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1 QUT Digital Repository: Frost, Ray L., Cejka, Jiri, Sejkora, Jiri, Plasil, Jakub, Reddy, B. Jagannadha, & Keeffe, Eloise C. (2011) Raman spectroscopic study of a hydroxy-arsenate mineral containing bismuth-atelestite Bi2O(OH)(AsO4). Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 78(1), pp Copyright 2011 Elsevier

2 Raman spectroscopic study of a hydroxy-arsenate mineral containing bismuth - atelestite Bi 2 O(OH)(AsO 4 ) Ray L. Frost, 1 Jiří Čejka, 1,2 Jiří Sejkora, 2. Jakub Plášil, 2 B. J. Reddy 1, Eloise C. Keeffe 1 1 Chemistry Discipline, Faculty of Science and Technology, Queensland University of Technology, GPO Box 2434, Brisbane Queensland 4001, Australia. 2 National Museum, Václavské náměstí 68, CZ Praha 1, Czech Republic. Abstract The Raman spectrum of atelestite Bi 2 O(OH)(AsO 4 ), a hydroxy-arsenate mineral containing bismuth, has been studied in terms of spectra-structure relations. The studied spectrum is compared with the Raman spectrum of atelestite downloaded from the RRUFF database. The sharp intense band at 834 cm -1 is assigned to the ν 1 AsO 3-4 (A 1 ) symmetric stretching mode and the three bands at 767, 782 and 802 cm to the ν 3 AsO 4 antisymmetric stretching modes. The bands at 310, 324, 353, 370, 395, 450, 480 and 623 cm -1 are assigned to the corresponding ν 4 and ν 2 bending modes and Bi-O-Bi (vibration of bridging oxygen) and Bi-O (vibration of non-bridging oxygen) stretching vibrations. Lattice modes are observed at 172, 199 and 218 cm -1. A broad low intensity band at 3095 cm -1 is attributed to the hydrogen bonded OH units in the atelestite structure. A weak band at 1082 cm -1 is assigned to δ (Bi-OH) vibration. Keywords: atelestite, arsenate, bismuth, Raman spectroscopy, hydroxy group Author to whom correspondence should be addressed (r.frost@qut.edu.au) 1

3 Introduction Atelestite, Bi 2 O(OH)(AsO 4 ), is a monoclinic mineral, space group P2 1 /c, forming tabular to prismatic crystals. The unit cell parameters are a (2), b 7.430(2), c Å, β (2) o, Z =4 [1,2]. Atelestite is formed as a rare supergene mineral in the oxide zone of bismuth- and arsenic- bearing mineral deposits; it is usually associated with bismutite, eulytite, erythrite, bismutostibiconite, beyerite, presingerite, walpurgite, mixite, conichalcite, torbernite etc. [2]. In atelestite-type minerals complete As-V-P isomorphism is observed [3]; its P- (smrkovecite) and V- (hechtsbergite) analogues have been recently described [4,5]. Crystal structure of atelestite was solved from single-crystal X-ray diffraction data by Mereiter and Preisinger [1]. In the asymmetric part of the unit-cell of atelestite, there are two symmetrically distinct bismuth atoms, one arsenic atom and six oxygen atoms; one of them adheres to OH - group. Arsenic atom is coordinated by four oxygen atoms forming slightly distorted (AsO 4 ) tetrahedron. Both bismuth atoms are coordinated by six ligands Biφ 6 (where φ = O, OH), thus forming strongly irregularly distorted Bi(O,OH) 6 polyhedra. The coordination polyhedra are strongly distorted namely due to lone-pair stereoactive electrons of bismuth atoms. In the structure, there are two types of bridges Bi-OH-Bi and Bi-O-Bi. Only Raman spectra of atelestite without any interpretation are available in the RRUFF database (Atelestite R080139). Corresponding wavenumbers of Raman bands were inferred from the Figure of R and used in this paper for comparison. Raman and infrared spectra of šreinite [6] and uranosphaerite [7] were also used to facilitate and help in the interpretation of the Raman spectra of atelestite. The objective of this paper is to report the Raman spectrum of atelestite and relate the spectrum with the molecular and crystal chemistry of this bismuth arsenate type mineral. The paper follows the systematic research of the large group of supergene minerals [8-11] and especially molecular structure of minerals containing oxyanions using Raman spectroscopy [12-16]. EXPERIMENTAL Minerals 2

4 The studied sample of the mineral atelestite was found at the type occurrence Schneeberg, Germany and it is deposited in the collection of National Museum, Prague (Czech Republic). The sample was analysed for phase purity by X-ray powder diffraction, and no significant impurities were found. Its refined unit-cell parameters for the monoclinic space group P2 1 /c: a = 6.983(1), b = 7.432(1), c = (1) Å, β = (1) o, V = (1) Å 3 are comparable with data from the crystal structure refinement [1]. The atelestite sample was quantitatively analysed by Cameca SX 100 microprobe system in wavelength dispersion mode for chemical composition. Studied sample was mounted into the epoxide resin and polished. The polished surface was coated with carbon layer 250 Å. An acceleration voltage of 15 kv, a specimen current of 10 na, and a beam diameter of 5 μm were used. The following lines and standards were used: Kα: andradite (Ca), fluorapatite (P), sanidine (Si), topaz (F), vanadinite (V); Lα: lammerite (As) and Mβ bismuth (Bi). Peak counting times (CTs) were 20 s for main elements and 60 s for minor ones, CT of each background was ½ of peak time. The raw intensities were converted to the concentrations using automatic PAP matrix correction software package. The H 2 O content was calculated from charge balance. The results (mean of 3 point analyses) are in weight % CaO (0.07), Bi 2 O 3 (78.22), SiO 2 (0.57), P 2 O 5 (0.12), As 2 O 5 (17.63), V 2 O 5 (0.02), F (0.11), H 2 O calc 1.54, the sum wt. % and empirical formula on the basis of (As+Si+V+P) = 1 apfu is (Bi 2.04 Ca 0.01 ) 2.05 O[(OH) 1.04 F 0.03 ] Σ1.07 [(AsO 4 ) 0.93 (SiO 4 ) 0.06 (PO 4 ) 0.01 ] Σ1.00. The observed chemical composition of atelestite sample is close to the ideal composition Bi 2 O(OH)(AsO 4 ) given for this compound [1,2]. Raman spectroscopy Crystal fragments of atelestite were placed on a polished metal surface on the stage of an Olympus BHSM microscope, which is equipped with 10x, 20x, and 50x objectives. The microscope is part of a Renishaw 1000 Raman microscope system, which also includes a monochromator, a filter system and a CCD detector (1024 pixels). The Raman spectra were excited by a Spectra-Physics model 127 He-Ne laser producing highly polarised light at 633 nm and collected at a nominal resolution of 2 cm -1 and a precision of ± 1 cm -1 in the range between 200 and 4000 cm -1. Repeated acquisition on the crystals using the highest magnification (50x) were accumulated to improve the signal to noise ratio in the spectra. Spectra were calibrated using the cm -1 line of a silicon wafer. Previous studies by the 3

5 authors provide more details of the experimental technique [12-16]. Alignment of all crystals in a similar orientation has been attempted and achieved. However, differences in intensity may be observed due to minor differences in the crystal orientation. A Raman spectrum of atelestite was downloaded from the RRUFF web site for comparative purposes (see Spectral manipulation such as baseline correction/adjustment and smoothing were performed using the Spectracalc software package GRAMS (Galactic Industries Corporation, NH, USA). Band component analysis was undertaken using the Jandel Peakfit software package that 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 Lorentzian-Gaussian crossproduct function with the minimum number of component bands used for the fitting process. The Gaussian-Lorentzian ratio was maintained at values greater than 0.7 and fitting was undertaken until reproducible results were obtained with squared correlations of r 2 greater than Results and discussion Arsenate and Bi-O vibrations According to Myneni et al. [17,18] and Nakamoto [19], AsO 3-4 is a tetrahedral unit, which exhibits four fundamental vibrations: the ν 1 symmetric stretching vibration (A 1 ) 818 cm -1, Raman active; the doubly degenerate ν 2 symmetric bending vibration (E) 350 cm -1, Raman active; the triply degenerate ν 3 antisymmetric stretching vibration (F 2 ) 786 cm -1, and the triply degenerate ν 4 bending vibration (F 2 ) 405 cm -1, both infrared and Raman active. Protonation, metal complexation, and/or adsorption on a mineral surface should cause change in AsO 3-4 symmetry from T d to lower symmetries, such as C 3v, C 2v or C 1. This loss of degeneracy causes splitting of degenerate vibrations of AsO 3-4 and the shifting of the As-OH 3- stretching vibrations to different wavenumbers. Such chemical interactions reduce AsO 4 tetrahedral symmetry, as mentioned above, to either C 3v /C 3 (corner-sharing), C 2v /C 2 (edgesharing, bidentate binuclear), or C 1 /C s (corner-sharing, edge-sharing, bidentate binuclear, multidentate) [17,18]. In association with AsO 3-4 symmetry and coordination changes, the A 1 4

6 band may shift to different wavenumbers and the doubly degenerate E and triply degenerate F modes may give rise to several new A 1, B 1, and/or E vibrations [17,18]. Bismuth ions, Bi 3+, can form BiO 3 pyramidal or BiO 6 octahedral polyhedra. Pyramidal polyhedra exhibit four fundamental vibrations (ν 1 /A 1, ν 2 /A 1, ν 3 /E, ν 4 /E), which are all Raman active, while octahedral polyhedra have six modes of vibrations (ν 1 /A 1g (RA), ν 2 /E g (RA), ν 3 /F 1u, ν 4 /F 1u, ν 5 /F 2g (RA), ν 6 /F 2u ), three of which are Raman active [19-21]. Raman spectroscopy of atelestite The Raman spectrum of atelestite in the 700 to 1200 cm -1 region is reported in Figure 1; in the 100 to 700 cm -1 region in Figure 2; in the 2700 to 3600 cm -1 region in Figure 3. In the Raman spectrum of atelestite (Figure 1) a set of overlapping bands at 767, 782, 802 and 834 cm -1 are observed. The sharp intense band at 834 cm -1 is assigned to the ν 1 AsO 3-4 (A 1 ) symmetric stretching mode. The three bands at 767, 782 and 802 cm -1 are 3- attributed to the triply degenerate ν 3 AsO 4 antisymmetric stretching modes. The Raman spectrum of atelestite in the RRUFF data base shows an intense sharp peak at 834 cm -1 in agreement with the results reported in this work. Three bands are observed for atelestite in the RRUFF spectrum at 765, 781 and 800 cm -1 which are in harmony with the results reported here. Low intensity bands are observed for RRUFF atelestite at ~900 and 948 cm -1 which are attributed to the presence of the phosphate anion in the atelestite structure. The observation of multiple bands at 767, 782 and 802 cm -1 in the Raman spectrum of the 3- studied sample provides evidence for the reduction of symmetry of the AsO 4 units. The spectrum of the low wavenumber region of atelestite is complex with sets of overlapping bands. The bands in the region from 300 to 600 cm -1 (310, 324, 352, 370, 395, 450, 480 cm ) are connected with the triply degenerate ν 4 and doubly degenerate ν 2 AsO 4 bending modes and the Bi-O-Bi symmetric anion vibration (vibration of bridging oxygen). A shoulder at 395 cm -1 may be connected with the Bi-O-Bi vibration of the [BiO 6 ] tetrahedral units. A weak band at 623 cm -1 can be attributed to the Bi-O stretching vibration (vibration of non- 5

7 bridging oxygen) of the octahedral units [20]. Raman bands at 118, 173, 200, 219 and 278 cm -1 may be connected with the Bi-O stretching vibrations and lattice vibrations. The Raman spectrum of RRUFF atelestite is practically identical with that studied in this paper. A low intensity Raman band is found at 3095 cm -1 and is assigned to the stretching mode of the hydrogen bonded OH units ( O-H...O hydrogen bond length ~2.68 Å) in the atelestite structure [22]. A weak band at 1082 cm -1 is assigned to the δ (Bi-OH) vibration. No bands above 1200 cm -1 are reported for the atelestite RRUFF Raman spectrum. CONCLUSIONS (1) The mineral atelestite is formed in the oxide zone of bismuth and arsenic-bearing mineral deposits. Raman spectroscopy has enabled the molecular structure of this mineral to be analysed. (2) The arsenate anion undergoes distortion in the structure as is evidenced by the multiple ν 3 stretching vibrations and the multiple ν 2 and ν 4 bending modes caused by splitting of the AsO 3-4 triply degenerate ν 3 stretching and ν 4 bending and doubly degenerate ν 2 bending vibrations. (3) Raman bands related to the Bi-O-Bi and Bi-O stretching and Bi-OH bending vibrations were observed. (4) A band connected with the OH stretching vibration of hydrogen bonded hydroxyls was attributed and approximate value of the corresponding O-H...O hydrogen bond was inferred. Acknowledgements The financial and infra-structure support of the Queensland University of Technology Inorganic Materials Research Program of the faculty of Science and Technology is gratefully acknowledged. The Australian Research Council (ARC) is thanked for funding the instrumentation. This work was financially supported by Ministry of Culture of the Czech Republic (MK ) to Jiří Sejkora and Jakub Plášil. The downloading of the Raman spectra of atelestite from the RRUFF web site is acknowledged. 6

8 REFERENCES [1] K. Mereiter, A. Presinger, Anz. Österr. Akad. Wiss., math.-naturwiss. Kl. 123 (1986) [2] J.W. Anthony, R.A. Bideaux, K.W. Bladh, M. C. Nichols, Handbook of Mineralogy, Vol. IV, Arsenates, Phosphates, Vanadates. Mineral Data Publishing Tucson, Arizona, USA. [3] J. Sejkora, J. Čejka, J. Hloušek, M. Novák, V. Šrein, Can. Mineral. 42 (2004) [4] T. Řídkošil, J. Sejkora, V. Šrein, N. Jb. Miner. Mh. (1996) [5] W. Krause, H.-J. Bernhardt, G. Blass, H. Effenberger, H. W. Graf, N. Jb. Miner. Mh. (1997) [6] J. Sejkora, J. Čejka, N. Jb. Miner. Abh. 184 (2007) [7] J. Sejkora, J. Čejka, U. Kolitsch, N. Jb. Miner. Abh. 185 (2008) [8] J. Plášil, J. Sejkora, J. Čejka, P. Škácha, V. Goliáš, J. Geosci. 54 (2009) 15. [9] J. Sejkora, J. Škovíra, J. Čejka, J. Plášil, J. Geosci. 54 (2009) 355. [10] J. Plášil, J. Čejka, J. Sejkora, P. Škácha, J. Geosci. 54 (2009) 385. [11] J. Plášil, J. Čejka, J. Sejkora, J. Hloušek, V. Goliáš, J. Geosci. I (2009) 373. [12] R. L. Frost, S. Bahfenne, J. Raman Spectrosc. 40, (2009) [13] R. L. Frost, S. Bahfenne, J. Graham, J. Raman Spectrosc. 40 (2009) [14] R. L. Frost, J. Čejka, J. Raman Spectrosc. 40 (2009) [15] R. L. Frost, J. Čejka, M. J. Dickfos, J. Raman Spectrosc. 40 (2009) [16] R. L. Frost, J. Čejka, M. J. Dickfos, J. Raman Spectrosc. 40 (2009) [17] S.C.B. Myneni, S.J. Traina, G.A. Waychunas, T.J. Logan, Geochim. Cosmochim. Acta 62 (1998) [18] S.C.B. Myneni, S.J. Traina, G.A. Waychunas, T.J. Logan, Geochim. Cosmochim. Acta 62 (1998) [19] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds., Wiley New York [20] S. Hazra, S. Mandal, A. Ghosh, Phys. Rev. B 56 (1997) [21] D. Sreenivasu, V. Chandramouli, Bull. Mater. Sci. 23 (2000) [22] E. Libowitzky, Monatsh. Chem. 130 (1999)

9 Figure Captions Fig. 1 Raman spectrum of atelestite (Schneeberg, this study) in the range from 700 to 1200 cm -1 Fig. 2 Raman spectrum of atelestite (Schneeberg, this study) in the range from 100 to 700 cm -1 Fig. 3 Raman spectrum of atelestite (Schneeberg, this study) in the range from 2700 to 3600 cm -1 8

10 Fig. 1 Raman spectrum of atelestite (Schneeberg, this study) in the range from 700 to 1200 cm -1 9

11 Fig. 2 Raman spectrum of atelestite (Schneeberg, this study) in the range from 100 to 700 cm -1 10

12 Fig. 3 Raman spectrum of atelestite (Schneeberg, this study) in the range from 2700 to 3600 cm -1 11