement of water molecules in the interlayer. This structure includes the hydrogen bonding to the uranyl-phosphate or uranyl arsenate surfaces and the hydration of the cation. Stretching vibrations of the XO4 and (UO2)2+ units The Raman spectra of the stretching region of (UO2)2+ and XO4 units of autunite, metautunite, metatorbernite and metazeunerite are shown in Figures 11 to 13. The results of the Raman spectroscopic analyses are reported in Tables 1 and 2. A set of overlapping bands is observed in the 980 to 1100 cm-1 region for each of the uranyl phosphates. These bands are attributed to the ν3 antisymmetric stretching vibrations. For autunite the bands are observed at 1018, 1007 and 988 cm-1 in the 298 K spectrum and at 1009, 998 and 989 cm-1 in the 77 K spectrum. More complexity exists in this region in the Raman spectra of metautunites. Five bands are observed in the 298 K spectrum at 1093, 1033, 1018, 1007, and 989 cm-1. In the 77 K spectrum bands are observed at 1093, 1039, 1022, 1013, 994 and 987 cm-1. Bands are observed in similar positions in the spectrum of the second metautunite sample. Reduction in the temperature from 298 to 77 K must lower the symmetry of the phosphate anion. For torbernite phosphate antisymmetric stretching vibrations are observed at 1004, 995, and 988 cm-1. The bands are observed at 1006, 999 and 991 cm-1 in the 77 K spectrum. A similar number of bands and their positions are observed for metatorbernite. One conclusion that may be reached is that there is a tendency for the bands to shift to higher wavenumbers upon cooling to 77 K. The Raman spectrum of saléeite show antisymmetric PO4 stretching vibrations at 1015, 1007,988 and 982 cm-1. At 77 K the bands are observed at 1005, 999 and 988 cm-1. A second saléeite sample showed bands at 1040, 1017 and 992 cm-1. No bands are observed in these positions for metazeunerite which is a uranyl arsenate. The Raman spectra of zeunerite at 298 and 77 K are shown in Figure 13. The symmetric stretching vibration of the aqueous arsenate anion (ν1) is observed at 810 cm-1 and coincides with the asymmetric stretching mode (ν3). The bending modes (ν2) and (ν4) are observed at 342 cm-1 and at 398 cm-1 respectively. In the Raman spectra of autunites, the band at around 809 cm-1 has been assigned to the AsO4 symmetric stretching vibration. This band is polarised. The structure of the

uranyl phosphate/arsenate sheets are formed from uranyl tetragonal -square - dipyramids and PO4 or AsO4 tetrahedra - four oxygens from four different tetrahedra are bonded to uranyl in its equatorial plane. The bands at 910 and 888 cm-1 are attributed to the AsO4 antisymmetric stretching vibrations. The bands shift to 892 and 884 cm-1 at 77 K. The Raman spectrum of autunite in the 700 to 1100 cm-1 region shows an intense sharp band at 833 cm-1 with additional bands observed at 822 and 816 cm-1. No band is observed in the Raman spectrum in the 930 to 950 cm-1 region where the PO4 symmetric stretching vibration would be expected to occur. The band at 833 cm-1 is assigned to this vibration. The second band which is also highly polarised at 822 cm-1 is attributed to the (UO2)2+ symmetric stretching vibration. These two bands are observed at 843 and 820 cm-1 at 77 K. The Raman spectrum of a second autunite sample gave bands in identical positions. The Raman spectrum of metautunite (Figure 11) showed intense bands at 833 and 818 cm-1 in the 298 K spectrum and the bands were found in identical positions in the 77 K spectra. The widths of these bands are 17.0 and 12.9 cm-1 in the 77 K spectrum. The Raman spectrum of metatorbernite (Figure 12) shows an intense band at 826 cm-1 in the 298 K spectrum and at 827 cm-1 in the 77 K spectrum. This latter band is extremely sharp with a band width of 4.95 cm-1. The Raman spectrum of torbernite shows bands in similar positions to that of metatorbernite. This is not unexpected since both minerals only differ in the degree of hydration. The assignment of this band may be given to the symmetric stretching mode of the (UO2)2+ stretching mode. The problem arises as to where the PO4 symmetric stretching mode may be found. Normally the band would be observed at around 935 cm-1. This band should be intense in the Raman spectrum. One possibility is that the symmetric stretching modes of the (UO2)2+ and PO4 symmetric stretching modes are coupled in some way such that the intense band at 827 cm-1 is that of both units. Similarly for saléeite two bands are observed at 833 and 818 cm-1. There is a small shift in obtaining the spectra at 77 K. The bands are observed at 837 and 818 cm-1. Raman bands are observed at 829 and 819 cm-1 for sabugalite. The Raman spectrum in this for metazeunerite is different (Figure 13). Two bands are observed at 819 and 809 cm-1. The first band is assigned to the (UO2)2+

stretching vibration and the second to the AsO4 symmetric stretching mode. It is apparent that by the comparison of the spectra of uranyl phosphates and arsenates a second band is observed on either side of the 819 cm-1 band. For phosphates the band is at a higher wavenumber than 819 cm-1 and for arsenates the band is at lower wavenumbers than 819 cm-1. Bending vibrations of the XO4 units The low wavenumber region of the uranyl micas are illustrated by Figures 14, 15 and 16. These figures show the low wavenumber region of metautunite, metatorbernite and metazeunerite. The lower wavenumber region displays the bending modes of the PO4, AsO4 and (UO2)2+ units. For the free anions, the phosphate ion will show ν2 modes at around 420 cm-1 and ν4 modes at around 567 cm-

1; similarly ν2 modes for AsO4 will be around 350 cm-1 and ν4 at around 463 cm-1; The bending modes of (UO2)2+ units are found in the 250 to 300 cm-1 range. Thus providing there is not too much anion isomorphic substitution, bands for the different anions should be well separated. The one possible overlap of bands is for that of ν2 of PO4 with ν4 of AsO4.

The Raman spectrum of autunite at 298 K shows a single broad band at 629 cm-1 which at 77 K can be resolved into bands at 624, 596, 552 and 533 cm-1. These bands are assigned to the PO4 ν4 bending modes. In the 298 K spectrum bands which may be assigned to the ν2 bending modes are observed at 464, 439, 406 and 399 cm-1. These bands are observed at 437, 403, 383, 343 and 329 cm-1 in the 77 K spectra. Two bands of considerable intensity are observed at 291 and 222 cm-1 and are assigned to the bending modes of (UO2)2+. These bands are observed at 291 and 238 cm-1 in the 77 K spectrum. For metautunite, Raman bands are observed at 643 and 507 cm-1 in both the 298 and 77 K spectra and are assigned to the ν4 bending modes. The ν2 bending region of metautunite appears less complex and bands are observed at 453 and 387 cm-1. A second metautunite sample from The Cunha Baixa Mine, Beira Alta province showed bands at 450, 385 and 369 cm-1. As for autunite strong Raman bands are observed at 263 and 222 cm-1 for metautunite and are attributed to (UO2)2+ bending modes. The Raman spectrum of torbernite shows a broad band at 629 cm-1 in the 298 K spectrum which may be curve resolved into bands at 668, 624, 576, 525 and 502 cm-1 in the 77 K spectrum. For the ν2 bending region bands are observed at 464, 439, 406 and 399 cm-1 in the 298 K spectrum and at 464, 443, 407 and 399 cm-1 in the 77 K spectrum. In the very low wavenumber region of torbernite at 298 K two bands are observed at 290 and 222 cm-1. At 77 K this region shows much greater complexity with bands observed at 313, 295, 286, 256 and 229 cm-1. In comparison the Raman spectra of metatorbernite bands are observed at 630, 529 and 508 cm-1 in the 298 K spectrum and at 668, 624, 550, 524 and 502 cm-1 in the 77 K spectrum. For the ν2 bending region bands are observed at 463, 440, 406 and 399 cm-1 in the 298 K spectrum and at 464, 443, 407 and 399 cm-1 in the 77 K spectrum. The spectrum of torbernite and metatorbernite are identical in this part of the spectrum. One possibility is that the torbernite has converted to metatorbernite in the laser excitation. Bands are observed at 288 and 222 cm-1 for metatorbernite at 298 K which become band separated into bands at 313, 292, 258 and 229 cm-1 at 77 K. In contrast the Raman spectrum of metazeunerite in the low wavenumber region shows bands at 449 and 398 cm-1 in the 298 K spectrum. These bands are band separated into components at 567, 456 and 394 cm-1 in the 77 K spectrum. The band at 567 cm-1 may be due to some minor phosphate isomorphic substitution. The bands at 449 and 398 (298 K) and at 456 and 394 cm-1 (77 K) are attributed to the ν4 AsO4 bending modes. The Raman spectrum of a second zeunerite sample gave bands at 482, 459, 438 and 403 cm-1. Intense Raman bands are found at 320 cm-1 and are attributed to the ν2 AsO4 bending modes. In the very low wavenumber region, three bands are observed in the 298 K spectrum at 276, 240 and 218 cm-1 and in the 77 K spectrum at 283, 265, 248 and 225 cm-1. A second zeunerite sample provided Raman spectra with bands at 286, 254 and 228 cm-1. CONCLUSIONS

Raman spectroscopy has proven most useful for the attribution of the spectra of the uranyl micas known as autunites. Raman spectroscopy avoids the difficulties in the infrared spectra of complex overlapping bands. One of the problems associated

with the collection of Raman data of autunites is their susceptibility to decomposition due to heating. In this work powers of less than 1 mW were used, together with some defocusing of the laser. Heating will cause partial dehydration and may convert the autunite to meta-autunite or a mineral with less water molecules in the structure. Collection of spectral data at 77 K helps avoid these difficulties.

It is envisaged that the following reactions (for example) might occur:

Ca(UO2)2(PO4)2.12H2O (autunite) →Ca(UO2)2(PO4)2.8H2O (metautunite)

Ca(UO2)2(PO4)2.8H2O (metautunite) →Ca(UO2)2(PO4)2.xH2O (partially dehydrated metautunite)

The bands for the vibrations of the PO4 and AsO4 units as well as the (UO2)2+ units are sharp with small bandwidths. Bands were identified at 900 and 818 cm-1 and assigned to the anti-symmetric and symmetric stretching modes of the (UO2)2+ units. Intense Raman bands were observed at around 285 and 222 cm-1 and were attributed to the antisymmetric and symmetric stretching modes of the (UO2)2+ units. One of the great advantages of Raman spectroscopy is the excellent band separation. This means that the bands due to the symmetric stretching modes of (UO2)2+ and PO4 or AsO4 can be readily obtained. Acknowledgment

The financial and infra-structure support of the Queensland University of Technology Inorganic Materials Research Program of the School of Physical and Chemical Sciences is gratefully acknowledged. The Australian Research Council (ARC) is thanked for funding.

Prof. Allan Pring, (Principal Curator of Minerals, South Australian Museum, North Terrace Adelaide, South Australia 5000) is thanked for the loan of some of the autunite minerals. Mr Dermot Henry of Museum Victoria is thanked for the loan of a collection of autunites and metautunites.

REFERENCES [1]. P. Burns, Reviews in mineralogy Vol 38 38 (1999) 23. [2]. H. Isobe, R. C. Ewing and T. Murakami, Materials Research Society

Symposium Proceedings 333 (1994) 653. [3]. A. J. Locock and P. C. Burns, American Mineralogist 88 (2003) 240. [4]. J. Cejka, Jr., A. Muck and J. Cejka, Physics and Chemistry of Minerals 11

(1984) 172. [5]. J. Cejka, J. Cejka, Jr. and A. Muck, Thermochimica Acta 86 (1985) 387. [6]. M. E. Zolensky, 1983. [7]. P. C. Burns, M. L. Miller and R. C. Ewing, Canadian Mineralogist 34 (1996)

845. [8]. J. Cejka, Reviews in mineralogy 38 (1999). [9]. J. Cejka, Jr., A. Muck and J. Cejka, Neues Jahrbuch fuer Mineralogie,

Monatshefte (1985) 115.

[10]. J. Cejka, Z. Urbanec, J. Cejka, Jr., J. Ederova and A. Muck, Journal of Thermal Analysis 33 (1988) 395.

[11]. V. C. Farmer, Mineralogical Society Monograph 4: The Infrared Spectra of Minerals, 1974.

[12]. J. Emsley, Chemical Society Reviews 9 (1980) 91. [13]. H. Lutz, Structure and Bonding (Berlin, Germany) 82 (1995) 85. [14]. W. Mikenda, Journal of Molecular Structure 147 (1986) 1. [15]. A. Novak, Structure and Bonding (Berlin) 18 (1974) 177. [16]. E. Libowitsky, Monatschefte fÜr chemie 130 (1999) 1047.

LIST OF FIGURES Figure 1 Polarised and depolarised Raman spectra of the OH stretching region

of autunite Figure 2 Polarised and depolarised Raman spectra of the (UO2)2+ and PO4

stretching region of autunite Figure 3 Raman spectra of the OH stretching region of water in autunite at 298

and 77 K. Figure 4 Raman spectra of the OH stretching region of water in torbernite at 298

and 77 K. Figure 5 Raman spectra of the OH stretching region of water in metatorbernite

at 298 and 77 K. Figure 6 Raman spectra of the OH stretching region of water in saléeite at 298

and 77 K. Figure 7 Raman spectra of the OH stretching region of water in metazeunerite at

298 and 77 K. Figure 8 Raman spectra of the 750 to 1100 cm-1 region of autunite at 298 and 77

K. Figure 9 Raman spectra of the 750 to 1100 cm-1 region of metautunite at 298 and

77 K. Figure 10 Raman spectra of the 750 to 1100 cm-1 region of metatorbernite at 298

and 77 K. Figure 11 Raman spectra of the 750 to 1100 cm-1 region of saléeite at 298 and 77

K. Figure 12 Raman spectra of the 750 to 1100 cm-1 region of zeunerite at 298 and

77 K. Figure 14 Raman spectra of the low wavenumber region of autunite at 298 and

77 K. Figure 15 Raman spectra of the low wavenumber region of zeunerite at 298 and

77 K.

LIST OF TABLES Table 1 Results of the Raman spectroscopic analysis of selected autunites and

meta-autunites Part 1 Table 2 Results of the Raman spectroscopic analysis of selected autunites and

meta-autunites Part 2

Table 1 Results of the Raman spectroscopic analysis of selected autunites and

meta-autunites Part 1 Autunite Metautunite torbernite metatorb

ernite 42016 27677 27680 46683 39006

Raman 298 K

Raman77 K

polarised

Raman 77 K

depolarised

Raman 298 K

Raman77 K

Raman 77 K

Raman 298 K

Raman77 K

Raman298 K

3511 3470 3268

3523 3489 3432 3400 3343 3262 3236 3077

3524 3490 3428 3400 3346 3264 3236

3508 3456 3244

3582 3510 3447

3307 3219

3518 3497 3441 3393 3327 3226

3359 3197 3032

3396 3338 3296 3166 3070 2992 2919

3362

3200 3020

1090 1018 1007 988

1009 998 989

1007 998 990

1093 1033 1018 1007 989

1093 1039 1022 1013 994 987

1112 1038 1022 1011 993 985

1004 995 988 957

1006 999 991

1005 996 988

915 915 890 890 900 901 900

833 822 816

843 820 739 673

842 820

850 833 818

838 832 818 782

837 818

826 808

827 838 826 817

629 624 596 552 533

Not measured

643 507

643 507

660 634 554

629 668 624 576 525 502

630 529 508

464 439 406 399

437 403 383 343 329

453

387

453

387

450

385 369

464 439 406 399

464 443 407 399

463 440 406 399

291 222

291 254 238

263 222

263 222

302 286 259 220

290 222

313 295 286 256 229

288 222

190 190 187

Table 2 Results of the Raman spectroscopic analysis of selected autunites and meta-autunites Part 2

saléeite sabugalite metazeun

erite

38558 456743 D40688 34627 Raman 298 K

Raman77 K

Raman 77 K

Raman 298 K

Raman 77 K

Raman 298 K

Raman 77 K

Raman 77 K

3512 3488

3296

3522 3489 3408

3340 3264 2979

3521 3488 3429 3397 3341 3263

3371 3238 3136

3350 3287 3191 3128 2952

3434 3373 3378 3288 3167 3055 2993 2923

1074 1015 1007 988 982

1005 999 988

1040 1017 992

1084 1017 994 975

1029 1012 990

910 888

892 884

847 833 818

843 840

837 819

835 829 819

832 829

819 809 793

821 812 797

643 624 597 551 533

Not measured

617 671 680 667 625 552 525

389 284

439 405 381 344 330

450 401 398

449 398

467 456 394

482 459 438 403

320 320 315 306

291

265 252 238

300 250

276 240 218

283 265 248 225

286 254 228

0

0.005

0.01

0.015

0.02

0.025

0.03

2900 3000 3100 3200 3300 3400 3500 3600 3700

Wavenumber/cm-1

Ram

an In

tens

ity

Polarised

Depolarised Autunite

3523

348934323400

33433262

3236

Figure 1

0

0.02

0.04

0.06

0.08

0.1

0.12

800 850 900 950 1000 1050

Wavenumber/cm-1

Ram

an In

tens

ity

Polarised

Depolarised

Autunite

1007

998

990

842

820

998

990

Figure 2

0

0.0005

0.001

0.0015

0.002

2900 3100 3300 3500 3700

Wavenumber/cm-1

Ram

an In

tens

ity

298 K

77 K

Autunite

Figure 3

0

0.001

0.002

0.003

0.004

0.005

0.006

0.007

270029003100330035003700

Wavenumber/cm-1

Ram

an In

tens

ity

298 K

77 KTorbernite

Figure 4

0

0.0005

0.001

0.0015

0.002

0.0025

260028003000320034003600

Wavenumber/cm-1

Ram

an In

tens

ity

298 K

77 K

Metatorbernite

Figure 5

0

0.0002

0.0004

0.0006

0.0008

0.001

0.0012

2500 2700 2900 3100 3300 3500 3700

Wavenumber/cm-1

Ram

an In

tens

ity

77 KSaléeite

298 K

Figure 6

0

0.0005

0.001

0.0015

0.002

0.0025

0.003

0.0035

260028003000320034003600

Wavenumber/cm-1

Ram

an In

tens

ity

298 K

77 K

Metazeunerite

Figure 7

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

750800850900950100010501100

Wavenumber/cm-1

Ram

an In

tens

ity

298 K

77 K Autunite

Figure 8

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0.045

0.05

750800850900950100010501100

Wavenumber/cm-1

Ram

an In

tens

ity

298 K

77 K Metautunite

Figure 9

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

75080085090095010001050

Wavenumber/cm-1

Ram

an In

tens

itymetatorbernite

298 K

77 K

Figure 10

0

0.01

0.02

0.03

0.04

0.05

0.06

70080090010001100

Wavenumber/cm-1

Ram

an In

tens

ity

298 K

77 K

Saléeite

Figure 11

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

750800850900950

Wavenumber/cm-1

Ram

an In

tens

ity

298 K

77 K

Metazeunerite

Figure 12

0

0.001

0.002

0.003

0.004

0.005

0.006

0.007

200300400500600700

Wavenumber/cm-1

Ram

an In

tens

ityMetatorbernite

298 K

77 K

Figure 14

0

0.0002

0.0004

0.0006

0.0008

0.001

0.0012

0.0014

200300400500600700

Wavenumber/cm-1

Ram

an In

tens

ityMetautunnite

298 K

77 K

Figure 13

0

0.001

0.002

0.003

0.004

0.005

200300400500600700

Wavenumber/cm-1

Ram

an In

tens

ity

298 K

77 K

Metazeunerite

Figure 15