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A vibrational spectroscopic study of the silicate mineral analcime – Na2(Al4SiO4O12)·2H2O – A natural zeolite
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 133 (2014) 521–525
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
A vibrational spectroscopic study of the silicate mineral analcime – Na2(Al4SiO4O12)2H2O – A natural zeolite Ray L. Frost a,⇑, Andrés López a, Frederick L. Theiss a, Antônio Wilson Romano b, Ricardo Scholz c a
School of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty, Queensland University of Technology, GPO Box 2434, Brisbane, Queensland 4001, Australia Geology Department, Federal University of Minas Gerais, Belo Horizonte, MG 31270-901, Brazil c Geology Department, School of Mines, Federal University of Ouro Preto, Campus Morro do Cruzeiro, Ouro Preto, MG 35400-00, Brazil b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
We have studied the mineral
analcime using SEM with EDX and vibrational spectroscopy. Analcime Na2(Al4SiO4O12)2H2O is a crystalline sodium silicate. Chemical analysis shows the mineral contains Na, Al, Fe2+ and Si. Multiple water bands indicate that water is involved with differing hydrogen bond strengths.
a r t i c l e
i n f o
Article history: Received 10 April 2014 Received in revised form 27 May 2014 Accepted 3 June 2014 Available online 14 June 2014 Keywords: Analcime Sodium silicate Raman spectroscopy Infrared spectroscopy Zeolite
a b s t r a c t We have studied the mineral analcime using a combination of scanning electron microscopy with energy dispersive spectroscopy and vibrational spectroscopy. The mineral analcime Na2(Al4SiO4O12)2H2O is a crystalline sodium silicate. Chemical analysis shows the mineral contains a range of elements including Na, Al, Fe2+ and Si. The mineral is characterized by intense Raman bands observed at 1052, 1096 and 1125 cm 1. The infrared bands are broad; nevertheless bands may be resolved at 1006 and 1119 cm 1. These bands are assigned to SiO stretching vibrational modes. Intense Raman band at 484 cm 1 is attributed to OSiO bending modes. Raman bands observed at 2501, 3542, 3558 and 3600 cm 1 are assigned to the stretching vibrations of water. Low intensity infrared bands are noted at 3373, 3529 and 3608 cm 1. The observation of multiple water bands indicate that water is involved in the structure of analcime with differing hydrogen bond strengths. This concept is supported by the number of bands in the water bending region. Vibrational spectroscopy assists with the characterization of the mineral analcime. Ó 2014 Elsevier B.V. All rights reserved.
Introduction The zeolite minerals form a complex group of aluminosilicates that often occur as accessory minerals in intermediate and basic rocks. Zeolites are microporous crystalline compounds with extre⇑ Corresponding author. Tel.: +61 7 3138 2407; fax: +61 7 3138 1804. E-mail address: [email protected] (R.L. Frost). http://dx.doi.org/10.1016/j.saa.2014.06.034 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
mely narrow pore size distributions, which have found several applications being used as ion exchangers, catalysts, among others. Analcime – Na2(Al2Si4O12)2H2O, is one of the most common minerals with the zeolite type structure. The mineral shows different symmetry of crystallization. The crystal structure was first determined by Taylor [1] with cubic symmetry and later refined by Pechar [2,3] and refined by neutron diffraction by Ferraris et al. [4] with triclinic symmetry. Analcime is usually classified as a zeo-
522
R.L. Frost et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 133 (2014) 521–525
lite mineral, but structurally and chemically it is more similar to the feldspathoids. Analcime occurs as a primary mineral in analcime basalt and other alkaline igneous rocks. It also occurs as cavity and vesicle fillings associated with prehnite, calcite and zeolites. The genesis of analcime may be related to the crystallization from a trachytic melt as a primary magmatic mineral, as proposed by Peterson et al. [5] and Roux and Hamilton [6]; or it may have been produced by ion exchange from preexisting leucite [7]. The mineral analcime as well as its similar synthetic analogs are of interest due to their application in different industries, such as the removal of nuclear wastes [8], the removal of heavy metals [9,10] and in ceramics [11]. Some Raman spectra of sodium aluminum silicates have been collected and a number of the spectra were shown to be dependent upon the number of condensed silica tetrahedra [12]. Such detailed assignment of infrared and Raman bands for a wide range of silicate structures was made by Dowty [13–16]. The thermal decomposition of sodium silicates has also been measured [17–19]. There is an apparent lack of information on the vibrational spectra of analcime. The reason for such a lack of information is not known; yet the mineral contains siloxane units. Such units lend themselves to vibrational spectroscopy. Raman spectroscopy has proven most useful for the study of mineral structure [20–25]. The objective of this research is to report the Raman and infrared spectra of analcime and to relate the spectra to the mineral structure.
lected 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 acquisitions on the crystals using the highest magnification (50) were accumulated to improve the signal to noise ratio of the Raman spectra. Raman spectra were calibrated using the 520.5 cm 1 line of a silicon wafer. Infrared spectroscopy
Experimental
Infrared spectra of analcime 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 128 scans with a resolution of 4 cm 1 and a mirror velocity of 0.6329 cm/s. Spectra were coadded to improve the signal to noise ratio. The infrared spectra are given in Supplementary information. 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 cross-product 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 r2 greater than 0.995.
Samples description and preparation
Results and discussion
The analcime sample studied in this work occurs as single crystals with tabular habitus up to 5 cm. The sample is part of the collection of the Geology Department of the Federal University of Ouro Preto, Minas Gerais, Brazil, with sample code SAD-014. The mineral sample originated from Moonen Bay, Dunvegan, Duirinish, Isle of Skye, North West Highlands (Inverness-shire), Scotland, UK. The sample was gently crushed and the associated minerals were removed under a stereomicroscope Leica MZ4. The analcime sample studied in this work was analyzed by scanning electron microscopy (SEM) in the EDS mode to support the mineral characterization.
Mineral characterization
Scanning electron microscopy (SEM) Experiments and analyses involving electron microscopy were performed in the Center of Microscopy of the Universidade Federal de Minas Gerais, Belo Horizonte, Minas Gerais, Brazil (http:// www.microscopia.ufmg.br). Analcime crystals were coated with a 5 nm layer of evaporated carbon. Secondary Electron and Backscattering Electron images were obtained using a JEOL JSM-6360LV equipment. Qualitative and semi-quantitative chemical analyses in the EDS mode were performed with a ThermoNORAN spectrometer model Quest and were applied to support the mineral characterization.
The SEM image of analcime sample studied in this work is shown in Fig. 1. The image shows a cleavage fragment up to 2 mm. Qualitative chemical analysis shows a homogeneous phase, composed by Na, Al, O and Si. The chemical data is in agreement with the chemical formula for the mineral. No other contaminant elements were observed and the sample can be considered as a pure single phase (Fig. 2). Vibrational spectroscopy The Raman spectrum of analcime over the 100–4000 cm 1 spectral range is shown in Fig. 3a. This figure shows the position and relative intensity of the Raman bands. It is noted there are
Raman microprobe spectroscopy Crystals of analcime 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 polarized light at 633 nm and col-
Fig. 1. Backscattered electron image (BSI) of an analcime single crystal up to 1.0 mm in length.
R.L. Frost et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 133 (2014) 521–525
Fig. 2. EDS analysis of analcime.
523
sists of three dimensional silicates with multiple linked silica tetrahedra [26]. Raman bands are observed at 1011, 1052, 1096 and 1125 cm 1 and are assigned to the SiO stretching vibrations. The position of these bands is typical of natural zeolites. Dowty calculated the band positions for the different ideal silicate units. Dowty showed that the –SiO3 units had a unique band position of 1025 cm 1 [16] (see Figs. 2 and 4 of this reference). Dowty calculated the Raman spectrum for these type of silicate networks and predicted two bands at around 1040 and 1070 cm 1 with an additional band at around 600 cm 1. We observe Raman bands at 1052 and 1096 cm 1 which is in close agreement with the predicted results of Dowty. The infrared spectrum over the 800–1300 cm 1 spectral range is reported in Fig. 4b. The infrared spectrum is quite broad and may be resolved into component bands at 916, 959, 975, 1006, 1047, 1081 and 1119 cm 1. These bands are assigned to SiO stretching vibrations. The Raman spectrum of analcime over the 350–700 cm 1 spectral range is shown in Fig. 5a. Intense Raman band is observed at 484 cm 1 and is assigned to OSiO bending vibrations. Bands of lesser intensity are observed at 385, 404, 511, 544, 591, 604, 612, 620 and 650 cm 1. These bands are of quite low intensity. These bands may also be attributed to displacement of the SiO and AlO tetrahedra [27,28]. It is likely that the Raman bands at 591, 604, 612, 620 and 650 cm 1 are related to the AlO and SiO chains. The Raman spectrum of analcime over the 100–350 cm 1 spectral range is shown in Fig. 5b. Intense Raman bands are observed at 115, 133 and 148 cm 1 with bands of lesser intensity noted at 184, 197, 207, 213, 226 and 243 cm 1. These bands are simply described as lattice vibrations. In addition, an intense Raman band is observed at 299 cm 1 with a shoulder at 308 cm 1. These bands may be assigned to metal oxygen vibrations.
Fig. 3. (a) Raman spectrum of analcime (upper spectrum) over the 100–4000 cm 1 spectral range and (b) infrared spectrum of analcime (lower spectrum) over the 500–4000 cm 1 spectral range.
large parts of the spectrum where little or no intensity is observed. The Raman spectrum is therefore subdivided into sections based upon the types of vibration being studied. It is noted that there is significant intensity in the hydroxyl stretching region (2500– 3800 cm 1 spectral range). The infrared spectrum of analcime over the 500–4000 cm 1 spectral range is displayed in Fig. 3b. This figure shows the position and relative intensities of the infrared bands. There is minimal intensity observed beyond 1500 cm 1. The infrared spectrum is subdivided into sections based upon the type of vibration being analyzed. The Raman spectrum of analcime over the 900–1250 cm 1 spectral range is shown in Fig. 4a. The structure of analcime con-
Fig. 4. (a) Raman spectrum of analcime over the 900–1250 cm 1 spectral range and (b) infrared spectrum of analcime over the 800–1300 cm 1 spectral range.
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R.L. Frost et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 133 (2014) 521–525
Fig. 5. (a) Raman spectrum of analcime (upper spectrum) in the 350–700 cm 1 spectral range and (b) Raman spectrum of analcime (lower spectrum) in the 100– 350 cm 1 spectral range.
Fig. 7. (a) Raman spectrum of analcime over the 1500–1750 cm 1 spectral range and (b) infrared spectrum of analcime over the 1500–1750 cm 1 spectral range.
The Raman spectrum of analcime over the 3300–3700 cm 1 spectral range is shown in Fig. 6a. Significant intensity is found in this spectral region of the Raman spectrum. Raman bands are observed at 3501, 3542, 3558 and 3600 cm 1. These bands are assigned to the water stretching vibrations. The number of observed bands suggests that water is involved in the structure of analcime in different hydrogen bond arrangements. The infrared spectrum of analcime over the 3000–3800 cm 1 spectral region is shown in Fig. 6b. The infrared spectrum is quite broad. The spectral profile may be resolved into component bands at 3373, 3529 and 3608 cm 1. These bands are assigned to water stretching vibrations. The observation of multiple water stretching bands shows that water is in different hydrogen bond arrangements. The Raman spectrum of analcime over the 1500–1750 cm 1 spectral range is reported in Fig. 7a. Three Raman bands are observed at 1585, 1629 and 1665 cm 1. These bands are assigned to water bending vibrations. The observation of multiple water bending modes is in harmony with the number of Raman bands in the water OH stretching region. The infrared spectrum of analcime over the 1500–1750 cm 1 spectral range is shown in Fig. 7b. Infrared bands are found at 1610, 1623, 1631 and 1647 cm 1. These bands are attributed to the water bending modes. Again, the observation of multiple bands supports the concept that water is in different molecular arrangements in the structure of analcime. Conclusions
Fig. 6. (a) Raman spectrum of analcime over the 3300–3700 cm 1 spectral range and (b) infrared spectrum of analcime over the 3000–3800 cm 1 spectral range.
We have studied the mineral analcime using a combination of scanning electron microscopy with energy dispersive spectroscopy (EDS) and vibrational spectroscopy. Chemical analysis shows a homogeneous phase, composed by Na, Al, O and Si. The mineral was characterized using Raman and infrared spectroscopy.
R.L. Frost et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 133 (2014) 521–525
Raman bands are observed at 1011, 1052, 1096 and 1125 cm 1 and are assigned to the SiO stretching vibrations. The position of these bands is typical of natural zeolites. These values are in agreement with the predicted values of Dowty. The intense Raman band observed at 484 cm 1 is assigned to OSiO bending vibrations. Raman bands observed at 3501, 3542, 3558 and 3600 cm 1 are assigned to the water stretching vibrations. Raman bands observed at 1585, 1629 and 1665 cm 1 are assigned to water bending vibrations. Vibrational spectroscopy enables the characterization of the zeolite mineral analcime. Acknowledgements The financial and infra-structure support of the Discipline of Nanotechnology and Molecular Science, Science and Engineering Faculty of the Queensland University of Technology, is gratefully acknowledged. The Australian Research Council (ARC) is thanked for funding the instrumentation. The authors would like to acknowledge the Center of Microscopy at the Universidade Federal de Minas Gerais (http://www.microscopia.ufmg.br) for providing the equipment and technical support for experiments involving electron microscopy. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2014.06.034. References [1] W.H. Taylor, Zeit. Krist. 74 (1930) 1–19. [2] F. Pechar, Acta Univ., Geol, Carolinae, 1989. pp. 469–487.
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[3] F. Pechar, Crystal Res. Tech. 24 (1989) 539–542. [4] G. Ferraris, D.W. Jones, J. Yerkess, Kristallgeom. Kristallphys. Kristallchem. 135 (1972) 240–252. [5] T.D. Petersen, K.L. Currie, E.D. Ghent, N.J. Begin, R.E. Beiersdorfer, Petrology and economic geology of the Crowsnest volcanics, Alberta, in: R.W. Macqueen (Ed.), Exploring for Minerals in Alberta, Geological Survey of Canada Geoscience Contributions, Canada-Alberta, Agreement on Mineral Development (1992–1995), Geol. Surv. Can., Mem., vol. 500, 1997, pp. 163– 184. [6] J. Roux, D.L. Hamilton, J. Petrol. 17 (1976) 244–257. [7] J.F. Luhr, B. Giannetti, Contrib. Mineral. Petrol. 95 (1987) 420–436. [8] M.H. Mallah, H. Soorchi, T.F. Jooybari, Ann. Nucl. Energy 47 (2012) 140–145. [9] S.N. Azizi, A.E. Tilami, J. Solid State Chem. 198 (2013) 138–142. [10] S.N. Azizi, M. Yousefpour, Zeit. Anorgan. Allgemeine Chem. 637 (2011) 759– 765. [11] D.A. Shushkov, O.B. Kotova, B.A. Goldin, Geomat. 1 (2011) 33–40. [12] W. Pilz, Acta Phys. Hung. 61 (1987) 27–30. [13] L. Ancillotti, E.M. Castellucci, M. Becucci, Proc. SPIE – The Int. Soc. Optical Eng. 5850 (2005) 182–189. [14] E. Dowty, Phys. Chem. Min. 14 (1987) 542–552. [15] E. Dowty, Phys. Chem. Min. 14 (1987) 122–138. [16] E. Dowty, Phys. Chem. Min. 14 (1987) 80–93. [17] Y. Okada, H. Shibasaki, T. Masuda, Onoda Kenkyu Hokoku 45 (1994) 126–141. [18] A. Winkler, W. Wieker, Zeit. Chem. 18 (1978) 375–376. [19] A.E. Zadov, N.V. Chukanov, N.I. Organova, O.V. Kuz’mina, D.I. Belokovskii, M.A. Litsarev, V.G. Nechai, F.S. Sokolovskii, Zap. Vser. Min. Obshchestva 130 (2001) 26–40. [20] R.L. Frost, Y. Xi, R. Scholz, M. Costa Alves Pereira, Carbonates Evaporites 29 (2014) 33–39. [21] R.L. Frost, Y. Xi, Spectrochim. Acta A117 (2014) 428–433. [22] R.L. Frost, R. Scholz, A. Lopez, Y. Xi, L.M. Graca, J. Mol. Struct. 1059 (2014) 20– 26. [23] R.L. Frost, A. Lopez, R. Scholz, Y. Xi, J. Mol. Struct. 1059 (2014) 40–43. [24] R.L. Frost, A. Lopez, G. de Oliveira Goncalves, R. Scholz, Y. Xi, J. Mol. Struct. 1056–1057 (2014) 267–272. [25] R.L. Frost, Z.Z. Gobac, A. Lopez, Y. Xi, R. Scholz, C. Lana, R.M.F. Lima, J. Mol. Struct. 1063 (2014) 251–258. [26] E. Thilo, H. Funk, Zeit. Anorgan. Chem. 262 (1950) 185–191. [27] D.A. Mckeown, Am. Min. 90 (2005) 1506–1517. [28] J.J. Freeman, A. Wang, K.E. Kuebler, B.L. Jolliff, L. Haskin, Can. Min. 46 (2008) 1477–1500.
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
A vibrational spectroscopic study of the silicate mineral analcime – Na2(Al4SiO4O12)2H2O – A natural zeolite Ray L. Frost a,⇑, Andrés López a, Frederick L. Theiss a, Antônio Wilson Romano b, Ricardo Scholz c a
School of Chemistry, Physics and Mechanical Engineering, Science and Engineering Faculty, Queensland University of Technology, GPO Box 2434, Brisbane, Queensland 4001, Australia Geology Department, Federal University of Minas Gerais, Belo Horizonte, MG 31270-901, Brazil c Geology Department, School of Mines, Federal University of Ouro Preto, Campus Morro do Cruzeiro, Ouro Preto, MG 35400-00, Brazil b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
We have studied the mineral
analcime using SEM with EDX and vibrational spectroscopy. Analcime Na2(Al4SiO4O12)2H2O is a crystalline sodium silicate. Chemical analysis shows the mineral contains Na, Al, Fe2+ and Si. Multiple water bands indicate that water is involved with differing hydrogen bond strengths.
a r t i c l e
i n f o
Article history: Received 10 April 2014 Received in revised form 27 May 2014 Accepted 3 June 2014 Available online 14 June 2014 Keywords: Analcime Sodium silicate Raman spectroscopy Infrared spectroscopy Zeolite
a b s t r a c t We have studied the mineral analcime using a combination of scanning electron microscopy with energy dispersive spectroscopy and vibrational spectroscopy. The mineral analcime Na2(Al4SiO4O12)2H2O is a crystalline sodium silicate. Chemical analysis shows the mineral contains a range of elements including Na, Al, Fe2+ and Si. The mineral is characterized by intense Raman bands observed at 1052, 1096 and 1125 cm 1. The infrared bands are broad; nevertheless bands may be resolved at 1006 and 1119 cm 1. These bands are assigned to SiO stretching vibrational modes. Intense Raman band at 484 cm 1 is attributed to OSiO bending modes. Raman bands observed at 2501, 3542, 3558 and 3600 cm 1 are assigned to the stretching vibrations of water. Low intensity infrared bands are noted at 3373, 3529 and 3608 cm 1. The observation of multiple water bands indicate that water is involved in the structure of analcime with differing hydrogen bond strengths. This concept is supported by the number of bands in the water bending region. Vibrational spectroscopy assists with the characterization of the mineral analcime. Ó 2014 Elsevier B.V. All rights reserved.
Introduction The zeolite minerals form a complex group of aluminosilicates that often occur as accessory minerals in intermediate and basic rocks. Zeolites are microporous crystalline compounds with extre⇑ Corresponding author. Tel.: +61 7 3138 2407; fax: +61 7 3138 1804. E-mail address: [email protected] (R.L. Frost). http://dx.doi.org/10.1016/j.saa.2014.06.034 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
mely narrow pore size distributions, which have found several applications being used as ion exchangers, catalysts, among others. Analcime – Na2(Al2Si4O12)2H2O, is one of the most common minerals with the zeolite type structure. The mineral shows different symmetry of crystallization. The crystal structure was first determined by Taylor [1] with cubic symmetry and later refined by Pechar [2,3] and refined by neutron diffraction by Ferraris et al. [4] with triclinic symmetry. Analcime is usually classified as a zeo-
522
R.L. Frost et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 133 (2014) 521–525
lite mineral, but structurally and chemically it is more similar to the feldspathoids. Analcime occurs as a primary mineral in analcime basalt and other alkaline igneous rocks. It also occurs as cavity and vesicle fillings associated with prehnite, calcite and zeolites. The genesis of analcime may be related to the crystallization from a trachytic melt as a primary magmatic mineral, as proposed by Peterson et al. [5] and Roux and Hamilton [6]; or it may have been produced by ion exchange from preexisting leucite [7]. The mineral analcime as well as its similar synthetic analogs are of interest due to their application in different industries, such as the removal of nuclear wastes [8], the removal of heavy metals [9,10] and in ceramics [11]. Some Raman spectra of sodium aluminum silicates have been collected and a number of the spectra were shown to be dependent upon the number of condensed silica tetrahedra [12]. Such detailed assignment of infrared and Raman bands for a wide range of silicate structures was made by Dowty [13–16]. The thermal decomposition of sodium silicates has also been measured [17–19]. There is an apparent lack of information on the vibrational spectra of analcime. The reason for such a lack of information is not known; yet the mineral contains siloxane units. Such units lend themselves to vibrational spectroscopy. Raman spectroscopy has proven most useful for the study of mineral structure [20–25]. The objective of this research is to report the Raman and infrared spectra of analcime and to relate the spectra to the mineral structure.
lected 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 acquisitions on the crystals using the highest magnification (50) were accumulated to improve the signal to noise ratio of the Raman spectra. Raman spectra were calibrated using the 520.5 cm 1 line of a silicon wafer. Infrared spectroscopy
Experimental
Infrared spectra of analcime 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 128 scans with a resolution of 4 cm 1 and a mirror velocity of 0.6329 cm/s. Spectra were coadded to improve the signal to noise ratio. The infrared spectra are given in Supplementary information. 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 cross-product 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 r2 greater than 0.995.
Samples description and preparation
Results and discussion
The analcime sample studied in this work occurs as single crystals with tabular habitus up to 5 cm. The sample is part of the collection of the Geology Department of the Federal University of Ouro Preto, Minas Gerais, Brazil, with sample code SAD-014. The mineral sample originated from Moonen Bay, Dunvegan, Duirinish, Isle of Skye, North West Highlands (Inverness-shire), Scotland, UK. The sample was gently crushed and the associated minerals were removed under a stereomicroscope Leica MZ4. The analcime sample studied in this work was analyzed by scanning electron microscopy (SEM) in the EDS mode to support the mineral characterization.
Mineral characterization
Scanning electron microscopy (SEM) Experiments and analyses involving electron microscopy were performed in the Center of Microscopy of the Universidade Federal de Minas Gerais, Belo Horizonte, Minas Gerais, Brazil (http:// www.microscopia.ufmg.br). Analcime crystals were coated with a 5 nm layer of evaporated carbon. Secondary Electron and Backscattering Electron images were obtained using a JEOL JSM-6360LV equipment. Qualitative and semi-quantitative chemical analyses in the EDS mode were performed with a ThermoNORAN spectrometer model Quest and were applied to support the mineral characterization.
The SEM image of analcime sample studied in this work is shown in Fig. 1. The image shows a cleavage fragment up to 2 mm. Qualitative chemical analysis shows a homogeneous phase, composed by Na, Al, O and Si. The chemical data is in agreement with the chemical formula for the mineral. No other contaminant elements were observed and the sample can be considered as a pure single phase (Fig. 2). Vibrational spectroscopy The Raman spectrum of analcime over the 100–4000 cm 1 spectral range is shown in Fig. 3a. This figure shows the position and relative intensity of the Raman bands. It is noted there are
Raman microprobe spectroscopy Crystals of analcime 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 polarized light at 633 nm and col-
Fig. 1. Backscattered electron image (BSI) of an analcime single crystal up to 1.0 mm in length.
R.L. Frost et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 133 (2014) 521–525
Fig. 2. EDS analysis of analcime.
523
sists of three dimensional silicates with multiple linked silica tetrahedra [26]. Raman bands are observed at 1011, 1052, 1096 and 1125 cm 1 and are assigned to the SiO stretching vibrations. The position of these bands is typical of natural zeolites. Dowty calculated the band positions for the different ideal silicate units. Dowty showed that the –SiO3 units had a unique band position of 1025 cm 1 [16] (see Figs. 2 and 4 of this reference). Dowty calculated the Raman spectrum for these type of silicate networks and predicted two bands at around 1040 and 1070 cm 1 with an additional band at around 600 cm 1. We observe Raman bands at 1052 and 1096 cm 1 which is in close agreement with the predicted results of Dowty. The infrared spectrum over the 800–1300 cm 1 spectral range is reported in Fig. 4b. The infrared spectrum is quite broad and may be resolved into component bands at 916, 959, 975, 1006, 1047, 1081 and 1119 cm 1. These bands are assigned to SiO stretching vibrations. The Raman spectrum of analcime over the 350–700 cm 1 spectral range is shown in Fig. 5a. Intense Raman band is observed at 484 cm 1 and is assigned to OSiO bending vibrations. Bands of lesser intensity are observed at 385, 404, 511, 544, 591, 604, 612, 620 and 650 cm 1. These bands are of quite low intensity. These bands may also be attributed to displacement of the SiO and AlO tetrahedra [27,28]. It is likely that the Raman bands at 591, 604, 612, 620 and 650 cm 1 are related to the AlO and SiO chains. The Raman spectrum of analcime over the 100–350 cm 1 spectral range is shown in Fig. 5b. Intense Raman bands are observed at 115, 133 and 148 cm 1 with bands of lesser intensity noted at 184, 197, 207, 213, 226 and 243 cm 1. These bands are simply described as lattice vibrations. In addition, an intense Raman band is observed at 299 cm 1 with a shoulder at 308 cm 1. These bands may be assigned to metal oxygen vibrations.
Fig. 3. (a) Raman spectrum of analcime (upper spectrum) over the 100–4000 cm 1 spectral range and (b) infrared spectrum of analcime (lower spectrum) over the 500–4000 cm 1 spectral range.
large parts of the spectrum where little or no intensity is observed. The Raman spectrum is therefore subdivided into sections based upon the types of vibration being studied. It is noted that there is significant intensity in the hydroxyl stretching region (2500– 3800 cm 1 spectral range). The infrared spectrum of analcime over the 500–4000 cm 1 spectral range is displayed in Fig. 3b. This figure shows the position and relative intensities of the infrared bands. There is minimal intensity observed beyond 1500 cm 1. The infrared spectrum is subdivided into sections based upon the type of vibration being analyzed. The Raman spectrum of analcime over the 900–1250 cm 1 spectral range is shown in Fig. 4a. The structure of analcime con-
Fig. 4. (a) Raman spectrum of analcime over the 900–1250 cm 1 spectral range and (b) infrared spectrum of analcime over the 800–1300 cm 1 spectral range.
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R.L. Frost et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 133 (2014) 521–525
Fig. 5. (a) Raman spectrum of analcime (upper spectrum) in the 350–700 cm 1 spectral range and (b) Raman spectrum of analcime (lower spectrum) in the 100– 350 cm 1 spectral range.
Fig. 7. (a) Raman spectrum of analcime over the 1500–1750 cm 1 spectral range and (b) infrared spectrum of analcime over the 1500–1750 cm 1 spectral range.
The Raman spectrum of analcime over the 3300–3700 cm 1 spectral range is shown in Fig. 6a. Significant intensity is found in this spectral region of the Raman spectrum. Raman bands are observed at 3501, 3542, 3558 and 3600 cm 1. These bands are assigned to the water stretching vibrations. The number of observed bands suggests that water is involved in the structure of analcime in different hydrogen bond arrangements. The infrared spectrum of analcime over the 3000–3800 cm 1 spectral region is shown in Fig. 6b. The infrared spectrum is quite broad. The spectral profile may be resolved into component bands at 3373, 3529 and 3608 cm 1. These bands are assigned to water stretching vibrations. The observation of multiple water stretching bands shows that water is in different hydrogen bond arrangements. The Raman spectrum of analcime over the 1500–1750 cm 1 spectral range is reported in Fig. 7a. Three Raman bands are observed at 1585, 1629 and 1665 cm 1. These bands are assigned to water bending vibrations. The observation of multiple water bending modes is in harmony with the number of Raman bands in the water OH stretching region. The infrared spectrum of analcime over the 1500–1750 cm 1 spectral range is shown in Fig. 7b. Infrared bands are found at 1610, 1623, 1631 and 1647 cm 1. These bands are attributed to the water bending modes. Again, the observation of multiple bands supports the concept that water is in different molecular arrangements in the structure of analcime. Conclusions
Fig. 6. (a) Raman spectrum of analcime over the 3300–3700 cm 1 spectral range and (b) infrared spectrum of analcime over the 3000–3800 cm 1 spectral range.
We have studied the mineral analcime using a combination of scanning electron microscopy with energy dispersive spectroscopy (EDS) and vibrational spectroscopy. Chemical analysis shows a homogeneous phase, composed by Na, Al, O and Si. The mineral was characterized using Raman and infrared spectroscopy.
R.L. Frost et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 133 (2014) 521–525
Raman bands are observed at 1011, 1052, 1096 and 1125 cm 1 and are assigned to the SiO stretching vibrations. The position of these bands is typical of natural zeolites. These values are in agreement with the predicted values of Dowty. The intense Raman band observed at 484 cm 1 is assigned to OSiO bending vibrations. Raman bands observed at 3501, 3542, 3558 and 3600 cm 1 are assigned to the water stretching vibrations. Raman bands observed at 1585, 1629 and 1665 cm 1 are assigned to water bending vibrations. Vibrational spectroscopy enables the characterization of the zeolite mineral analcime. Acknowledgements The financial and infra-structure support of the Discipline of Nanotechnology and Molecular Science, Science and Engineering Faculty of the Queensland University of Technology, is gratefully acknowledged. The Australian Research Council (ARC) is thanked for funding the instrumentation. The authors would like to acknowledge the Center of Microscopy at the Universidade Federal de Minas Gerais (http://www.microscopia.ufmg.br) for providing the equipment and technical support for experiments involving electron microscopy. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2014.06.034. References [1] W.H. Taylor, Zeit. Krist. 74 (1930) 1–19. [2] F. Pechar, Acta Univ., Geol, Carolinae, 1989. pp. 469–487.
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