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Molecular structure of the mineral svanbergite SrAl3(PO4,SO4)2(OH)6 – A vibrational spectroscopic study
Journal of Molecular Structure 994 (2011) 232–237
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Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc
Molecular structure of the mineral svanbergite SrAl3(PO4,SO4)2(OH)6 – A vibrational spectroscopic study Ray L. Frost ⇑, Sara J. Palmer Chemistry Discipline, Faculty of Science and Technology, Queensland University of Technology, GPO Box 2434, Brisbane Queensland 4001, Australia
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Article history: Received 8 February 2011 Received in revised form 9 March 2011 Accepted 9 March 2011 Available online 12 March 2011 Keywords: Svanbergite Woodhouseite Phosphate Sulphate Raman spectroscopy Infrared spectroscopy
a b s t r a c t The mineral svanbergite SrAl3(PO4,SO4)2(OH)6 is a hydroxy phosphate–sulphate mineral belonging to the beudantite subgroup of alunites and has been characterised by vibrational spectroscopy. Bands at various wavenumbers were assigned to the different vibrational modes of svanbergite, which were then associated with the structure of the mineral. Bands were primarily assigned to phosphate and sulphate stretching and bending modes. Two symmetric stretching modes for both phosphate and sulphate supported the concept of non-equivalent phosphate and sulphate units in the mineral structure. Bands in the OH stretching region enabled hydrogen bond distances to be calculated. Comparison of the hydrogen bond distances and the calculated hydrogen bond distances from the structure models indicates that hydrogen bonding in svanbergite occurs between the two OH units rather than OH to SO2 4 units. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Svanbergite SrAl3(PO4,SO4)2(OH)6 is a colourless mineral but maybe yellowish–red, depending upon the composition [1,2]. The mineral is a strontium aluminium phosphate sulphate hydroxide and belongs to the beaudantite mineral subgroup of the alunite supergroup [3,4]. The mineral is hexagonal with point group D53d m ) [5,6]. The mineral is isostructural with alunite [6] and forms (R3 a continuous series with woodhouseite [7–9]. It is interesting that one of the products of the corrosion of bauxite is svanbergite and the mineral may be found in bauxite wastes. Svanbergite is a mineral belonging to the beudantite subgroup of the alunite–jarosite supergroup [10,11]. From a spectroscopic point of view, the mineral is the most interesting because it contains both sulphate and phosphate anions together with OH units. Very limited vibrational spectroscopic data are available on this mineral [12,13]. Previous studies by the authors on the Raman spectroscopy of alunites [14] and jarosites [15,16] have been published. The mineral svanbergite is related to minerals of the crandallite group [17,18]. Raman spectroscopy has proven very useful for the study of minerals [19–25]. Indeed, Raman spectroscopy has proven most useful for the study of diagenetically related minerals as often occurs with minerals containing sulphate and phosphate groups, including svanbergite, corkite and hinsdalite. Raman spectroscopy is especially useful when the minerals are X-ray non-diffracting or ⇑ Corresponding author. E-mail address: [email protected] (R.L. Frost). 0022-2860/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2011.03.023
poorly diffracting and very useful for the study of amorphous and colloidal minerals. Interest in the mineral svanbergite SrAl3(PO4,SO4)2(OH)6 is based upon our studies of minerals that are found in soils. Further, our interest in svanbergite and related minerals including crandallite is that the minerals are found in caves such as the Jenolan caves. Caves such as the Jenolan caves have been in existence for some 350 million years. These caves can function as a mechanism for the formation of minerals. Thus, the calcite in the caves can function as a substrate for chemical reactions bringing the various components of the mineral svanbergite together to form the mineral. This paper is a part of systematic studies of vibrational spectra of minerals of secondary origin in the oxide supergene zone. In this work, we attribute bands at various wavenumbers to vibrational modes of svanbergite using Raman spectroscopy complemented with infrared spectroscopy and relate the spectra to the structure of the mineral.
2. Experimental 2.1. Mineral The mineral svanbergite was supplied by The Mineralogical Research Company. The samples originated from Mt. Brussilof Mine, Radium, British Columbia, Canada. The mineral data on svanbergite are available [1]. The mineral purity was confirmed by X-ray powder diffraction and the composition determined using an electron probe.
R.L. Frost, S.J. Palmer / Journal of Molecular Structure 994 (2011) 232–237
2.2. Raman spectroscopy Crystals of svanbergite were placed on a polished metal surface on the stage of an Olympus BHSM microscope, which is equipped with 10, 20, and 50 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 cm1 and a precision of ±1 cm1 in the range between 100 and 4000 cm1. Repeated acquisition on the crystals using the highest magnification (50) was accumulated to improve the signal-to-noise ratio in the spectra. Spectra were calibrated using the 520.5 cm1 line of a silicon wafer. Further details of the technique have been published [19–25]. 2.3. 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 cm1 range were obtained by the co-addition of 128 scans with a resolution of 4 cm1 and a mirror velocity of 0.6329 cm/s. Spectra were co-added to improve the signal-to-noise ratio. Band component analysis was undertaken using the Jandel ‘Peakfit’ (Erkrath, Germany) software package which enabled the type of fitting function to be selected and allowed specific parameters to be fixed or varied accordingly. Band fitting was done using a Lorentz–Gauss cross-product function with the minimum number of component bands used for the fitting process. The Lorentz–Gauss ratio was maintained at values greater than 0.7, and fitting was undertaken until reproducible results were obtained with squared correlations (r2) greater than 0.995. Band fitting of the spectra is quite reliable providing there is some band separation or changes in the spectral profile.
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using vibrational spectroscopy. A good starting point to study the position of the expected bands is to observe where the bands occur in aqueous solutions and then to observe the position of the bands resulting from the vibrational spectroscopy of minerals containing the individual anions. In aqueous systems, the sulphate anion is of Td symmetry and has symmetric stretching mode (m1) at 981 cm1, the antisymmetric stretching mode (m3) at 1104 cm1, the symmetric bending mode (m2) at 451 cm1 and the m4 mode at 613 cm1 [5]. In aqueous systems, Raman spectra of phosphate oxyanions show a symmetric stretching mode (m1) at 938 cm1, the antisymmetric stretching mode (m3) at 1017 cm1, the symmetric bending mode (m2) at 420 cm1 and the m4 mode at 567 cm1 [5]. The Raman spectroscopy of some phosphate minerals has been studied [26–30]. Kato and Miura discussed the crystal structure of svanbergite and compared the structure of svanbergite with alunites and jarosites [5]. These authors showed that the behaviour of the alunites with Al octahedra behaved differently from jarosites with ferric octahedra. Further, the authors described a lowering of the symmetry of the anions in svanbergite to C3v or even C2v. S.D. Ross in Farmer’s treatise [31] reported the infrared spectra of the jarosite–alunite minerals (Table 18, IX page 433). This table compares the infrared spectra of minerals from the alunite–jarosite supergroups. Ross reported infrared bands for alunite at 1030 cm1 (m1), 475 cm1 (m2), 1086, 1170 cm1 (m3), 605, 632 cm1 (m4) and infrared bands for jarosite at 1018, 1028 cm1 (m1), 482 cm1 (m2), 1100, 1190 cm1 (m3), 638, 685 cm1 (m4). OH vibrations for alunite were reported at 3485 and 505, 780, 802 cm1 attributed to the stretching and bending of the OH units, and OH vibrations. For jarosite bands were reported at 3260, 3355, 3430 and 512,790 cm-1 attributed to the stretching and bending of the OH units. Raman spectra of these minerals have also been published [14,15,32,33]. These results serve to show the positions of the bands, which may be assigned to sulphate and phosphate.
3. Results and discussion
4. Spectroscopy
3.1. Factor group analysis
The Raman spectrum of svanbergite in the 850–1150 cm1 region and the infrared spectrum in the 800–1300 cm1 region are displayed in Figs. 1a and 1b, respectively. The Raman spectrum is characterised by an intense sharp band at 1022 cm1 assigned to the m1 (SO4)2 symmetric stretching mode. The band is asymmetric on the high wavenumber side, and a component band at 1034 cm1 may be resolved. The broad band at 1098 cm1 may be attributed to the m3 (SO4)2 antisymmetric stretching mode. Previous studies by the first author have shown that the position of the symmetric stretching mode of the sulphate anion for jarosites and alunites is cation dependent [14]. Such a result is in harmony with the results of the crystal structure of svanbergite [5] where the S tetrahedral is affected by the cation in the cation octahedra [5]. A comparison of the band position may be made with the structurally related alunite group. The position of the sulphate stretching mode was found at 1026.4 cm1 for K-alunite and 1027.0 cm1 for Na-alunite. The effect of obtaining the spectra at liquid nitrogen temperatures was to shift the band to higher wavenumbers [14]. Three bands are observed for jarosite [15] at 1009.8, 1011.9 and 1026.3 cm1 and are assigned to the (m1) SO2 4 symmetric stretching mode. The observation of more than one band was accounted for by the non-equivalence of the SO2 4 units in the jarosite structure [15]. Two overlapping Raman bands are found at 981 and 998 cm1, which are assigned to the m1 (PO4)3symmetric stretching mode. The observation of two phosphate symmetric stretching modes suggests that there are two non-equivalent phosphate units in the structure of svanbergite. The question arises as to the position
The splitting in Table 1 is expected to be applicable for sulphate and arsenate. However, the Davydov splitting is likely to be weak due to mixed sulphate and phosphate in the C3v site; it is possible that only site group splitting would be detected. For instance, the m3 antisymmetric stretch may not be detected as an A1g + Eg in the Raman spectrum, instead it would be detected as a single vibration. The irreducible representation for svanbergite is given by
Cirred ¼ 8A1g ðRÞ þ 10A2u ðIRÞ þ 11Eg ðRÞ þ 14Eu ðIRÞ þ 3A2g ðinactiv eÞ þ 4A1u ðinactiv eÞ
3.2. Background The mineral svanbergite contains both sulphate and phosphate anions, and therefore, the presence of these anions can be observed Table 1 Factor group splitting of the sulphate/arsenate on a C3v site in a D53d crystal.
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Fig. 1a. Raman spectrum of svanbergite in the 850–1150 cm1 region.
in this work at 981 and 998 cm1. In the RRUFF, spectrum of svanbergite two overlapping bands at 1089 and 1108 cm1 is identified and is attributed to the m3 (PO4)3 and (SO4)2 antisymmetric stretching modes. In the infrared spectrum, an intense band at 1021 cm1 is found that is infrared equivalent to the Raman band at 1022 cm1. This band is assigned to the m1 (SO4)2 symmetric stretching mode. The band is asymmetric on the low wavenumber side, and a band may be resolved at 961 cm1 and is attributed to the m1 (PO4)3 symmetric stretching mode. A series of bands at 1095, 1183 and 1211 cm1 were found. These bands are attributed to the m3 (PO4)3 and (SO4)2 antisymmetric stretching modes. Raman bands for K-alunite [14] were found at 1082, 1158, and 1194 cm1. The observation of multiple bands in this spectral region is evidence for the reduction in symmetry of the sulphate anion. The Raman spectrum of the K-jarosite at 298 K shows two bands at 1111.0 and 1153.1 cm1. These bands are attributed to the m3 antisymmetric stretching mode of the sulphate units. Two bands were reported by Frost et al. at 1102.6 and 1153.3 cm1 for a synthetic K-jarosite which is in excellent agreement with these values [15]. Low intensity bands at 820, 851 and 889 cm1 are observed and may be attributed to the O3POH stretching vibrations. Breitinger et al. reported the spectra of a series of synthetic crandallites and assigned bands in these positions to the symmetric and antisymmetric stretching vibrations of the O3POH units. The Raman spectrum of svanbergite in the 350–700 cm1 region and the infrared spectrum in the 550–800 cm1 region are shown in Figs. 2. This spectral region is where the sulphate and phosphate bending vibrations are observed. Multiple Raman bands are found at 616, 633, 654 cm1. These Raman bands are assigned to the m4 (SO4)2 bending modes. For alunites, an intense Raman band is observed at 655.3 cm1 with a shoulder at 644.0 cm1 for K-alunite, 653.6 cm1 for Naalunite and 655.1 cm1 for NH4-alunite. These bands are ascribed to the m4 bending modes of the sulphate anion [14]. The Raman spectra of the K and Na jarosites [15] in this spectral region show bands at around 575, 625 and 641 cm1. The observation of multiple bands in this spectral region supports the concept of reduction
Fig. 1b. Infrared spectrum of svanbergite in the 800–1300 cm1 region.
of the m3 (PO4)3 antisymmetric stretching mode. The broad feature at 1034 cm1 may be assigned to this mode. Alternatively, the band at 1098 cm1 through coupling of the phosphate and sulphate vibrations may also be due to the m3 (PO4)3 antisymmetric stretching mode. A Raman spectrum of svanbergite is available in the RRUFF data base. This spectrum is similar to that reported in this work. An intense Raman peak is observed at 1024 cm1 with a shoulder at 1033 cm1. Further, two Raman bands are observed at 983 and 1004 cm1, which are in similar positions to that found
Fig. 2a. Raman spectrum of svanbergite in the 350–700 cm1 region.
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Fig. 3a. Raman spectrum of svanbergite in the 100–350 cm1 region. Fig. 2b. Infrared spectrum of svanbergite in the 550–800 cm1 region.
in symmetry of the sulphate anion in these structures. Other component bands are observed at 572, 588 and 602 cm1. In the infrared spectrum, multiple bands are found. Infrared bands are observed at 591, 603, 615, 649 and 658 cm1. These bands are ascribed to the m4 (SO4)2 bending modes. The observation of multiple m4 (SO4)2 bands provides evidence for the reduction in symmetry of the sulphate anion in the svanbergite structure. The Raman band at 522 cm1 is assigned to the triply degenerate m4 (PO4)3 bending mode. It is possible that the Raman bands at 474 and 486 cm1 are attributable to the doubly degenerate m2 (SO4)2 bending mode. The assignment of these bands is supported by the position of bands in the alunite Raman spectrum. In the Raman spectrum of alunites, bands are observed for K-alunite at 505, 488 and 482 cm1; for Na-alunite at 517, 486 and 485.5 cm1 and for NH4-alunite at 508 and 486.5 cm1 [14]. A comparison may also be made with the position of the bands for jarosites [15]. The Raman spectrum of K-jarosite shows two overlapping bands at 443.7 and 452.8 cm1 assigned to the m2 bending mode. The observation of two bands was attributed to symmetry lowering. It is possible that the two Raman bands at 369 and 392 cm1 are assignable to the doubly degenerate m2 (PO4)3 bending modes. In the infrared spectrum, bands below 550 cm1 are not observed as the bands occur at positions below the detection limit of the ATR instrumentation. The Raman spectrum of svanbergite in the 100– 350 cm1 region is displayed in Fig. 3b. Raman bands are observed at 179, 246 and 280 cm1 and are simply described as lattice vibrations. Quite intense bands are observed for alunites and jarosites in the very low wavenumber region. In the spectra of the low wavenumber region of the K and Na jarosites, two bands are found at around 366 and 299 cm1. One probable assignment of these bands is to FeO stretching vibrations. An intense band for jarosite is also found at around 230 cm1. It may be attributed to OH H hydrogen bond vibrations (see Fig. 3a). Fig. 3b shows the region of the water bending modes. The infrared bands are of a quite low intensity. Two bands are observed at 1642 and 1729 cm1 and are assigned to the water bending modes of strongly hydrogen-bonded water. In addition, two bands are ob-
Fig. 3b. Infrared spectrum of svanbergite in the 1300–1800 cm–1 region.
served at 1426 and 1483 cm1. It is not certain as to the assignment of the bands; however, it is possible that these bands are due to carbonate antisymmetric stretching vibrations. It is possible that water is involved in the structure of the mineral. The Raman spectrum in the 2900–3600 cm1 region and infrared spectrum over the 2400–3800 cm1 range are displayed in Figs. 4a and 4b, respectively. The intense Raman band at 3467 cm1 is assigned to the OH stretching vibration of the OH units of the svanbergite structure. Two Raman side bands are observed at 3415 and 3518 cm1. These three bands provide evidence
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for the concept that all the OH units are not identical in the svanbergite structure. Raman bands of alunites [14] showed bands at 3509, 3482 cm1 (K-al) 3492, 3457 cm1 (Na-al), 3511 cm1and 3484 cm1 (NH4-al). Infrared spectra of alunites show bands at 3479 cm1 (K-al) 3454 with a shoulder at 3486 cm1 (Na-al), 3481 cm1 with a shoulder at 3513 cm1 (NH4-al). This concept of non-identical OH units is supported by the infrared spectrum where two bands at 3409 and 3459 cm1 are observed, showing that there are two non-equivalent OH units in the svanbergite structure. Raman bands are centred upon 3151 cm1. These bands are assigned to O3POH stretching vibrations. In the infrared spectrum, a series of bands are also observed centred upon 3072 cm1. The bands are also assigned to O3POH stretching vibrations. A comparison may be made with the position of the OH stretching vibrations in alunites. Raman bands for alunites are observed at 3509, 3482 cm1 (K-al) 3492, 3457 cm1 (Na-al), 3511 and 3484 cm1 (NH4-al). Infrared spectra show bands at 3479 cm1 (K-al) 3454 with a shoulder at 3486 cm1 (Na-al), 3481 with a shoulder at 3513 cm1 (NH4-al). The position of the bands suggests that significant hydrogen bonding exists in the svanbergite structure. It is inferred that there is more than one distinct OH unit in the unit cell. These OH units may not necessarily be explicitly symmetrically distinct but there is the possibility that the two water molecules are not equivalent. If we use a Libowitzky-type empirical equation [34] and we assume that we can use infrared data in the equation, estimates of the hydrogen bond distances can be obtained. Studies have shown a strong correlation between OH stretching frequencies and both O O bond distances and H O hydrogen bond distances [35– 38]. Libowitzky showed that a regression function can be employed relating the hydroxyl stretching frequencies with regression coefficients better than 0.96 using infrared spectroscopy [39]. The function is described as: dðOOÞ m1 ¼ ð3592 304Þ 109 0:1321 cm1 . Thus, OH O hydrogen bond distances may be calculated using this Libowitzky empirical function. The values for the OH stretching vibrations for the OH units give calculated approximate O–H O hydrogen bond lengths of 2.92
Fig. 4b. Infrared spectrum of svanbergite in the 2400–3800 cm–1 region.
and 3.1 Å. These values suggest that the hydroxyl units are only weakly hydrogen bonding to the adjacent sulphate or phosphate units as mentioned. The approximate O–H O hydrogen bond lengths calculated for the water stretching vibrations are 2.7 Å (3072 cm1) and 2.8 Å (3409 cm1). Thus, OH units with different hydrogen bond distances are found in the svanbergite structure. These results suggest that the OH units belonging to the O3POH units are involved in hydrogen bonding to varying strengths according to their position in the svanbergite structure. This variation is also observed in the width of the Raman bands. Further, O3POH units are significantly more strongly hydrogen bonded than the OH units. Variation in the hydrogen bond distance of jarosites for the 3411 cm1 band is between 2.760 and 2.867 Å. The variation for the 3292 cm1 band is between 2.678 and 2.857 Å. In contrast, the variation in hydrogen bond distance for the 3388 cm1 band of the natural K-jarosite is between 2.778 and 2.796 Å and for the 3357 cm1 band is between 2.764 and 2.778 Å.
5. Conclusions
Fig. 4a. Raman spectrum of svanbergite in the 2900–3600 cm1 region.
Bands associated with phosphate and sulphate vibration modes dominate the Raman and infrared spectra of svanbergite. The majority of the Raman and infrared bands were assigned to the symmetric stretching modes (m1), the antisymmetric stretching modes (m3), the symmetric bending modes (m2) and the m4 bending modes of hydrogen phosphate, phosphate and sulphate. A comparison is made with the Raman spectra of selected alunites and jarosites. The most intense modes in the Raman spectrum are the symmetric stretching modes of phosphate and sulphate. Two symmetric stretching modes for both phosphate and sulphate support the concept of non-equivalent phosphate and sulphate units in the svanbergite structure. Bands in the OH stretching region enabled the hydrogen bond distances to be calculated. Comparison of the hydrogen bond distances and the calculated hydrogen bond distances from the structure models indicates that hydrogen bonding in svanbergite occurs between the two OH units rather than OH to SO2 4 units.
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Acknowledgements The financial and infrastructure support of the Chemistry Discipline of the Faculty of Science and Technology, Queensland University of Technology is gratefully acknowledged. The Australian Research Council (ARC) is thanked for funding the instrumentation. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.molstruc.2011.03.023. References [1] J.W. Anthony, R.A. Bideaux, K.W. Bladh, M.C. Nichols, Handbook of Mineralogy, Arsenates, phosphates, vanadates – Mineral, vol. IV, Mineral Data Publishing, Tucson, Arizona, 2000. [2] F. Novak, P. Paulis, B. Moravec, Vestnik Ceskeho Geologickeho Ustavu 72 (1997) 373–380. [3] J.E. Dutrizac, J.L. Jambor, Reviews in Mineralogy & Geochemistry 40 (2000) 405–452. [4] A.G. Hikov, Dokladi na Bulgarskata Akademiya na Naukite 57 (2004) 61–68. [5] T. Kato, Y. Miura, Mineralogical Journal 8 (1977) 419–430. [6] E. Sato, I. Nakai, R. Miyawaki, S. Matsubara, Neues Jahrbuch fuer Mineralogie, Abhandlungen 185 (2009) 313–322. [7] S. Qin, J. Sheng, J. Liu, L. Guo, Zhongguo Dizhi Kexueyuan Kuangchuang Dizhi Yanjiuso Sokan 11 (1983) 119–125. [8] V.I. Silaev, V.N. Filippov, M.Y. Sokerin, Zapiski Vserossiiskogo Mineralogicheskogo Obshchestva 130 (2001) 99–110. [9] I. Velinov, M. Gorova, K. Neikov, Dokladi na Bulgarskata Akademiya na Naukite 44 (1991) 45–48. [10] S. Gaboreau, D. Beaufort, P. Vieillard, P. Patrier, P. Bruneton, Canadian Mineralogist 43 (2005) 813–827. [11] S. Gaboreau, M. Cuney, D. Quirt, D. Beaufort, P. Patrier, R. Mathieu, American Mineralogist 92 (2007) 267–280. [12] E.I. Nikitina, V.I. Sotnikov, G.A. Golubova, Geologiya i Geofizika (1968) 114– 118. [13] S.V. Gevorkyan, A.A. Petrunina, A.S. Povarennykh, Konstitutsiya i Svoistva Mineralov 10 (1976) 51–59.
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[14] R.L. Frost, R.-A. Wills, M.L. Weier, W. Martens, J.T. Kloprogge, Journal of Molecular Structure 785 (2006) 123–132. [15] R.L. Frost, R.-A. Wills, M.L. Weier, W. Martens, S. Mills, Part A: Molecular and Biomolecular Spectroscopy 63A (2006) 1–8. [16] R.L. Frost, R.-A. Wills, W. Martens, Journal of Raman Spectroscopy 36 (2005) 1106–1112. [17] D.K. Breitinger, G. Brehm, J. Mohr, D. Colognesi, S.F. Parker, A. Stolle, T.H. Pimpl, R.G. Schwab, Journal of Raman Spectroscopy 37 (2006) 208–216. [18] D.K. Breitinger, R. Krieglstein, A. Bogner, R.G. Schwab, T.H. Pimpl, J. Mohr, H. Schukow, Journal of Molecular Structure 408–409 (1997) 287–290. [19] R.L. Frost, S. Bahfenne, Journal of Raman Spectroscopy 41 (2010) 207–211. [20] R.L. Frost, S. Bahfenne, Journal of Raman Spectroscopy 41 (2010) 325–328. [21] R.L. Frost, S. Bahfenne, J. Cejka, J. Sejkora, S.J. Palmer, R. Skoda, Journal of Raman Spectroscopy 41 (2010) 690–693. [22] R.L. Frost, S. Bahfenne, J. Cejka, J. Sejkora, J. Plasil, S.J. Palmer, Journal of Raman Spectroscopy 41 (2010) 814–819. [23] R.L. Frost, K.H. Bakon, S.J. Palmer, Journal of Raman Spectroscopy 41 (2010) 78–83. [24] R.L. Frost, J. Cejka, J. Sejkora, J. Plasil, S. Bahfenne, S.J. Palmer, Journal of Raman Spectroscopy 41 (2010) 571–575. [25] R.L. Frost, J. Cejka, J. Sejkora, J. Plasil, S. Bahfenne, S.J. Palmer, Journal of Raman Spectroscopy 41 (2010) 566–570. [26] R.L. Frost, J. Sejkora, E.C. Keeffe, J. Plasil, J. Cejka, S. Bahfenne, Journal of Raman Spectroscopy 41 (2010) 202–206. [27] R.L. Frost, J. Cejka, Journal of Raman Spectroscopy 40 (2009) 591–594. [28] R.L. Frost, A.W. Musumeci, M.O. Adebajo, W. Martens, Journal of Thermal Analysis and Calorimetry 89 (2007) 95–99. [29] R.L. Frost, M.L. Weier, K.L. Erickson, O. Carmody, S.J. Mills, Journal of Raman Spectroscopy 35 (2004) 1047–1055. [30] R.L. Frost, P.A. Williams, W. Martens, J.T. Kloprogge, P. Leverett, Journal of Raman Spectroscopy 33 (2002) 260–263. [31] V.C. Farmer, Mineralogical Society Monograph 4, The Infrared Spectra of Minerals, 1974. [32] R.L. Frost, R.-A. Wills, M.L. Weier, W. Martens, Journal of Raman Spectroscopy 36 (2005) 435–444. [33] R.L. Frost, D.L. Wain, Part A: Molecular and Biomolecular Spectroscopy 71 (2008) 490–495. [34] E. Libowitzky, Monatshefte Fur Chemie 130 (1999) 1047–1059. [35] J. Emsley, Chemical Society Reviews 9 (1980) 91–124. [36] H. Lutz, Structure and Bonding (Berlin, Germany) 82 (1995) 85–103. [37] W. Mikenda, Journal of Molecular Structure 147 (1986) 1–15. [38] A. Novak, Structure and Bonding (Berlin) 18 (1974) 177–216. [39] E. Libowitsky, Monatschefte fÜr chemie 130 (1999) 1047–1049.
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Molecular structure of the mineral svanbergite SrAl3(PO4,SO4)2(OH)6 – A vibrational spectroscopic study Ray L. Frost ⇑, Sara J. Palmer Chemistry Discipline, Faculty of Science and Technology, Queensland University of Technology, GPO Box 2434, Brisbane Queensland 4001, Australia
a r t i c l e
i n f o
Article history: Received 8 February 2011 Received in revised form 9 March 2011 Accepted 9 March 2011 Available online 12 March 2011 Keywords: Svanbergite Woodhouseite Phosphate Sulphate Raman spectroscopy Infrared spectroscopy
a b s t r a c t The mineral svanbergite SrAl3(PO4,SO4)2(OH)6 is a hydroxy phosphate–sulphate mineral belonging to the beudantite subgroup of alunites and has been characterised by vibrational spectroscopy. Bands at various wavenumbers were assigned to the different vibrational modes of svanbergite, which were then associated with the structure of the mineral. Bands were primarily assigned to phosphate and sulphate stretching and bending modes. Two symmetric stretching modes for both phosphate and sulphate supported the concept of non-equivalent phosphate and sulphate units in the mineral structure. Bands in the OH stretching region enabled hydrogen bond distances to be calculated. Comparison of the hydrogen bond distances and the calculated hydrogen bond distances from the structure models indicates that hydrogen bonding in svanbergite occurs between the two OH units rather than OH to SO2 4 units. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Svanbergite SrAl3(PO4,SO4)2(OH)6 is a colourless mineral but maybe yellowish–red, depending upon the composition [1,2]. The mineral is a strontium aluminium phosphate sulphate hydroxide and belongs to the beaudantite mineral subgroup of the alunite supergroup [3,4]. The mineral is hexagonal with point group D53d m ) [5,6]. The mineral is isostructural with alunite [6] and forms (R3 a continuous series with woodhouseite [7–9]. It is interesting that one of the products of the corrosion of bauxite is svanbergite and the mineral may be found in bauxite wastes. Svanbergite is a mineral belonging to the beudantite subgroup of the alunite–jarosite supergroup [10,11]. From a spectroscopic point of view, the mineral is the most interesting because it contains both sulphate and phosphate anions together with OH units. Very limited vibrational spectroscopic data are available on this mineral [12,13]. Previous studies by the authors on the Raman spectroscopy of alunites [14] and jarosites [15,16] have been published. The mineral svanbergite is related to minerals of the crandallite group [17,18]. Raman spectroscopy has proven very useful for the study of minerals [19–25]. Indeed, Raman spectroscopy has proven most useful for the study of diagenetically related minerals as often occurs with minerals containing sulphate and phosphate groups, including svanbergite, corkite and hinsdalite. Raman spectroscopy is especially useful when the minerals are X-ray non-diffracting or ⇑ Corresponding author. E-mail address: [email protected] (R.L. Frost). 0022-2860/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2011.03.023
poorly diffracting and very useful for the study of amorphous and colloidal minerals. Interest in the mineral svanbergite SrAl3(PO4,SO4)2(OH)6 is based upon our studies of minerals that are found in soils. Further, our interest in svanbergite and related minerals including crandallite is that the minerals are found in caves such as the Jenolan caves. Caves such as the Jenolan caves have been in existence for some 350 million years. These caves can function as a mechanism for the formation of minerals. Thus, the calcite in the caves can function as a substrate for chemical reactions bringing the various components of the mineral svanbergite together to form the mineral. This paper is a part of systematic studies of vibrational spectra of minerals of secondary origin in the oxide supergene zone. In this work, we attribute bands at various wavenumbers to vibrational modes of svanbergite using Raman spectroscopy complemented with infrared spectroscopy and relate the spectra to the structure of the mineral.
2. Experimental 2.1. Mineral The mineral svanbergite was supplied by The Mineralogical Research Company. The samples originated from Mt. Brussilof Mine, Radium, British Columbia, Canada. The mineral data on svanbergite are available [1]. The mineral purity was confirmed by X-ray powder diffraction and the composition determined using an electron probe.
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2.2. Raman spectroscopy Crystals of svanbergite were placed on a polished metal surface on the stage of an Olympus BHSM microscope, which is equipped with 10, 20, and 50 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 cm1 and a precision of ±1 cm1 in the range between 100 and 4000 cm1. Repeated acquisition on the crystals using the highest magnification (50) was accumulated to improve the signal-to-noise ratio in the spectra. Spectra were calibrated using the 520.5 cm1 line of a silicon wafer. Further details of the technique have been published [19–25]. 2.3. 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 cm1 range were obtained by the co-addition of 128 scans with a resolution of 4 cm1 and a mirror velocity of 0.6329 cm/s. Spectra were co-added to improve the signal-to-noise ratio. Band component analysis was undertaken using the Jandel ‘Peakfit’ (Erkrath, Germany) software package which enabled the type of fitting function to be selected and allowed specific parameters to be fixed or varied accordingly. Band fitting was done using a Lorentz–Gauss cross-product function with the minimum number of component bands used for the fitting process. The Lorentz–Gauss ratio was maintained at values greater than 0.7, and fitting was undertaken until reproducible results were obtained with squared correlations (r2) greater than 0.995. Band fitting of the spectra is quite reliable providing there is some band separation or changes in the spectral profile.
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using vibrational spectroscopy. A good starting point to study the position of the expected bands is to observe where the bands occur in aqueous solutions and then to observe the position of the bands resulting from the vibrational spectroscopy of minerals containing the individual anions. In aqueous systems, the sulphate anion is of Td symmetry and has symmetric stretching mode (m1) at 981 cm1, the antisymmetric stretching mode (m3) at 1104 cm1, the symmetric bending mode (m2) at 451 cm1 and the m4 mode at 613 cm1 [5]. In aqueous systems, Raman spectra of phosphate oxyanions show a symmetric stretching mode (m1) at 938 cm1, the antisymmetric stretching mode (m3) at 1017 cm1, the symmetric bending mode (m2) at 420 cm1 and the m4 mode at 567 cm1 [5]. The Raman spectroscopy of some phosphate minerals has been studied [26–30]. Kato and Miura discussed the crystal structure of svanbergite and compared the structure of svanbergite with alunites and jarosites [5]. These authors showed that the behaviour of the alunites with Al octahedra behaved differently from jarosites with ferric octahedra. Further, the authors described a lowering of the symmetry of the anions in svanbergite to C3v or even C2v. S.D. Ross in Farmer’s treatise [31] reported the infrared spectra of the jarosite–alunite minerals (Table 18, IX page 433). This table compares the infrared spectra of minerals from the alunite–jarosite supergroups. Ross reported infrared bands for alunite at 1030 cm1 (m1), 475 cm1 (m2), 1086, 1170 cm1 (m3), 605, 632 cm1 (m4) and infrared bands for jarosite at 1018, 1028 cm1 (m1), 482 cm1 (m2), 1100, 1190 cm1 (m3), 638, 685 cm1 (m4). OH vibrations for alunite were reported at 3485 and 505, 780, 802 cm1 attributed to the stretching and bending of the OH units, and OH vibrations. For jarosite bands were reported at 3260, 3355, 3430 and 512,790 cm-1 attributed to the stretching and bending of the OH units. Raman spectra of these minerals have also been published [14,15,32,33]. These results serve to show the positions of the bands, which may be assigned to sulphate and phosphate.
3. Results and discussion
4. Spectroscopy
3.1. Factor group analysis
The Raman spectrum of svanbergite in the 850–1150 cm1 region and the infrared spectrum in the 800–1300 cm1 region are displayed in Figs. 1a and 1b, respectively. The Raman spectrum is characterised by an intense sharp band at 1022 cm1 assigned to the m1 (SO4)2 symmetric stretching mode. The band is asymmetric on the high wavenumber side, and a component band at 1034 cm1 may be resolved. The broad band at 1098 cm1 may be attributed to the m3 (SO4)2 antisymmetric stretching mode. Previous studies by the first author have shown that the position of the symmetric stretching mode of the sulphate anion for jarosites and alunites is cation dependent [14]. Such a result is in harmony with the results of the crystal structure of svanbergite [5] where the S tetrahedral is affected by the cation in the cation octahedra [5]. A comparison of the band position may be made with the structurally related alunite group. The position of the sulphate stretching mode was found at 1026.4 cm1 for K-alunite and 1027.0 cm1 for Na-alunite. The effect of obtaining the spectra at liquid nitrogen temperatures was to shift the band to higher wavenumbers [14]. Three bands are observed for jarosite [15] at 1009.8, 1011.9 and 1026.3 cm1 and are assigned to the (m1) SO2 4 symmetric stretching mode. The observation of more than one band was accounted for by the non-equivalence of the SO2 4 units in the jarosite structure [15]. Two overlapping Raman bands are found at 981 and 998 cm1, which are assigned to the m1 (PO4)3symmetric stretching mode. The observation of two phosphate symmetric stretching modes suggests that there are two non-equivalent phosphate units in the structure of svanbergite. The question arises as to the position
The splitting in Table 1 is expected to be applicable for sulphate and arsenate. However, the Davydov splitting is likely to be weak due to mixed sulphate and phosphate in the C3v site; it is possible that only site group splitting would be detected. For instance, the m3 antisymmetric stretch may not be detected as an A1g + Eg in the Raman spectrum, instead it would be detected as a single vibration. The irreducible representation for svanbergite is given by
Cirred ¼ 8A1g ðRÞ þ 10A2u ðIRÞ þ 11Eg ðRÞ þ 14Eu ðIRÞ þ 3A2g ðinactiv eÞ þ 4A1u ðinactiv eÞ
3.2. Background The mineral svanbergite contains both sulphate and phosphate anions, and therefore, the presence of these anions can be observed Table 1 Factor group splitting of the sulphate/arsenate on a C3v site in a D53d crystal.
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Fig. 1a. Raman spectrum of svanbergite in the 850–1150 cm1 region.
in this work at 981 and 998 cm1. In the RRUFF, spectrum of svanbergite two overlapping bands at 1089 and 1108 cm1 is identified and is attributed to the m3 (PO4)3 and (SO4)2 antisymmetric stretching modes. In the infrared spectrum, an intense band at 1021 cm1 is found that is infrared equivalent to the Raman band at 1022 cm1. This band is assigned to the m1 (SO4)2 symmetric stretching mode. The band is asymmetric on the low wavenumber side, and a band may be resolved at 961 cm1 and is attributed to the m1 (PO4)3 symmetric stretching mode. A series of bands at 1095, 1183 and 1211 cm1 were found. These bands are attributed to the m3 (PO4)3 and (SO4)2 antisymmetric stretching modes. Raman bands for K-alunite [14] were found at 1082, 1158, and 1194 cm1. The observation of multiple bands in this spectral region is evidence for the reduction in symmetry of the sulphate anion. The Raman spectrum of the K-jarosite at 298 K shows two bands at 1111.0 and 1153.1 cm1. These bands are attributed to the m3 antisymmetric stretching mode of the sulphate units. Two bands were reported by Frost et al. at 1102.6 and 1153.3 cm1 for a synthetic K-jarosite which is in excellent agreement with these values [15]. Low intensity bands at 820, 851 and 889 cm1 are observed and may be attributed to the O3POH stretching vibrations. Breitinger et al. reported the spectra of a series of synthetic crandallites and assigned bands in these positions to the symmetric and antisymmetric stretching vibrations of the O3POH units. The Raman spectrum of svanbergite in the 350–700 cm1 region and the infrared spectrum in the 550–800 cm1 region are shown in Figs. 2. This spectral region is where the sulphate and phosphate bending vibrations are observed. Multiple Raman bands are found at 616, 633, 654 cm1. These Raman bands are assigned to the m4 (SO4)2 bending modes. For alunites, an intense Raman band is observed at 655.3 cm1 with a shoulder at 644.0 cm1 for K-alunite, 653.6 cm1 for Naalunite and 655.1 cm1 for NH4-alunite. These bands are ascribed to the m4 bending modes of the sulphate anion [14]. The Raman spectra of the K and Na jarosites [15] in this spectral region show bands at around 575, 625 and 641 cm1. The observation of multiple bands in this spectral region supports the concept of reduction
Fig. 1b. Infrared spectrum of svanbergite in the 800–1300 cm1 region.
of the m3 (PO4)3 antisymmetric stretching mode. The broad feature at 1034 cm1 may be assigned to this mode. Alternatively, the band at 1098 cm1 through coupling of the phosphate and sulphate vibrations may also be due to the m3 (PO4)3 antisymmetric stretching mode. A Raman spectrum of svanbergite is available in the RRUFF data base. This spectrum is similar to that reported in this work. An intense Raman peak is observed at 1024 cm1 with a shoulder at 1033 cm1. Further, two Raman bands are observed at 983 and 1004 cm1, which are in similar positions to that found
Fig. 2a. Raman spectrum of svanbergite in the 350–700 cm1 region.
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Fig. 3a. Raman spectrum of svanbergite in the 100–350 cm1 region. Fig. 2b. Infrared spectrum of svanbergite in the 550–800 cm1 region.
in symmetry of the sulphate anion in these structures. Other component bands are observed at 572, 588 and 602 cm1. In the infrared spectrum, multiple bands are found. Infrared bands are observed at 591, 603, 615, 649 and 658 cm1. These bands are ascribed to the m4 (SO4)2 bending modes. The observation of multiple m4 (SO4)2 bands provides evidence for the reduction in symmetry of the sulphate anion in the svanbergite structure. The Raman band at 522 cm1 is assigned to the triply degenerate m4 (PO4)3 bending mode. It is possible that the Raman bands at 474 and 486 cm1 are attributable to the doubly degenerate m2 (SO4)2 bending mode. The assignment of these bands is supported by the position of bands in the alunite Raman spectrum. In the Raman spectrum of alunites, bands are observed for K-alunite at 505, 488 and 482 cm1; for Na-alunite at 517, 486 and 485.5 cm1 and for NH4-alunite at 508 and 486.5 cm1 [14]. A comparison may also be made with the position of the bands for jarosites [15]. The Raman spectrum of K-jarosite shows two overlapping bands at 443.7 and 452.8 cm1 assigned to the m2 bending mode. The observation of two bands was attributed to symmetry lowering. It is possible that the two Raman bands at 369 and 392 cm1 are assignable to the doubly degenerate m2 (PO4)3 bending modes. In the infrared spectrum, bands below 550 cm1 are not observed as the bands occur at positions below the detection limit of the ATR instrumentation. The Raman spectrum of svanbergite in the 100– 350 cm1 region is displayed in Fig. 3b. Raman bands are observed at 179, 246 and 280 cm1 and are simply described as lattice vibrations. Quite intense bands are observed for alunites and jarosites in the very low wavenumber region. In the spectra of the low wavenumber region of the K and Na jarosites, two bands are found at around 366 and 299 cm1. One probable assignment of these bands is to FeO stretching vibrations. An intense band for jarosite is also found at around 230 cm1. It may be attributed to OH H hydrogen bond vibrations (see Fig. 3a). Fig. 3b shows the region of the water bending modes. The infrared bands are of a quite low intensity. Two bands are observed at 1642 and 1729 cm1 and are assigned to the water bending modes of strongly hydrogen-bonded water. In addition, two bands are ob-
Fig. 3b. Infrared spectrum of svanbergite in the 1300–1800 cm–1 region.
served at 1426 and 1483 cm1. It is not certain as to the assignment of the bands; however, it is possible that these bands are due to carbonate antisymmetric stretching vibrations. It is possible that water is involved in the structure of the mineral. The Raman spectrum in the 2900–3600 cm1 region and infrared spectrum over the 2400–3800 cm1 range are displayed in Figs. 4a and 4b, respectively. The intense Raman band at 3467 cm1 is assigned to the OH stretching vibration of the OH units of the svanbergite structure. Two Raman side bands are observed at 3415 and 3518 cm1. These three bands provide evidence
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for the concept that all the OH units are not identical in the svanbergite structure. Raman bands of alunites [14] showed bands at 3509, 3482 cm1 (K-al) 3492, 3457 cm1 (Na-al), 3511 cm1and 3484 cm1 (NH4-al). Infrared spectra of alunites show bands at 3479 cm1 (K-al) 3454 with a shoulder at 3486 cm1 (Na-al), 3481 cm1 with a shoulder at 3513 cm1 (NH4-al). This concept of non-identical OH units is supported by the infrared spectrum where two bands at 3409 and 3459 cm1 are observed, showing that there are two non-equivalent OH units in the svanbergite structure. Raman bands are centred upon 3151 cm1. These bands are assigned to O3POH stretching vibrations. In the infrared spectrum, a series of bands are also observed centred upon 3072 cm1. The bands are also assigned to O3POH stretching vibrations. A comparison may be made with the position of the OH stretching vibrations in alunites. Raman bands for alunites are observed at 3509, 3482 cm1 (K-al) 3492, 3457 cm1 (Na-al), 3511 and 3484 cm1 (NH4-al). Infrared spectra show bands at 3479 cm1 (K-al) 3454 with a shoulder at 3486 cm1 (Na-al), 3481 with a shoulder at 3513 cm1 (NH4-al). The position of the bands suggests that significant hydrogen bonding exists in the svanbergite structure. It is inferred that there is more than one distinct OH unit in the unit cell. These OH units may not necessarily be explicitly symmetrically distinct but there is the possibility that the two water molecules are not equivalent. If we use a Libowitzky-type empirical equation [34] and we assume that we can use infrared data in the equation, estimates of the hydrogen bond distances can be obtained. Studies have shown a strong correlation between OH stretching frequencies and both O O bond distances and H O hydrogen bond distances [35– 38]. Libowitzky showed that a regression function can be employed relating the hydroxyl stretching frequencies with regression coefficients better than 0.96 using infrared spectroscopy [39]. The function is described as: dðOOÞ m1 ¼ ð3592 304Þ 109 0:1321 cm1 . Thus, OH O hydrogen bond distances may be calculated using this Libowitzky empirical function. The values for the OH stretching vibrations for the OH units give calculated approximate O–H O hydrogen bond lengths of 2.92
Fig. 4b. Infrared spectrum of svanbergite in the 2400–3800 cm–1 region.
and 3.1 Å. These values suggest that the hydroxyl units are only weakly hydrogen bonding to the adjacent sulphate or phosphate units as mentioned. The approximate O–H O hydrogen bond lengths calculated for the water stretching vibrations are 2.7 Å (3072 cm1) and 2.8 Å (3409 cm1). Thus, OH units with different hydrogen bond distances are found in the svanbergite structure. These results suggest that the OH units belonging to the O3POH units are involved in hydrogen bonding to varying strengths according to their position in the svanbergite structure. This variation is also observed in the width of the Raman bands. Further, O3POH units are significantly more strongly hydrogen bonded than the OH units. Variation in the hydrogen bond distance of jarosites for the 3411 cm1 band is between 2.760 and 2.867 Å. The variation for the 3292 cm1 band is between 2.678 and 2.857 Å. In contrast, the variation in hydrogen bond distance for the 3388 cm1 band of the natural K-jarosite is between 2.778 and 2.796 Å and for the 3357 cm1 band is between 2.764 and 2.778 Å.
5. Conclusions
Fig. 4a. Raman spectrum of svanbergite in the 2900–3600 cm1 region.
Bands associated with phosphate and sulphate vibration modes dominate the Raman and infrared spectra of svanbergite. The majority of the Raman and infrared bands were assigned to the symmetric stretching modes (m1), the antisymmetric stretching modes (m3), the symmetric bending modes (m2) and the m4 bending modes of hydrogen phosphate, phosphate and sulphate. A comparison is made with the Raman spectra of selected alunites and jarosites. The most intense modes in the Raman spectrum are the symmetric stretching modes of phosphate and sulphate. Two symmetric stretching modes for both phosphate and sulphate support the concept of non-equivalent phosphate and sulphate units in the svanbergite structure. Bands in the OH stretching region enabled the hydrogen bond distances to be calculated. Comparison of the hydrogen bond distances and the calculated hydrogen bond distances from the structure models indicates that hydrogen bonding in svanbergite occurs between the two OH units rather than OH to SO2 4 units.
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