An infrared and Raman Spectroscopic Study of the uranyl micas

Ray L. Frost• Inorganic Materials Research Program, School of Physical and Chemical Sciences, Queensland University of Technology, GPO Box 2434, Brisbane Queensland 4001, Australia. Published as: Frost Ray, L., An infrared and Raman spectroscopic study of the uranyl micas. Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy, 2004. 60(7): p. 1469-80. Copyright 2004 Elsevier Abstract: Vibrational spectroscopy using a combination of infrared and Raman spectroscopy has been used to study the uranyl micas also known as the autunite minerals, 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. Included in these minerals are autunite, metautunite, torbernite, metatorbernite, metazeunerite, saléeite and sabugalite. Compared with the results of infrared spectroscopy, Raman microscopy shows excellent band separation enabling the separation and identification of bands attributed to (UO2)2+ units, PO4 and AsO4 units. Common to all spectra were bands at around 900 and 818 cm-1, attributed to the antisymmetric and symmetric stretching vibrations of the (UO2)2+ units. Water in autunites is in a highly structured arrangement in the interlayer of the uranyl micas. Water molecules are differentiated according to the strength of the hydrogen bonds formed between the water and the adjacent uranyl-phosphate or uranyl-arsenate surfaces and the hydration sphere of the interlayer cation. Keywords: autunite, metautunite, arsenate, phosphate, vanadate, Raman spectroscopy 1. Introduction The minerals known as autunites have been studied for some considerable time [1-3]. Many of these minerals are found in Australia. 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) and X is As, or P or V. These minerals are often known as uranium micas. Autunites are common minerals, yet have been rarely studied. The minerals have a layer-like structure [4-6]. 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

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

Ca[(UO2)2(PO4)]2.12(H2O). 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 [7]. 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.

One characteristic of the minerals is the variability of the degree of hydration. Many of these minerals contain varies amounts of water depending upon the temperature and vapour pressure. A characteristic feature of these minerals is the variability in water content. This variability is no doubt related to the hydration of the cations in the interlayer space. A second form of these minerals known as the meta-autunites simply have less water in the molecule. Normally the number of molecules of water is reduced from 12 or some number approaching 12 to 8 molecules in the formula unit [8, 9] . Walenta found that members of the torbernite-meta-torbernite group have the same basic composition but differ in H2O content [9]. The highest hydration stage was found with 12H2O in uranocircite I, followed by uranocircite II with 10 H2O, meta-uranocircite I with 8 H2O, and meta-uranocircite II with 6 H2O. The 1st 2 stages are very unstable and are rapidly converted to meta-uranocircite I at normal temperatures and in dry atmospheres. The transformations were found to be reversible. The transformation of uranocircite II in meta-uranocircite I may be considered as irreversible. 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 application of vibrational spectroscopy to the study of these minerals has been lacking. There have been several definitive infrared works [4, 6]. A Raman spectrum of metatorbernite has been published [4]. Such an observation of the lack of vibration spectroscopic studies in particular Raman spectroscopy is difficult to understand when this group of minerals all contain oxyanions which lend themselves to vibrational spectroscopy [4]. Farmer reported the infrared spectral results of some autunite minerals [10]. The ν1 mode of PO4 in the phosphate autunites 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. 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 ν2 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 [11] . The ν2 bands of the (UO2)2+ unit were found at 298 and 254 cm-1. Herein lies the difficulty in that both the ν1 bands of PO4 and (UO2)2+ are found at the same spectral positions making interpretation by infrared spectroscopy difficult. Many of these minerals can be readily synthesised. Some infrared spectroscopic measurements have been undertaken [4, 5, 12]. However the interpretation of the infrared spectra is difficult to interpret because of overlapping bands [13]. The infrared spectra of many of the

minerals have been determined, without interpretation or analysis, and only to form an IR spectroscopic data base [14] [15, 16]. Some infrared spectra of the water molecules in autunites has been determined and related to the crystal structures [17] . In this paper we report a comparison of the infrared and Raman spectra of selected autunites and meta-autunites and relate the spectra to the autunite structures.

2. Experimental 2.1 Minerals:

The following minerals were obtained from 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 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

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 microprobe spectroscopy

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.

Infrared spectroscopy

Infrared spectra were obtained using a Nicolet Nexus 870 FTIR spectrometer with a smart endurance single bounce diamond ATR cell. Spectra over the 4000 to 525 cm-1 range were obtained by the co-addition of 64 scans with a resolution of 4 cm-1 and a mirror velocity of 0.6329 cm/s.

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.

3. Results and discussion The infrared and Raman spectra of the hydroxyl stretching region of autunite is shown in Figure1. The results of the band component analysis are reported in Table 1. Three bands are observed in the infrared spectra at 3566, 3444 and 3248 cm-1. Two low intensity bands are observed at around 2900 cm-1 and are considered to be organic impurities. The Raman spectrum of the hydroxyl stretching region also displays three bands at 3511, 3470 and 3268 cm-1. Previously published data on the infrared spectrum of autunite gave two bands at 3435 and 2935 cm-1[10]. Although the first band is in some agreement with the band observed at 3444 cm-1 in this work the second band does not. The problem of studying the uranyl micas as suggested above is the indeterminate nature of the moles of water in the formula unit. For example autunite has 12 moles whilst metautunite 8 moles. A comparison of the infrared and Raman spectrum of metautunite is given in Figure 2. Infrared bands are observed at 3569, 3464, 3279 and 3054 cm-1. The pattern strongly resembles that for autunite. Two infrared bands are observed at 3620 and 3695 cm-1 and are assigned to the hydroxyl stretching vibrations of kaolinite impurity. The low intensity bands at around 2900 cm-1 are also ascribed to impurity of an organic nature. The intensity of the band at 3279 cm-1 has increased in intensity in going from autunite to metautunite. This observation suggests the number of moles of water with stronger hydrogen bonding has increased. A previous study of the infrared spectrum of metautunite gave a band at 3480 cm-1. The position of this band corresponds to the maximum in the profile of our infrared spectrum. The infrared and Raman spectra of the hydroxyl stretching region of torbernite and metatorbernite are shown in Figures 3 and 4. The infrared spectrum of torbernite displays intense bands at 3339 and 2920 cm-1 with other lower intensity bands at 3418 and 3295 cm-1. Raman bands in the hydroxyl stretching region are observed at 3359, 3197 and 3032 cm-1. What is clearly observed is that the infrared and Raman bands are mutually exclusive; suggesting that the infrared bands are the antisymmetric OH stretching vibrations and the Raman is the OH symmetric stretching vibrations. Thus it is possible that we are observing three different water molecules in both the infrared and Raman spectra. The three water molecules differ in that the OH…O hydrogen

bond distances differ accordingly. Farmer reported the infrared spectrum of torbernite with bands in the hydroxyl stretching region at 3570, 3430 and 2940 cm-1 [10]. The two bands at 3418 and 2920 cm-1 are in agreement with these previously reported data. The infrared spectrum of metatorbernite shows strong resemblance to that of torbernite. Infrared bands are observed in the OH stretching region at 3413, 3339, 3295 and 2920 cm-1. A previous study gave a broad band centred upon 3360 cm-1 with a second band at 2930 cm-1 [4]. The values are in reasonable agreement with our data. Similarly the Raman spectrum of metatorbernite shows similarity to that of torbernite. Raman bands in the hydroxyl stretching region may be observed at 3508, 3455 and 3244 cm-1. The bands are however observed in different positions. The difficulty with the study of the vibrational spectra of these types of minerals is the degree of hydration. Some of these minerals including torbernite are known to dehydrate or partially dehydrate in the atmosphere depending upon the external conditions such as water vapour pressure, temperature and air flow. The effect of infrared absorption on the minerals is not known as is also the effect of laser beams with high power density on the minerals. It is possible that sample degradation of the minerals can occur during measurement. The infrared and Raman spectra of saléeite are shown in Figure 5. The infrared spectra show considerable complexity with a band profile made up of several overlapping bands. Infrared bands are observed at 3574, 3508, 3425, 3335, 3255 and 2968 cm-1. A previous study suggested a broad band centred upon 3450 cm-1 [4]. The Raman spectrum is much simpler with bands observed at 3512, 3488 and 3296 cm-1. The results suggest that some of the infrared bands are Raman inactive. The infrared and Raman spectra of the hydroxyl stretching region metazeunerite are shown in Figure 6. The infrared spectrum shows bands at 3340, 3275 and 2925 cm-1. Additional bands are observed at 3620 and 3695 cm-1 and are attributed to kaolinite impurity. In the Raman spectrum bands are observed at 3371, 3238 and 3136 cm-1. The infrared band at 2925 cm-1 is apparently Raman inactive. HOH bending vibrations

The presence of water in the uranyl mica structure is important and it’s bonding with [(UO2)(PO4)] surface is of necessity for the stability of the autunite layered structure. The region from around 1600 to 1750 cm-1 is a window, which is clear of other possible vibrations and is unique in that this is the region where the HOH bending modes of water are found. Minerals containing physically adsorbed water give strong infrared bands at ~3550 cm-1, the water hydroxyl stretching vibration, and at ~ 1630 cm-1, the water bending vibrations. These frequencies are influenced by the amount of adsorbed water, the mineral type and the exchangeable cation to which the water is bonded. For monomeric non-hydrogen bonded water as occurs in the vapour phase, these bands are found at 3755 and 1595 cm-1. For liquid water the bands occur at 3455 and 1645 cm-1 and for water molecules in ice, the bands are at 3255 and 1655 cm-1. When water molecules are very tightly bound to the mineral surface as may occur with autunites, then bands occur in the 3200 to 3250 cm-1 region. What is being distinguished here is the formation of strong and weak hydrogen bonds. The hydroxyl stretching modes of weak hydrogen bonds occur in the 3580 to 3500 cm-1 region and the hydroxyl stretching modes of strong hydrogen bonds occurs below 3420 cm-1. When the water is coordinated to the cation in the

clays as occurs in certain minerals, then the water OH stretching frequency occurs at 3220 cm-1. A simple observation can be made that as the water OH stretching frequency decreases then the HOH bending frequency increases. The 3220 cm-1 band corresponds to an ice-like structure with O-H….O bond distances of 2.77 Å. Thus the water hydroxyl stretching and the water HOH bending 1610 cm-1 frequencies provide a measure of the strength of the bonding of the water molecules either chemically or physically to the autunite surfaces or to the interlayer anions. Likewise the position of the water bending vibration also provides a measure of this strength of water hydrogen bonding. Bands that occur at frequencies above 1650 cm-1 are indicative of coordinated water and chemically bonded water. Bands that occur below 1630 cm-1 are indicative of water molecules that are not as tightly bound. In this case the hydrogen bonding is less as the wavenumber decreases. Much information about the behaviour of water in the uranyl mica minerals can be obtained from the study of the water HOH bending region centred upon 1630 cm-1. The infrared spectra of this region for selected uranyl micas are shown in Figure 7. The analysis of the spectral data is recorded in Table 1. The spectra clearly show complex patterns with often more than one band observed in the spectral profile. Importantly bands are observed at high wavenumbers in the 1650 to 1680 cm-1 region, indicating the water molecules are very strongly hydrogen bonded to both the

[(UO2)(PO4)]- surface and to other water molecules in the hydration sphere of the

cation. The IR spectrum of the HOH deformation region of autunite shows three bands observed at 1660, 1631 and 1600 cm-1. It should be noted that three bands were also observed in the hydroxyl stretching region (at 3566, 3444 and 3248 cm-1). This simply means that there is water molecules involved in three different hydrogen bonded states. The IR spectrum of the second autunite shows even more complexity with bands observed at 1678, 1658, 1630 and 1595 cm-1. The difference in the spectra in the HOH deformation region is significant. It means that not all autunites are the same. Even though the autunites may have the same chemical formula, the arrangement of cations and water in the interlayer between the [(UO2)(PO4)] sheets are different. The HOH deformation spectrum of metautunite is similar to that of autunite with bands observed at 1660 and 1630 cm-1. It is apparent that the reduction of the number of water molecules from 12 to 8 does not change the spectral profile in this region.

The IR spectra of the two torbernites are very similar. Two bands are observed at around 1668 and 1623 cm-1. The spectrum of metatorbernite is less complex with a single band observed at 1643 cm-1. The spectrum of saléeite from the South Alligator River, NT, Australia shows a single band at 1630 cm-1; whereas the spectrum of the saléeite from Jabiru, NT, Australia shows more complexity with bands observed at 1679, 1658 and 1620 cm-1. The conclusion is reached that the arrangement of water molecules between the [(UO2)(PO4)] sheets is different for the different saléeite [18, 19]. Such a conclusion was also reached for other layered structures such as hydrotalcites. Hydrotalcites consist of modified brucite layers with anions and water molecules between the layered double hydroxide sheets [18, 19]. The infrared spectrum of metazeunerite in the water bending region shows an intense band at 1645 cm-1. PO4 and (UO2)2+ stretching region

One of the difficulties associated with the vibrational spectroscopy of autunites is the overlap of bands resulting from the stretching vibrations of the PO4 and (UO2)2+ units. Such complexity is illustrated by the infrared and Raman spectra of the stretching vibrations of the PO4 and (UO2)2+ units of autunite as shown in Figure 8 and subsequent figures. The results of the band component analyses are reported in Table 1. What is readily apparent is that the Raman spectra are less complex with more well separated bands.

It has been reported that the (UO2)2+ bands occur at 915, 810, 298 and 254 cm-1 [4]. 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 [5, 6] . The ν2 bands of the (UO2)2+ units were found at 298 and 254 cm-1. These assignments differ from that reported by Farmer [10]. Phosphate vibrations are well known. In aqueous systems, Raman spectra of phosphate oxyanions show a symmetric stretching mode (ν1) at 938 cm-1, the antisymmetric stretching mode (ν3) at 1017 cm-1, the symmetric bending mode (ν2) at 420 cm-1 and the ν4 mode at 567 cm-1. The pseudomalachite vibrational spectrum consists of ν1 at 953, ν2 at 422 and 450 cm-1, ν3 at 1025 and 1096 and ν4 at 482, 530, 555 and 615 cm-1 [20, 21]. Libethenite vibrational modes occur at 960 (ν1), 445 (ν2), 1050 (ν3) and 480, 522, 555, 618 and 637cm-1 (ν4) [20, 21] . The infrared spectrum of autunite shows a complex set of overlapping bands. Infrared bands are observed at 1118, 1074, 985, 916, 888 and 821 cm-1. The first three bands are assigned to the infrared antisymmetric stretching vibrations, the band at 916 cm-1 may be attributed to the PO4 symmetric stretching mode. An alternative and more likely assignment is to the ν3 vibrational mode of the (UO2)2+ units. The band at 888 cm-1 may be assigned to a water librational mode. The high wavenumber position of this band is not unexpected because the strong hydrogen bonding of the water molecules in the interlayer of the autunites. Such a concept is supported by the lack of any intensity in this region in the Raman spectrum. The Raman spectrum of water librational modes is inherently weak. The Raman spectrum of autunite shows a set of low intensity bands at 1090, 1018, 1007 and 988 cm-1. These bands are attributed to the PO4 antisymmetric stretching vibrations. Two intense bands are observed in the Raman spectrum of autunite at 833 and 822 cm-1. One possible assignment is that the first band is due to the PO4 symmetric stretching mode and the second to the (UO2)2+ symmetric stretching modes. The position of a PO4 symmetric stretching mode at 833 cm-1 is very low. A comparison may be made with the position of the band in other phosphate minerals around 935 cm-1. One possible explanation is that the vibrations of the PO4 and (UO2)2+ units are coupling. The infrared and Raman spectra of metautunite is shown in Figure 9. A comparison may be made with the spectra for autunite (Figure 8). The infrared spectrum of metautunite shows subtle differences from that of autunite. Infrared bands are observed at 1118, 1048, 983, 917, 892 and 810 cm-1. A previous study reported infrared bands at 1120 and 1000 cm-1. These bands are in reasonable agreement with the results of this work considering that a complex profile has been band deconvoluted. The assignment of these bands is as for autunite. The Raman spectra show bands at 1093, 1033, 1018, 1007 and 989 cm-1, all of which are assigned to the PO4 antisymmetric stretching modes. Two intense bands are observed at 833

and 818 cm-1, the attribution of these bands is as for autunite. If the band at 833 cm-1 is not due to the PO4 symmetric stretching vibration, the question arises as to which band is the PO4 symmetric stretching vibration. The infrared and Raman spectra of torbernite and metatorbernite are shown in Figures 10 and 11. The infrared spectrum of torbernite displays a complex set of overlapping bands in the 800 to 1200 cm-1 region. Infrared bands are observed at 1128, 1100, 1042 and 985 cm-1, all of which are attributed to ν3 antisymmetric PO4 stretching modes. The most intense band is observed at 910 cm-1. This band is probably attributable to the ν3 antisymmetric stretching modes of (UO2)2+ units. Such an assignment would be in agreement with the assignations of Cejka et al. [4]. Other infrared bands are observed at 895, 782 and 681 cm-1. These three bands may be attributed to water librational modes. The Raman spectra of torbernite show bands at 1004, 995, 988 and 957 cm-1, all of which are attributed to the antisymmetric stretching modes. A low intensity band is observed at 900 cm-1 and is assigned to the ν3 antisymmetric stretching mode of (UO2)2+ units. A low intensity band is found in this position in nearly all of the Raman spectra of the uranyl micas. This band corresponds to the most intense band in the infrared spectrum of this region. The most intense band in the Raman spectrum is observed at 826 cm-1. The band is highly polarised. The infrared spectrum of metatorbernite shows bands at 1104, 1022 and 978 cm-1. These bands are assigned to the PO4 ν3 antisymmetric stretching modes. Published data gave bands at 1100, 990 and 930 cm-1 [4]. Two of these bands are in agreement with our data. A low intensity band is found at 933 cm-1 which may be due to the PO4 symmetric stretching mode. The difficulty is that the band is not observed in the Raman spectrum. The most intense band in the infrared spectrum in this region is observed at 902 cm-1 and is assigned to the ν3 antisymmetric stretching vibration of the (UO2)2+ units. Another intense band is observed at 865 cm-1. Other bands are observed at 787, 755 and 675 cm-1. In the Raman spectrum of metatorbernite, bands are observed at 1005, 996 and 988 cm-1 and are ascribed to the PO4 antisymmetric stretching vibrations. In the Raman spectrum of metatorbernite the most intense band is observed at 826 cm-1. This band is attributed to the coupled symmetric stretching vibration of PO4 and (UO2)2+ units. The infrared and Raman spectrum of saléeite are shown in Figure 12. The spectra show a resemblance to that of autunite which is perhaps not unexpected. Infrared bands are observed at 1117, 1054 and 985 cm-1 which compare favourably with the published band positions at 1115 and 1000 cm-1 [4]. In addition two intense absorption bands are observed at 918 and 900 cm-1. These bands are assigned to the ν3 antisymmetric stretching modes of (UO2)2+ units. In the Raman spectrum of saléeite a complex set of overlapping bands are observed at 1074, 1015, 1007, 988 and 982 cm-1, all of which are assigned to the PO4 antisymmetric stretching vibrations of (UO2)2+ units. The Raman and infrared spectra of metazeunerite is shown in Figure 13. A set of low intensity bands arte observed at 1100, 1047, 1031 and 1006 cm-1. These bands are attributed to PO4 symmetric stretching modes. It should be kept in mind that zeunerite is the AsO4 equivalent of torbernite (Cu[(UO2)2(PO4)]2.12(H2O). Metazeunerite thus will be Cu[(UO2)2(AsO4)]2.8(H2O) . Hence no phosphate bands should be found for a pure metazeunerite. Thus what is being determined is the isomorphic substitution of arsenate by phosphate. The symmetric stretching vibration

of the aqueous arsenate anion (ν1) is observed at 810 cm-1 and coincides with the antisymmetric stretching mode (ν3). The bending modes (ν2) and (ν4) are observed at 342 cm-1 and at 398 cm-1 respectively. In solids the antisymmetric stretching mode (ν3) can occur at higher wavenumbers ~880 cm-1 [22, 23]. In the infrared spectrum of metazeunerite an intense band is observed at 924 cm-1 and is assigned to the antisymmetric stretching mode of (UO2)2+ units. Bands are observed at 880, 845, 803, 785, 767 and 684 cm-1. It is possible that the first two bands are due to the AsO4 antisymmetric stretching modes. In the Raman spectrum two bands are observed at 910 and 888 cm-1. The first band is of low intensity and is ascribed to the ν3 antisymmetric stretching vibration of (UO2)2+ units. The second band is likely to be the Raman band of the ν3 antisymmetric stretching modes. In the Raman spectrum two bands are observed at 819 and 809 cm-1. The first band is attributed to the ν1 symmetric stretching vibration of the (UO2)2+ units and the second band is assigned to the ν1 symmetric stretching mode of the AsO4 units. No Raman bands were observed in the 980 to 1100 cm-1 region. 4. Conclusion

Vibrational spectroscopy has proven most useful for the assignment of bands in the spectra of the uranyl micas also known as autunites. Raman spectroscopy avoids the difficulties in se 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. Heating will cause partial dehydration and may convert the autunite to meta-autunite or a mineral with less water molecules in the structure. Such experimental effects require further research. 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 Raman 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. One of the great advantages of Raman spectroscopy is the narrow bandwidths. This means that the bands due to the symmetric stretching modes of (UO2)2+ and PO4 or AsO4 can be separated. Acknowledgments

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. Mr M. Weier is thanked for collecting some of the spectral data. Mr Dermot Henry of Museum Victoria is thanked for a collection of autunites and metautunites.

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LIST OF FIGURES Figure 1 Infrared and Raman spectra of the hydroxyl stretching region of autunite Figure 2 Infrared and Raman spectra of the hydroxyl stretching region of metautunite Figure 3 Infrared and Raman spectra of the hydroxyl stretching region of torbernite Figure 4 Infrared and Raman spectra of the hydroxyl stretching region of

metatorbernite Figure 5 Infrared and Raman spectra of the hydroxyl stretching region of saléeite Figure 6 Infrared and Raman spectra of the hydroxyl stretching region of

metazeunerite Figure 7 Infrared spectrum of the water HOH bending region of autunite, metautunite,

torbernite, metatorbernite and saléeite. Figure 8 Infrared and Raman spectra of the PO4 and (UO2)2+ stretching region of

autunite Figure 9 Infrared and Raman spectra of the PO4 and (UO2)2+ stretching region of

metautunite Figure 10 Infrared and Raman spectra of the PO4 and (UO2)2+ stretching region of

torbernite Figure 11 Infrared and Raman spectra of the PO4 and (UO2)2+ stretching region of

metatorbernite Figure 12 Infrared and Raman spectra of the PO4 and (UO2)2+ stretching region of

saléeite. Figure 11 Infrared and Raman spectra of the PO4 and (UO2)2+ stretching region of

metazeunerite.

LIST OF TABLES Table 1 Infrared and Raman spectral data of selected autunites and metautunites.

Table 1 Infrared and Raman spectral data of selected autunites and metautunites. Autunite Metautunite torbernite metatorbernit

e

IR Raman IR published

[10]

IR Raman IR published

[4]

IR Raman IR Raman IR published

[4] 3566 3444 3248

3511 3470 3268

3400 3569 3464 3279 3054

3400 3418 3339 3295 2920

3359 3197 3032

3413 3340 3298 2919

3508 3455 3244

3360 2930

1660 1630 1595

1640 1660 1630 1590

1640 1668 1623 1595

1645 1645

1118 1074

985

1090 1018 1007 988

1123 1023

1118 1048

983

1093 1033 1018 1007 989

1120 1000

1128 1100 1042

985

1004 995 988 957

1104

1022 978 931

1005 996 988

1100 990 930

916 915 917 915 915 910 902 910 888 892 890 895 900 865 900 821 833

822 816

820

810 745

850 833 818

820

782 681

826 808

787 755 675

838 826 817

845 800

680

629 540 643 507

540 605 629 630 630 529

615 550

508 464

439 406 399

465 453

387

465 464 439 406 399

463 440 406 399

465 405

291 222

295 253

263 222

295 253

290 222

288 222

295 258

190

Table 1 Infrared and Raman spectral data of selected autunites and metautunites (continued).

saléeite metazeunerite IR Raman IR

published [4]

IR Raman

3574 3508 3425 3335 3255 2968

3512 3488 3296

3450 3340 3275 2925

3371 3238 3136

1679 1658 1630

1635 1645

1117 1054 985

1074 1015 1007 988 982

1115 1000

1100 1047 1031 1006

918 920 924 880

910 888

900 756 686

847 833 818

848 845 803 785 767 684

819 809 793

643 545 389 465 449

284 398 320 300

275 276

240 218

0

0.0002

0.0004

0.0006

0.0008

0.001

0.0012

0.0014

0.0016

0.0018

0.002

2700 2900 3100 3300 3500 3700 3900

Wavenumber/cm-1

Inte

nsity

Raman

Infrared Autunite

Figure 1

0

0.0005

0.001

0.0015

0.002

0.0025

2700 2900 3100 3300 3500 3700 3900

Wavenumber/cm-1

Inte

nsity

Metautunite

Raman

Infrared

Figure 2 Raman and Infrared spectrum of the hydroxyl stretching region of

metautunite

0

0.0005

0.001

0.0015

0.002

0.0025

2500 2700 2900 3100 3300 3500 3700

Wavenumber/cm-1

Inte

nsity

Raman

Infrared Torbernite

Figure 3

0

0.0002

0.0004

0.0006

0.0008

0.001

0.0012

0.0014

0.0016

0.0018

2500 2700 2900 3100 3300 3500 3700

Wavenumber/cm-1

Inte

nsity

Raman

InfraredMetatorbernite

Figure 4

0

0.0002

0.0004

0.0006

0.0008

0.001

0.0012

0.0014

2500 2700 2900 3100 3300 3500 3700

Wavenumber/cm-1

Inte

nsity

Raman

InfraredSaléeite

Figure 5

0

0.0005

0.001

0.0015

0.002

0.0025

0.003

2500 2700 2900 3100 3300 3500 3700

Wavenumber/cm-1

Inte

nsity

Raman

Infrared Metazeunerite

Figure 6

0.00656

0.01156

0.01656

0.02156

0.02656

0.03156

1500 1550 1600 1650 1700 1750

Wavenumber/cm-1

Ref

lect

ance

Autunite

Metautunite

Metatorbernite

Saléeite

Torbernite

Figure 7

0

0.01

0.02

0.03

0.04

0.05

0.06

600 700 800 900 1000 1100 1200

Wavenumber/cm-1

Inte

nsity

Raman

Infrared Autunite

Figure 8

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

600 700 800 900 1000 1100 1200

Wavenumber/cm-1

Inte

nsity

Raman

Infrared metautunite

Figure 9

0

0.01

0.02

0.03

0.04

0.05

0.06

600 700 800 900 1000 1100 1200

Wavenumber/cm-1

Inte

nsity

Raman

InfraredTorbernite

Figure 10

0

0.01

0.02

0.03

0.04

0.05

0.06

600 700 800 900 1000 1100 1200

Wavenumber/cm-1

Inte

nsity

Raman

Infrared Metatorbernite

Figure 11

0

0.01

0.02

0.03

0.04

0.05

0.06

600 700 800 900 1000 1100 1200

Wavenumber/cm-1

Inte

nsity

Raman

Infrared Saléeite

Figure 12

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

700 750 800 850 900 950 1000

Wavenumber/cm-1

Inte

nsity

Raman

InfraredMetazeunerite

Figure 13