QUT Digital Repository: http://eprints.qut.edu.au/ Frost, Ray L. and Cejka, Jiri and Keeffe, Eloise C. and Dickfos, Marilla J. (2008) Raman spectroscopic study of the uranyl selenite mineral marthozite Cu[(UO2)3(SeO3)2O2].8H2O. Journal of Raman Spectroscopy, 39(10). pp. 1413-1418. © Copyright 2008 John Wiley & Sons Raman spectroscopic study of the uranyl selenite mineral marthozite Cu[(UO2)3(SeO3)2O2].8H2O Ray L. Frost, 1• Jiří Čejka, 1,2 Eloise C. Keeffe 1 and Marilla J. Dickfos 1 1 Inorganic Materials Research Program, School of Physical and Chemical Sciences, Queensland University of Technology, GPO Box 2434, Brisbane Queensland 4001, Australia. 2 National Museum, Václavské náměstí 68, CZ-115 79 Praha 1, Czech Republic. Abstract The mineral marthozite, a uranyl selenite, has been characterised by Raman spectroscopy at 298 K. The bands at 812 and 797 cm-1 were assigned to 2+ 2- the symmetric stretching modes of the (UO2) and (SeO3) units, respectively. These values gave calculated U-O bond lengths in uranyl of 1.799 and/or 1.814 Å. Average U-O bond length in uranyl is 1.795 Å, inferred from the X-ray single crystal structure analysis of marthozite by Cooper and Hawthorne. The broad -1 band at 869 cm was assigned to the ν3 antisymmetric stretching mode of the 2+ -1 (UO2) (calculated U-O bond length 1.808 Å). The band at 739 cm was 2- attributed to the ν3 antisymmetric stretching vibration of the (SeO3) units. The 2- -1 ν4 and the ν2 vibrational modes of the (SeO3) units were observed at 424 cm and 473 cm-1. Bands observed at 257, and 199 and 139 cm-1 were assigned to OUO bending vibrations and lattice vibrations, respectively. O-H…O hydrogen bond lengths were inferred using the Libowiztky‘s empirical relation. The infrared spectrum of marthozite was studied for complementation. KEYWORDS: marthozite, selenite, Raman spectroscopy, U-O bond length, uranyl. Introduction 1-4 Marthozite is a uranyl hydroxy selenite of formula [Cu[(UO2)3(SeO3)2O2].8H2O 5. The mineral is one of a number of known uranyl selenite minerals which includes 4,6 7 derriksite [Cu4UO2(SeO3)2(OH)5.H2O] , haynesite (UO2)3(OH)2(SeO3)2.5H2O , 8-10 piretite [Ca(UO2)3(SeO3)2(OH)4.4H2O] , guilleminite 11-13 [Ba(UO2)3(SeO3)2(OH)4.3H2O] , demesmaekerite 6,9,14 [Pb2Cu5(UO2)2(SeO3)5(OH)5.2H2O] and larisaite 15 [Na(H3O)(UO2)3(SeO3)2O2.4H2O] . These minerals are naturally occuring, however it is possible to synthesise uranyl selenites with different cations such as strontium 13. 9 Marthozite is orthorhombic with point group mm2 and space group Pbn21 = C 2v, Z = 4. There are three symmetrically distinct, U6+, and two symmetrically distinct Se4+ in the crystal structure of marthozite 3. Marthozite is topologically related to the phosphuranylite anion sheet topology. Each sheet has chains of uranyl pentagonal and hexagonal dipyramids which are linked by bonds to SeO3 pyramids with a base of three O atoms and Se4+ at the fourth apex 16. According to Burns 16, the strongly distorted coordination environment about the Se4+ cation is due to the presence of a stereoactive lone-pair of electrons that are directed away from the coordinating • Author to whom correspondence should be addressed ([email protected]) 1 anions. In the structure of marthozite low-valence cations and water molecules provide linkages between the sheets. O-H…O hydrogen bond lengths were inferred from the wavenumbers of the OH stretching vibrations 17. Čejka reported the infrared spectrum of haynesite and piretite 18 both by DRIFT and KBr absorption 19. The DRIFT spectrum showed much greater 2+ complexity. Čejka reported the (UO2) symmetric stretching mode as two bands at -1 2+ 850 and 820 cm . The (UO2) antisymmetric stretching mode was observed as two -1 bands at 905 and 862 cm . Čejka reported the selenite ν1 mode of haynesite at ~820 -1 -1 cm and the ν3 mode at 738 and 800 cm . In light of the values for the divalent -1 cationic selenites above, the value of ν1 at ~820 cm appears high. However, this corresponds with Bäumer´s conclusions. Čejka also suggested that the ν2 band was at -1 471 cm . This value was high compared with the values of the ν2 bending modes above, however, it was in agreement with Khandelwal´s assignment 20. The value of 19 20 ν4 was not reported but is available from the paper by Khandelwal . There was 2- the possibility of overlap between the symmetric stretching bands of (SeO3) and 2+ (UO2) . Čejka suggested that the bands in the infrared spectrum at 820, 471 and 738 2- were attributable to the ν1, ν2 and ν3 bands of (SeO3) . However, the potential overlap of bands makes assignment difficult. Čejka also points out that the mineral - + haynesite may contain not only OH units but also H3O units, which will complicate its spectra. It was noted from Table 6.34 of Reference 12, that the band positions for hydrated selenites are at higher positions than for the anhydrous compounds. Deliens and Piret also assumed that the chemical formula of haynesite is possibly 5 (H3O)2(UO2)3(OH)4(SeO3)2.H2O . This possible arrangement of water molecules in the formula could be excluded. Chukanov et al. concluded from the X-ray crystal + structure analysis and infrared spectra that (H3O) ions were present in the crystal structure of larisaite. From this, the authors assumed that these ions were also present + in the crystal structure of haynesite, and that the total amount of the (H3O) ions is lowered in the order haynesite → larisaite → piretite 15. Raman spectroscopy was proven most useful for the characterisation of secondary uranyl containing minerals 7,21-26. In order to identify and characterise the vibrational spectrum of marthozite, this research reports the Raman spectrum of marthozite and relates the spectra to the structure of the mineral. EXPERIMENTAL Minerals Marthozite Cu[(UO2)3(SeO3)2O2 originated from Musoni Mine, near Kolwezi, Shaba Province, Zaire and is a type mineral 27. The sample was analysed by X-ray diffraction and EDX measurements. No mineral impurities were detected. Raman microprobe spectroscopy The crystals of marthozite were placed and orientated on the stage of an Olympus BHSM microscope, equipped with 10x and 50x objectives which was part of a Renishaw 1000 Raman microscope system, also including a monochromator, filter system and Charge Coupled Device (CCD). Raman spectra were excited by a HeNe laser (633 nm) at a resolution of 2 cm-1 in the range between 100 and 4000 2 cm-1. Repeated acquisition using the highest magnification was accumulated to improve the signal to noise ratio. Spectra were calibrated using the 520.5 cm-1 line of a silicon wafer. Details of the technique have been published by the authors 23,24,28,29. 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 4000−525 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. Spectra were co-added to improve the signal to noise ratio. Spectroscopic manipulation such as baseline adjustment, smoothing and normalisation were performed using the Spectracalc software package GRAMS (Galactic Industries Corporation, NH, USA). 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 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 correlations of r2 greater than 0.995. Further details on the manipulation of the data has been published 7,24,30-37 . RESULTS AND DISCUSSION The spectroscopy of selenites is interesting in that, like many mineral arsenates, the symmetric stretching mode was observed at higher wavenumbers than 38 the antisymmetric stretching mode . The values of ν1 for sodium, calcium and copper selenites occured at 788, 784 and 774 cm-1 respectively. In contrast the values -1 for the ν3 antisymmetric stretching modes occured at 740, 713 and 714 cm -1 respectively. The value for ν2 bands occured between 449 and 461 cm and ν4 between 387 and 427 cm-1 38. Bäumer et al. proved that in the case of infrared spectra of M2+ selenite monohydrates, the stretching vibrations of selenite units were located -1 -1 39 in the regions 760 ≤ ν1 SeO3 ≤ 855 cm and 680 ≤ ν3 SeO3 ≤ 775 cm . Khandelwal and Verma and Verma attributed the Raman (infrared) bands in the spectra of -1 2- -1 synthetic UO2SeO3 at 381 and 389 (390) cm to the ν4 (SeO3) , at 497 (500) cm to 2- -1 2- -1 the ν2 (SeO3) , 724 and 731 (700) cm to the ν3 (SeO3) , 829 (820-840) cm to the 2- -1 ν1 (SeO3) and bands at 878 (875 sh) and 884 (920-940) cm to the ν1 and ν3 2+ 20,40 (UO2) , respectively . The same authors assigned observed Raman and infrared bands, for example, (NH4)2(UO2)2(SeO3)3.6H2O at 384, 390 (395), 475 (498), 731 and -1 829 (700 and 830), and 800 (808) cm to the ν4, ν2, ν3 and ν1 modes, respectively and -1 2+ those at 879 and 883 (872) and (900) cm to the ν1 and ν3 (UO2) modes, respectively.