- Email: [email protected]
Raman spectroscopy of nacrite single crystals at 298 and 77 K
Spectrochimica Acta Part A 56 (2000) 931 – 939 www.elsevier.nl/locate/saa
Raman spectroscopy of nacrite single crystals at 298 and 77 K Ray L. Frost *, J. Theo Kloprogge Centre for Instrumental and De6elopmental Chemistry, Queensland Uni6ersity of Technology, 2 George Street, GPO Box 2434, Brisbane Qld 4001, Australia Accepted 22 July 1999
Abstract A Raman microscope in conjunction with a thermal stage has been used to determine the Raman spectra of single crystals of nacrite at 298 and 77 K. The spectra obtained are a function of the physics of the spectrometer and were orientation dependent. Bands are observed at 3710, 3646, 3630 and 3623 cm − 1. Upon obtaining the Raman spectra at liquid nitrogen temperature, the band at 3648 cm − 1 was not observed but an additional band at 3603 cm − 1 appeared. This latter band may be attributed to the hydroxyl stretching of non-hydrogen bonded interlayer hydroxyls in the nacrite. The bands attributed to both the inner and inner surface hydroxyls moved to lower frequencies upon cooling to liquid nitrogen temperatures. Low frequency bands also showed orientation dependence. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Nacrite; Lattice; Raman microprobe; Dehydroxylation; FT Raman; Hydroxyl
1. Introduction Nacrite is one of the kaolin polytypes and as a mineral is extremely rare. Nacrite has the same formulation as kaolinite (Al(OH)4 · 2SiO2) but basically has six layers in the unit cell. Structural studies of nacrite have relied heavily on X-ray diffraction and infrared studies [1,2]. The scarcity of spectral studies depends on the rarity of this clay mineral. Dickite, one of the other kaolin polytypes, has been studied by infrared spec* Corresponding author. Tel.: +61-7-38642407; fax: + 617-38641804. E-mail address: [email protected] (R.L. Frost)
troscopy including single crystal FTIR microscopy [3]. However the crystal structure of dickite and nacrite differ considerably [4]. The stacking sequence of nacrite layers is different from that of either kaolinite or dickite. The nacrite sequence is that of 6R polytype in which each layer is shifted by one third of the 8.9 A, lateral repeat unit relative to the layer immediately below [5]. Alternate layers are rotated by 180°. This is entirely different from the layer sequence in kaolinite and dickite where the layer shifts are one third of 5.1 A, . Thus, it is expected that the Raman spectra of nacrite would be different from that of dickite. Certainly the infrared spectra of nacrite is very different from that of dickite or kaolinite [2].
1386-1425/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 6 - 1 4 2 5 ( 9 9 ) 0 0 1 8 3 - 3
932
R.L. Frost, J.T. Kloprogge / Spectrochimica Acta Part A 56 (2000) 931–939
Raman spectroscopy has been used to a limited extent for the study of the kaolin polytypes [6– 10]. Most of the scientific research has focussed on the study of kaolinites and little work has been published on the Raman spectroscopy of the other kaolinite polytypes although FT Raman spectroscopy has been used to elucidate kaolin polytype structures [11,12]. Wiewiora et al. reported the Raman spectra of the kaolin polytypes and showed a spectrum of the hydroxyl stretching region of nacrite with bands at 3618, 3628, 3634, 3650 and 3698 cm − 1 [9]. Johnston reported single crystal Raman spectra of dickite [4]. Qualitative single crystal spectra for dickite and nacrite have been reported [13,14]. Infrared spectra of nacrite have also been reported [12] and over a wide temperature range [2]. There has been little published on the Raman spectra of the low frequency region of nacrite and few attempts to obtain single crystal spectra. This scarcity of Raman spectra of clay minerals means that important structural information is lacking. The purpose of this paper is to report the single crystal Raman spectrum of nacrite at 298 and 77 K.
2. Experimental
2.1. Nacrite samples Nacrites were also obtained from The Netherlands Institute of Sea Research (NIOZ) and from Professor Radko Kuhnel of the International Institute for Aerospace and Earth Sciences, Delft, Holland. The minerals were dried in a desiccator to remove adsorbed water and were used without further treatment. Samples were analysed for phase purity using X-ray diffraction techniques before Raman spectroscopic analysis.
2.2. Raman microprobe spectroscopy For spectra at 298 K, nacrite crystals were placed on a polished stainless steel surface on the stage of an Olympus BHSM microscope, equipped with 5 × , 20 × and 50 × objective lenses [10]. Crystal faces were adjusted perpendicular to the incident laser light by using a fine
needle. The microscope is part of a Renishaw 1000 Raman microscope system, which also includes a monochromator, a filter system and a charge coupled device (CCD). Raman spectra were excited by a Spectra-Physics model 127 HeNe laser (633 nm), recorded at a resolution of 2 cm − 1 in sections of 1000 cm − 1 for 633 nm excitation. Repeated acquisitions using the highest magnification, were accumulated to improve the signal to noise ratio in the spectra. For the 298 K spectra, data were collected at 20-s intervals for 10 min at maximum magnification (× 50). Spectra were calibrated using the 520.5 cm − 1 line of a silicon wafer. Spectra at liquid nitrogen temperature were obtained using a Linkam thermal stage (Scientific Instruments Ltd, Waterfield, Surrey, England) [15]. Samples were placed in a stainless steel cup, fitted over the silver plate of the thermal stage. For spectra at 77 K, nitrogen gas from liquid nitrogen passed through a small hole in this plate immediately below the centre of the glass disc. It was found that the best method of obtaining 77 K was to cool rapidly at 50 K min − 1. Because of the increased optical path, spectra at 77 K are noisier and require longer accumulation times. Spectra were obtained using 12-s scans for 20 min using the special short 50 × (ULWD) objective. The intensity of the nacrite spectra were found to be deceased in intensity by 0.2 using the ULWD objective compared with the normal ×50 objective. A lower Raman signal was obtained using this objective owing to the low numerical aperture of this long working distance objective. This, combined with the spherical aberration of the stage window, results in a decreased signal. Spectral manipulation such as baseline adjustment, smoothing and normalisation was performed using the Spectracalc software package ® GRAMS (Galactic Industries Corporation, Salem, NH). Band component analysis was undertaken using the Jandel ‘PEAKFIT’ software package which enabled the type of fitting function to be selected and allows specific parameters to be fixed or varied accordingly. Band fitting was done using a Lorentz–Gauss cross-product function with the minimum number of component bands used for the fitting process. The Gauss–Lorentz ratio was
R.L. Frost, J.T. Kloprogge / Spectrochimica Acta Part A 56 (2000) 931–939
maintained at values greater than 0.7 and fitting was undertaken until reproducible results were obtained with squared correlations of r 2 greater than 0.995.
3. Results and discussion
3.1. Single crystal Raman spectrum of nacrite at 298 K The nacrite crystals are large \ 1 mm and are observed easily and the crystal faces positioned in the Raman microprobe. The spectra are reported according to the Porto notation [16,17]: the propagation directions of the incident and scattered light are described in terms of the crystallographic axes a, b, c. The notation may read, for example A(BC)D. Here the A is the direction of the incident radiation and the D the direction of the collection of the scattered radiation. B is the direction of the polarisation of the electric vector of the incident radiation and the C is the direction of the analyser [18,19]. In this paper we report
933
only the spectra of the type C(AA)C. Thus, the incident radiation and collected radiation are in the same direction and the direction of the polariser and analyser are also in the same direction. We are using 180° scattering geometry. This means for example that if the incident radiation is perpendicular to the 001 face of the nacrite crystal and the electric vector is aligned with the direction of the polarisability tensor of the hydroxyl stretching vibration, then the intensity of the Raman band will be maximised. If the electric vector of the incident radiation and the electric vector are at right angles then the intensity will be at a minimum and approach zero. The Raman spectra of the hydroxyl-stretching region of nacrite at 298 and 77 K are shown in Figs. 1 and 2 and the results of the band component analyses in Table 1. Raman spectra of nacrite are characterised by bands at 3622, 3630, 3648 and 3701 cm − 1. The relative intensities of these bands depend strongly on the orientation of the crystal and scattering geometry. Spectrum (a) in Fig. 1 corresponds to the A(CC)A experiment. This experiment determines the hydroxyl stretch-
Fig. 1. Raman spectra of the hydroxyl-stretching region of nacrite at 298 K. Spectrum (a) is the experiment A(CC)A, (b) is B(CC)B and (c) is C(AA)C.
934
R.L. Frost, J.T. Kloprogge / Spectrochimica Acta Part A 56 (2000) 931–939
Fig. 2. Raman spectra of the hydroxyl-stretching region of nacrite at 77 K. Spectrum (a) is the experiment A(CC)A, (b) is B(CC)B and (c) is C(AA)C.
ing frequencies, which are from the hydroxyls at small angles to the c axis. The inner hydroxyl group of kaolinite and likewise nacrite lies parallel to the ab plane and the resultant hydroxyl stretching frequency will be determined in this experiment. Spectrum (b) results from the B(CC)B experiment. Band fitting of the 298 K spectra show that for spectrum (b), bands are found at 3701, 3648, 3630 and 3619 cm − 1 with relative intensities of 16, 25.4, 28.3 and 27.1%. The bandwidths of these hydroxyl stretching frequencies are 15.1, 21.8, 5.5 and 4.5 cm − 1. This spectrum corresponds well with the B(CC)B spectrum reported for dickite [14]. The principal feature of this spectrum is the two bands at 3630 and 3619 cm − 1 of equal relative intensities. Spectrum (c) is the C(AA)C spectrum. Bands are observed at 3702, 3646, 3630 and 3622 cm − 1. The relative intensities of these bands are 25.5, 13.6, 59.0 and 1.7% and the bandwidths are 12.5, 12.0, 5.3 and 7.5, respectively. The significant feature of this spectrum is the predominance of the intensity of the band at 3630 cm − 1.
3.2. Single crystal Raman spectra of nacrite at 77 K The Raman spectra of the hydroxyl-stretching region of nacrite at 77 K are shown in Fig. 2 and the band component analyses reported in Table 1. There are some significant differences observed between the 298 K spectrum and the 77 K spectrum. Hydroxyl-stretching bands are observed at 3700, 3630, 3621 and 3603 cm − 1. A band is observed at 3603 cm − 1 in the spectra. Compared with the inner surface hydroxyl stretching frequencies of kaolinite the bands observed at 3710 cm − 1 in the 298 K spectrum are found at lower wavenumbers at around 3699 cm − 1. For kaolinites a shift to longer wavenumbers was observed upon cooling to liquid nitrogen temperatures. The band at 3622 cm − 1 attributed to the inner hydroxyl of nacrite did not shift to lower wavenumbers. Such a result contrasts with the shift of the Raman frequency of the inner hydroxyl to 3615 cm − 1 for kaolinites [15].
R.L. Frost, J.T. Kloprogge / Spectrochimica Acta Part A 56 (2000) 931–939
935
Table 1 Results of the band component analysis of Raman spectra of the hydroxyl stretching region of nacrite at 298 and 77 K Nacrite
Band parameters
n1 Band 1
Nacrite A(CC)A 298 K
Band centre (cm−1) %area Bandwidth (cm−1)
3710 10.0 9.8%
Nacrite B(CC)B 298 K
Band centre (cm−1) %area Bandwidth (cm−1)
3711 11.0 3.0%
Nacrite C(AA)C 298 K
Band centre (cm−1) %area Bandwidth (cm−1)
3701 12.5 25.5%
Nacrite A(CC)A 77 K
Band centre (cm−1) %area Bandwidth (cm−1)
3700 8.2 7.8%
Nacrite B(CC)B 77 K
Band centre (cm−1) %area Bandwidth (cm−1)
3699 9.5 16.8%
Nacrite C(AA)C 77 K
Band centre (cm−1) %area Bandwidth (cm−1)
3697 9.2 21.6%
Nacrite C(aa)C
Band centre (cm−1) %area Bandwidth (cm−1)
n2
n3
n4
n5
n6
3653 9.6 16.8%
3642 9.1 15.3%
3633 6.9 9.7%
3623 5.6 48.5%
3646 21.8 25.4%
3630 5.5 28.3%
3622 4.5 27.1%
3648 12.0 13.6%
3630 5.3 59.0%
3619 7.5 1.9%
3630 8.8 15.0%
3621 6.4 66.0%
3604 9.7 11.8
3634 6.3 39.0
3621 6.7 35.6%
3603 8.2 7.2%
3634 4.8 74%
3621 5.8 1.7%
3603 7.2 2.4%
Band 2
3702 15.1 16.0%
3677 9.1 1.3%
3.3. Spectrum (a), the A(CC)A experiment Spectrum (a) results from the A(CC)A experiment where the incident radiation is directed at the nacrite crystal along the a axis. Bands are observed at 3700, 3630, 3621 and 3603 cm − 1. The relative intensities of these bands were determined as 7.8, 15.0, 66.0 and 11.8% with bandwidths of 8.2, 8.8, 6.4 and 9.7 cm − 1. A band at 3642 cm − 1 was not observed. It is not known what the assignment of the 3603 cm − 1 band is but one likely possibility is that of non-hydrogen bonded interlayer water in the nacrite crystals. Another possibility is non-hydrogen bonded nacrite inner surface hydroxyls, which are bonded to water. Nacrite crystals are under considerable strain since only one basal oxygen is located such that it can pair up ideally with a directed hydrogen bond from the adjacent layer [5,20]. It is possible that water molecules may be incorporated into the
structure so as to relieve this strain. Such water molecules are required to stabilise halloysite structures [5]. Zheng and Bailey identified one of the three hydroxyls in the unit cell as not participating in the interlayer hydrogen bonding [20]. If this inner surface hydroxyl was hydrogen bonded to the water molecule then the band at 3603 cm − 1 could be assigned to this hydroxyl group. The 3700 cm − 1 band is narrowed upon cooling to liquid nitrogen temperatures. It is not known why the band ascribed to the inner surface hydroxyls moves to lower frequency upon cooling to liquid nitrogen temperatures as may be compared for kaolinite [15]. Nacrite consists of layers, which are repeated but shifted by 1/3 of the 8.9 A, lateral repeat unit. Such a figure illustrating the repeat units has been shown by Bailey [15]. Upon cooling to liquid nitrogen temperatures, the layer shift may be reduced and so the strain in the nacrite crystals is relieved.
936
R.L. Frost, J.T. Kloprogge / Spectrochimica Acta Part A 56 (2000) 931–939
3.4. Spectrum (b), the BCCB experiment Spectrum (b) results from the BCCB experiment. This experiment is the more difficult experiment and probably is the experiment with the most likely to be inaccurate. The reason is that the laser is being directed at the end of the elongated hexagonal nacrite crystal. Bands are observed at 3697, 3634, 3621 and 3603 cm − 1 with relative intensities of 16.8, 39.0, 35.6 and 7.2%. The bandwidths are 9.5, 6.3, 6.7 and 8.2 cm − 1 respectively. A band of low intensity is also observed at 3677 cm − 1 with 1.3% of the total band intensity. The predominant feature of this spectrum is the two bands at 3634 and 3621 cm − 1 with about equal intensity. Electrostatic energy calculations by Giese and Datta showed that the three inner surface hydroxyls in nacrite are not identical with two OHs quasi-normal to the 001 plane and the third at 37° to the 001 plane. Further their studies showed that the inner hydroxyl group was at 20° to the 001 plane [20]. Thus in this experiment, we are measuring the Raman spectra of both the inner and inner surface hydroxyls.
3.5. Spectrum (c), the C(AA)C experiment Spectrum (c) results from the C(AA)C experiment. In this experiment, the incident and scattered radiation are on the c axis, which is at right angles to the 001 plane. The plane of polarisation of the laser and the analyser are parallel. Spectroscopically this is the easiest experiment, as we are simply collecting the spectrum at right angles to the flat side of the nacrite crystals. The predominant feature of this spectrum is the lack of intensity in the 3621 cm − 1 peak and the high relative intensity of the 3634 cm − 1 band. Zheng and Bailey suggested all three hydroxyls were at angles of 50–66° to the 001 plane [20]. All three OH– H–O contacts were bent with angles of 132 and 141° and with contacts between 2.94 and 3.12 A, . Not all three inner surface hydroxyls participated in the interlayer hydrogen bonding. The interlayer separation was 2.915 A, , which is slightly larger than for dickite. This was interpreted as a less favourable meshing of the oxygen and hydroxyl
surfaces which is a direct consequence of the layer shifts along the 8.9 A, axis. The band at 3634 cm − 1 is therefore assigned to the inner hydroxyls, which have a significant differential polarisability vector along the C axis. Such a band was reported by Johnson et al. for dickite at 3643 cm − 1 [4]. In their spectrum, significant intensity remained in the 3624 cm − 1 band. In this work, some intensity (1.7%) remains in the 3622 cm − 1 band. Such a small value would indicate that the projection of the polarisability vector of the inner hydroxyl on the C axis is very small, thus confirming the position of the inner hydroxyl as lying in the ab plane. The infrared spectra of nacrite showed peaks at 3700, 3648 and 3622 cm − 1. Whilst there is a no difference in the frequency of the 3700 and 3622 cm − 1 bands between the Raman and the infrared spectra, there is a significant difference between in wavenumbers between the 3648 cm − 1 infrared band and the 3630 cm − 1 Raman band. Such a difference might be attributed to the in-phase vibrations of the inner surface hydroxyls, such that the symmetric vibration is found in the Raman spectrum at 3630 cm − 1 and the antisymmetric vibration predominantly in the infrared at 3648 cm − 1. In crystals which have a centrosymmetric or pseudosymmetric as in the case of nacrite, structure, molecular modes that appear to be both Raman and infrared active, arise from different components in the correlation multiplet. In the nacrite unit cell, four hydroxyl groups exist and may be labelled OH1 to OH4. OH1 is assigned to the inner hydroxyl and the frequency is observed at 3622 cm − 1. OH3 has been attributed to one of the inner surface hydroxyl groups which Johnston et al. have assigned in dickite to the 3700 cm − 1 band [3]. The two remaining hydroxyl groups OH2 and OH4 combine to give the 3630 cm − 1 band. The reason why the frequencies of the inner hydroxyl groups occur at different positions is related to the strength of the hydrogen bond formed between the inner surface hydroxyls and the adjacent oxygens of the next siloxane layer. The OH3 hydroxyl is weakly hydrogen bonded and therefore the stretching frequency occurs at 3710 cm − 1. The OH2 and OH4 hydroxyls are strongly hydrogen bonded and the
R.L. Frost, J.T. Kloprogge / Spectrochimica Acta Part A 56 (2000) 931–939
937
Fig. 3. Raman spectra of the low frequency region of nacrite at 298 K. Spectrum (a) is the experiment A(CC)A, (b) is B(CC)B and (c) is C(AA)C.
frequency of these two hydroxyl groups occurs at 3630 cm − 1. It is interesting to note that in the A(CC)A spectrum the band is observed at 3630 and in the B(CC)B spectrum, the band is observed at 3634 cm − 1. This suggests that there are two overlapping bands at similar frequencies.
3.6. Spectra of the low frequency region The Raman spectra of the low frequency region corresponding to the spectra obtained at 90° to the three crystal faces of nacrite are shown in Fig. 3 and the spectral data reported in Table 2. With the use of the Raman microprobe it is difficult to accurately determine bands which occur below 150 cm − 1. The reason for this is that the Rayleigh line filters cut in and prevent the measurement of spectra in this region. The three spectra shown correspond to the three experiments A(CC)A, C(BB)C and C(AA)C. The data in Table 2 is in excellent agreement with the data originally pub-
Table 2 Raman spectral data for the low frequency region of nacrite Spectrum (a)
Spectrum (b)
Spectrum (c)
179 197.7 239 271 332 366 421.5 432.9 461.3 481.6 543.3
179 198.4 238.9 270.5 332.2 367.4 431.9 461.4 478
179 198.1 239.3 270.2 331.8 367.4 430.9 463 475
538
646.4 706 748.2 799 827 915.4 987.5 1039.9
641.2 747 800.3
542 604 646 705 751 800.5
915.1 986.8 1039
915.6 988 1036
938
R.L. Frost, J.T. Kloprogge / Spectrochimica Acta Part A 56 (2000) 931–939
lished by Wiewiora et al. for polycrystalline nacrite. One method of analysing the vibrations of sheet silicates is to consider the molecular vibrations to be composed of four parts: firstly the vibrations of the distorted octahedron AlO6 with S6 symmetry where Al is the octahedral cation, secondly the H –O –H triangle of C26 symmetry, thirdly the distorted tetrahedron of SiO4 of C36 symmetry and fourthly the vibrations of the OH group. The vibrations of the first molecular unit occur below 210 cm − 1, the second group between 200 and 300 cm − 1 and the third group above 300 cm − 1. If the AlO6 moiety has octahedral symmetry, then the point group is Oh for a perfect octahedron or S6 for the distorted octahedron. In which case one would predict Raman active bands for AlO6 octahedron at 196 cm − 1 for the A1g (n1) vibration, 162 cm − 1 for the Eg (n2) vibration and 107 cm − 1 for the F2g (n5) mode. The other vibrations for this molecular unit are predicted at 162 and 92 cm − 1 for the 2F1u (n3 and n4) modes which are infrared active but not Raman active. The mode predicted for the F2u (n6) vibration at 89 cm − 1 is neither Raman or Infrared active. The observed Raman spectra show a peak at 197 cm − 1 which is attributed to the A1g (n1) of the AlO6 in the octahedral layer. For the O – H – O triangle, the vibrational modes are A1(n1) +A1(n2) +B2(n3) all of which are both Raman and Infrared active. The predicted values for these vibrations are calculated to be at 265, 187 and 240 cm − 1, respectively. Two prominent bands are easily observed at 267 and 239 cm − 1 and these are attributed to the A1(n1) and the B2(n3) of the O – H – O triangle. These bands are the symmetric stretching and asymmetric stretching vibrations. The B2(n3) mode is not theoretically Raman active but because of symmetry reduction, is observed. The third predicted band is observed at 180.6 cm − 1 and is attributed to the B2(n3) vibrational mode. Another weak band is also observed at 256.5 cm − 1. This band may result from distortions of the O – H –O unit. One of the major differences in the spectra from the different crystal faces of the nacrite crystal occurs in this part of the spectrum. The 267 cm − 1 band varies in intensity and is most intense for the
c axis spectrum and less intense for the b axis spectrum. The predicted internal vibrations of the SiO4 tetrahedral units are known and are the n1(a1), n2(e), n3(f2) and n4(f2) modes. These vibrations are all Raman active and are predicted to occur at 678, 363, 1055 and 1020 and 455 and 470 cm − 1, respectively for the point group Td. The splitting of the F2 modes resulting in the doublets at 1055 and 1020 and 455 and 470 cm − 1 occurs when the symmetry is reduced from Td to C36. The band observed at 646 cm − 1 is attributed to the n1(a1) mode of the SiO4 tetrahedral molecular unit. This band is very weak and varies between the spectra from the different crystal faces. The n2(e) mode is observed as an intense peak at 367 cm − 1. The n4(f2) modes are observed at 432 and 463 cm − 1. The band profile in this part of the spectrum is complex with prominent shoulders at 421 and 482 cm − 1. The intensity of these two bands varies with the particular crystal face and are most intense for the a axis spectrum. These bands are also observed in the infrared spectrum at 427 and 471 cm − 1. A further shoulder is observed at 509 cm − 1. The band at 543 cm − 1 is broad and has not been assigned. Prominent peaks are observed at 706, 748 and 799 cm − 1. These bands are attributed to Al–O–Si lattice-flexing vibrations. The 748 and 799 cm − 1 bands are found in the IR absorption spectra but are weak in the IR. In the Raman spectrum of nacrite an intense sharp band is observed at 988 cm − 1 and the intensity of this band varies greatly across the spectra of the three axes and varies from a band of high intensity to zero. A band was not observed at 1055 cm − 1. The band at 915 cm − 1 has a strong shoulder at 932 cm − 1 and these bands are attributed to the OH deformation bands
4. Conclusion Single crystal Raman spectra of nacrite at 298 and 77 K have been obtained. The spectra are orientation dependent and the spectra depend on the physics of the experimental set-up for the collection of the Raman spectra. Bands are observed at 3710, 3646, 3630 and 3623 cm − 1. Upon
R.L. Frost, J.T. Kloprogge / Spectrochimica Acta Part A 56 (2000) 931–939
obtaining the Raman spectra at liquid nitrogen temperature, the band at 3648 cm − 1 was not observed but an additional band at 3603 cm − 1 appeared. This latter band is attributed to the hydroxyl stretching of water in the nacrite layers. The band attributed to the hydroxyl OH3 shifted to lower frequency upon cooling to liquid nitrogen temperature. A slight shift of the bands attributed to OH2 and 4 and also the inner hydroxyl OH1 was observed.
Acknowledgements The financial and infra-structure support of the Queensland University of Technology Centre for Instrumental and Developmental Chemistry is gratefully acknowledged. Normandy Industrial Minerals through the Chief Geologist, L. Barnes, is also acknowledged for his financial assistance.
References [1] A.M. Blount, I.M. Threadgold, S.W. Bailey, Clays Clay Miner. 17 (1969) 185. [2] R. Prost, A.S. Damene, E. Huard, J. Driard, J.P. Leydecker, Clays Clay Miner. 37 (1989) 464.
.
939
[3] C.T. Johnston, S.F. Agnew, D.L. Bish, Clays Clay Miner. 38 (1990) 573. [4] C.T. Johnston, J. Helsen, R.A. Schoonheydt, D.L. Bish, S.F. Agnew, Am. Miner. 83 (1998) 75. [5] S.W. Bailey, Review of the structural relationships of the kaolin minerals, in: H. Murray (Ed.), Kaolin Genesis and Utilisation, Specialist Publication No. 1, The Clay Minerals Society, Boulder, Colorado, USA. 1993. [6] C.T. Johnston, G. Sposito, R.R. Birge, Clays Clay Miner. 33 (1985) 483. [7] K.H. Michaelian, Can. J. Chem. 64 (1986) 285. [8] V. Pajcini, P. Dhamelincourt, Appl. Spectrosc. 48 (1994) 638. [9] A. Wiewiora, T. Wieckowski, A. Sokolowska, Arch. Miner. 135 (1979) 5. [10] R.L. Frost, S.J. van der Gaast, Clay Miner. 32 (1997) 471. [11] R.L. Frost, Clays Clay Miner. 43 (1995) 191 – 195. [12] R.L. Frost, P.M. Fredericks, J.R. Bartlett, Spectrochim. Acta 20 (1994) 667. [13] R.L. Frost, P.M. Fredericks, H.F. Shurvell, Can. J. Appl. Spectrosc. 41 (1996) 10 – 14. [14] R.L. Frost, T.H.T. Tran, L. Rintoul, J. Kristof, Analyst 123 (1998) 611. [15] U. Johansson, R.L. Frost, W. Forsling, J.T. Kloprogge, Appl. Spectrosc. 52 (1998) 1277. [16] S.P.S. Porto, Spex Speaker 13 (1968) 1. [17] T.R. Gilson, P.J. Hendra, Laser Raman Spectroscopy, Wiley – Interscience, London, 1970, pp. 97 – 99. [18] A. De Paepe, P.J. Hendra, Internet J. Vibrational Spectrosc. 3 (1999) 25. [19] B.I. Swanson, Appl. Spectrosc. 27 (1973) 382. [20] H. Zheng, S.W. Bailey, Clays Clay Miner. 42 (1994) 46.
Raman spectroscopy of nacrite single crystals at 298 and 77 K Ray L. Frost *, J. Theo Kloprogge Centre for Instrumental and De6elopmental Chemistry, Queensland Uni6ersity of Technology, 2 George Street, GPO Box 2434, Brisbane Qld 4001, Australia Accepted 22 July 1999
Abstract A Raman microscope in conjunction with a thermal stage has been used to determine the Raman spectra of single crystals of nacrite at 298 and 77 K. The spectra obtained are a function of the physics of the spectrometer and were orientation dependent. Bands are observed at 3710, 3646, 3630 and 3623 cm − 1. Upon obtaining the Raman spectra at liquid nitrogen temperature, the band at 3648 cm − 1 was not observed but an additional band at 3603 cm − 1 appeared. This latter band may be attributed to the hydroxyl stretching of non-hydrogen bonded interlayer hydroxyls in the nacrite. The bands attributed to both the inner and inner surface hydroxyls moved to lower frequencies upon cooling to liquid nitrogen temperatures. Low frequency bands also showed orientation dependence. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Nacrite; Lattice; Raman microprobe; Dehydroxylation; FT Raman; Hydroxyl
1. Introduction Nacrite is one of the kaolin polytypes and as a mineral is extremely rare. Nacrite has the same formulation as kaolinite (Al(OH)4 · 2SiO2) but basically has six layers in the unit cell. Structural studies of nacrite have relied heavily on X-ray diffraction and infrared studies [1,2]. The scarcity of spectral studies depends on the rarity of this clay mineral. Dickite, one of the other kaolin polytypes, has been studied by infrared spec* Corresponding author. Tel.: +61-7-38642407; fax: + 617-38641804. E-mail address: [email protected] (R.L. Frost)
troscopy including single crystal FTIR microscopy [3]. However the crystal structure of dickite and nacrite differ considerably [4]. The stacking sequence of nacrite layers is different from that of either kaolinite or dickite. The nacrite sequence is that of 6R polytype in which each layer is shifted by one third of the 8.9 A, lateral repeat unit relative to the layer immediately below [5]. Alternate layers are rotated by 180°. This is entirely different from the layer sequence in kaolinite and dickite where the layer shifts are one third of 5.1 A, . Thus, it is expected that the Raman spectra of nacrite would be different from that of dickite. Certainly the infrared spectra of nacrite is very different from that of dickite or kaolinite [2].
1386-1425/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 6 - 1 4 2 5 ( 9 9 ) 0 0 1 8 3 - 3
932
R.L. Frost, J.T. Kloprogge / Spectrochimica Acta Part A 56 (2000) 931–939
Raman spectroscopy has been used to a limited extent for the study of the kaolin polytypes [6– 10]. Most of the scientific research has focussed on the study of kaolinites and little work has been published on the Raman spectroscopy of the other kaolinite polytypes although FT Raman spectroscopy has been used to elucidate kaolin polytype structures [11,12]. Wiewiora et al. reported the Raman spectra of the kaolin polytypes and showed a spectrum of the hydroxyl stretching region of nacrite with bands at 3618, 3628, 3634, 3650 and 3698 cm − 1 [9]. Johnston reported single crystal Raman spectra of dickite [4]. Qualitative single crystal spectra for dickite and nacrite have been reported [13,14]. Infrared spectra of nacrite have also been reported [12] and over a wide temperature range [2]. There has been little published on the Raman spectra of the low frequency region of nacrite and few attempts to obtain single crystal spectra. This scarcity of Raman spectra of clay minerals means that important structural information is lacking. The purpose of this paper is to report the single crystal Raman spectrum of nacrite at 298 and 77 K.
2. Experimental
2.1. Nacrite samples Nacrites were also obtained from The Netherlands Institute of Sea Research (NIOZ) and from Professor Radko Kuhnel of the International Institute for Aerospace and Earth Sciences, Delft, Holland. The minerals were dried in a desiccator to remove adsorbed water and were used without further treatment. Samples were analysed for phase purity using X-ray diffraction techniques before Raman spectroscopic analysis.
2.2. Raman microprobe spectroscopy For spectra at 298 K, nacrite crystals were placed on a polished stainless steel surface on the stage of an Olympus BHSM microscope, equipped with 5 × , 20 × and 50 × objective lenses [10]. Crystal faces were adjusted perpendicular to the incident laser light by using a fine
needle. The microscope is part of a Renishaw 1000 Raman microscope system, which also includes a monochromator, a filter system and a charge coupled device (CCD). Raman spectra were excited by a Spectra-Physics model 127 HeNe laser (633 nm), recorded at a resolution of 2 cm − 1 in sections of 1000 cm − 1 for 633 nm excitation. Repeated acquisitions using the highest magnification, were accumulated to improve the signal to noise ratio in the spectra. For the 298 K spectra, data were collected at 20-s intervals for 10 min at maximum magnification (× 50). Spectra were calibrated using the 520.5 cm − 1 line of a silicon wafer. Spectra at liquid nitrogen temperature were obtained using a Linkam thermal stage (Scientific Instruments Ltd, Waterfield, Surrey, England) [15]. Samples were placed in a stainless steel cup, fitted over the silver plate of the thermal stage. For spectra at 77 K, nitrogen gas from liquid nitrogen passed through a small hole in this plate immediately below the centre of the glass disc. It was found that the best method of obtaining 77 K was to cool rapidly at 50 K min − 1. Because of the increased optical path, spectra at 77 K are noisier and require longer accumulation times. Spectra were obtained using 12-s scans for 20 min using the special short 50 × (ULWD) objective. The intensity of the nacrite spectra were found to be deceased in intensity by 0.2 using the ULWD objective compared with the normal ×50 objective. A lower Raman signal was obtained using this objective owing to the low numerical aperture of this long working distance objective. This, combined with the spherical aberration of the stage window, results in a decreased signal. Spectral manipulation such as baseline adjustment, smoothing and normalisation was performed using the Spectracalc software package ® GRAMS (Galactic Industries Corporation, Salem, NH). Band component analysis was undertaken using the Jandel ‘PEAKFIT’ software package which enabled the type of fitting function to be selected and allows specific parameters to be fixed or varied accordingly. Band fitting was done using a Lorentz–Gauss cross-product function with the minimum number of component bands used for the fitting process. The Gauss–Lorentz ratio was
R.L. Frost, J.T. Kloprogge / Spectrochimica Acta Part A 56 (2000) 931–939
maintained at values greater than 0.7 and fitting was undertaken until reproducible results were obtained with squared correlations of r 2 greater than 0.995.
3. Results and discussion
3.1. Single crystal Raman spectrum of nacrite at 298 K The nacrite crystals are large \ 1 mm and are observed easily and the crystal faces positioned in the Raman microprobe. The spectra are reported according to the Porto notation [16,17]: the propagation directions of the incident and scattered light are described in terms of the crystallographic axes a, b, c. The notation may read, for example A(BC)D. Here the A is the direction of the incident radiation and the D the direction of the collection of the scattered radiation. B is the direction of the polarisation of the electric vector of the incident radiation and the C is the direction of the analyser [18,19]. In this paper we report
933
only the spectra of the type C(AA)C. Thus, the incident radiation and collected radiation are in the same direction and the direction of the polariser and analyser are also in the same direction. We are using 180° scattering geometry. This means for example that if the incident radiation is perpendicular to the 001 face of the nacrite crystal and the electric vector is aligned with the direction of the polarisability tensor of the hydroxyl stretching vibration, then the intensity of the Raman band will be maximised. If the electric vector of the incident radiation and the electric vector are at right angles then the intensity will be at a minimum and approach zero. The Raman spectra of the hydroxyl-stretching region of nacrite at 298 and 77 K are shown in Figs. 1 and 2 and the results of the band component analyses in Table 1. Raman spectra of nacrite are characterised by bands at 3622, 3630, 3648 and 3701 cm − 1. The relative intensities of these bands depend strongly on the orientation of the crystal and scattering geometry. Spectrum (a) in Fig. 1 corresponds to the A(CC)A experiment. This experiment determines the hydroxyl stretch-
Fig. 1. Raman spectra of the hydroxyl-stretching region of nacrite at 298 K. Spectrum (a) is the experiment A(CC)A, (b) is B(CC)B and (c) is C(AA)C.
934
R.L. Frost, J.T. Kloprogge / Spectrochimica Acta Part A 56 (2000) 931–939
Fig. 2. Raman spectra of the hydroxyl-stretching region of nacrite at 77 K. Spectrum (a) is the experiment A(CC)A, (b) is B(CC)B and (c) is C(AA)C.
ing frequencies, which are from the hydroxyls at small angles to the c axis. The inner hydroxyl group of kaolinite and likewise nacrite lies parallel to the ab plane and the resultant hydroxyl stretching frequency will be determined in this experiment. Spectrum (b) results from the B(CC)B experiment. Band fitting of the 298 K spectra show that for spectrum (b), bands are found at 3701, 3648, 3630 and 3619 cm − 1 with relative intensities of 16, 25.4, 28.3 and 27.1%. The bandwidths of these hydroxyl stretching frequencies are 15.1, 21.8, 5.5 and 4.5 cm − 1. This spectrum corresponds well with the B(CC)B spectrum reported for dickite [14]. The principal feature of this spectrum is the two bands at 3630 and 3619 cm − 1 of equal relative intensities. Spectrum (c) is the C(AA)C spectrum. Bands are observed at 3702, 3646, 3630 and 3622 cm − 1. The relative intensities of these bands are 25.5, 13.6, 59.0 and 1.7% and the bandwidths are 12.5, 12.0, 5.3 and 7.5, respectively. The significant feature of this spectrum is the predominance of the intensity of the band at 3630 cm − 1.
3.2. Single crystal Raman spectra of nacrite at 77 K The Raman spectra of the hydroxyl-stretching region of nacrite at 77 K are shown in Fig. 2 and the band component analyses reported in Table 1. There are some significant differences observed between the 298 K spectrum and the 77 K spectrum. Hydroxyl-stretching bands are observed at 3700, 3630, 3621 and 3603 cm − 1. A band is observed at 3603 cm − 1 in the spectra. Compared with the inner surface hydroxyl stretching frequencies of kaolinite the bands observed at 3710 cm − 1 in the 298 K spectrum are found at lower wavenumbers at around 3699 cm − 1. For kaolinites a shift to longer wavenumbers was observed upon cooling to liquid nitrogen temperatures. The band at 3622 cm − 1 attributed to the inner hydroxyl of nacrite did not shift to lower wavenumbers. Such a result contrasts with the shift of the Raman frequency of the inner hydroxyl to 3615 cm − 1 for kaolinites [15].
R.L. Frost, J.T. Kloprogge / Spectrochimica Acta Part A 56 (2000) 931–939
935
Table 1 Results of the band component analysis of Raman spectra of the hydroxyl stretching region of nacrite at 298 and 77 K Nacrite
Band parameters
n1 Band 1
Nacrite A(CC)A 298 K
Band centre (cm−1) %area Bandwidth (cm−1)
3710 10.0 9.8%
Nacrite B(CC)B 298 K
Band centre (cm−1) %area Bandwidth (cm−1)
3711 11.0 3.0%
Nacrite C(AA)C 298 K
Band centre (cm−1) %area Bandwidth (cm−1)
3701 12.5 25.5%
Nacrite A(CC)A 77 K
Band centre (cm−1) %area Bandwidth (cm−1)
3700 8.2 7.8%
Nacrite B(CC)B 77 K
Band centre (cm−1) %area Bandwidth (cm−1)
3699 9.5 16.8%
Nacrite C(AA)C 77 K
Band centre (cm−1) %area Bandwidth (cm−1)
3697 9.2 21.6%
Nacrite C(aa)C
Band centre (cm−1) %area Bandwidth (cm−1)
n2
n3
n4
n5
n6
3653 9.6 16.8%
3642 9.1 15.3%
3633 6.9 9.7%
3623 5.6 48.5%
3646 21.8 25.4%
3630 5.5 28.3%
3622 4.5 27.1%
3648 12.0 13.6%
3630 5.3 59.0%
3619 7.5 1.9%
3630 8.8 15.0%
3621 6.4 66.0%
3604 9.7 11.8
3634 6.3 39.0
3621 6.7 35.6%
3603 8.2 7.2%
3634 4.8 74%
3621 5.8 1.7%
3603 7.2 2.4%
Band 2
3702 15.1 16.0%
3677 9.1 1.3%
3.3. Spectrum (a), the A(CC)A experiment Spectrum (a) results from the A(CC)A experiment where the incident radiation is directed at the nacrite crystal along the a axis. Bands are observed at 3700, 3630, 3621 and 3603 cm − 1. The relative intensities of these bands were determined as 7.8, 15.0, 66.0 and 11.8% with bandwidths of 8.2, 8.8, 6.4 and 9.7 cm − 1. A band at 3642 cm − 1 was not observed. It is not known what the assignment of the 3603 cm − 1 band is but one likely possibility is that of non-hydrogen bonded interlayer water in the nacrite crystals. Another possibility is non-hydrogen bonded nacrite inner surface hydroxyls, which are bonded to water. Nacrite crystals are under considerable strain since only one basal oxygen is located such that it can pair up ideally with a directed hydrogen bond from the adjacent layer [5,20]. It is possible that water molecules may be incorporated into the
structure so as to relieve this strain. Such water molecules are required to stabilise halloysite structures [5]. Zheng and Bailey identified one of the three hydroxyls in the unit cell as not participating in the interlayer hydrogen bonding [20]. If this inner surface hydroxyl was hydrogen bonded to the water molecule then the band at 3603 cm − 1 could be assigned to this hydroxyl group. The 3700 cm − 1 band is narrowed upon cooling to liquid nitrogen temperatures. It is not known why the band ascribed to the inner surface hydroxyls moves to lower frequency upon cooling to liquid nitrogen temperatures as may be compared for kaolinite [15]. Nacrite consists of layers, which are repeated but shifted by 1/3 of the 8.9 A, lateral repeat unit. Such a figure illustrating the repeat units has been shown by Bailey [15]. Upon cooling to liquid nitrogen temperatures, the layer shift may be reduced and so the strain in the nacrite crystals is relieved.
936
R.L. Frost, J.T. Kloprogge / Spectrochimica Acta Part A 56 (2000) 931–939
3.4. Spectrum (b), the BCCB experiment Spectrum (b) results from the BCCB experiment. This experiment is the more difficult experiment and probably is the experiment with the most likely to be inaccurate. The reason is that the laser is being directed at the end of the elongated hexagonal nacrite crystal. Bands are observed at 3697, 3634, 3621 and 3603 cm − 1 with relative intensities of 16.8, 39.0, 35.6 and 7.2%. The bandwidths are 9.5, 6.3, 6.7 and 8.2 cm − 1 respectively. A band of low intensity is also observed at 3677 cm − 1 with 1.3% of the total band intensity. The predominant feature of this spectrum is the two bands at 3634 and 3621 cm − 1 with about equal intensity. Electrostatic energy calculations by Giese and Datta showed that the three inner surface hydroxyls in nacrite are not identical with two OHs quasi-normal to the 001 plane and the third at 37° to the 001 plane. Further their studies showed that the inner hydroxyl group was at 20° to the 001 plane [20]. Thus in this experiment, we are measuring the Raman spectra of both the inner and inner surface hydroxyls.
3.5. Spectrum (c), the C(AA)C experiment Spectrum (c) results from the C(AA)C experiment. In this experiment, the incident and scattered radiation are on the c axis, which is at right angles to the 001 plane. The plane of polarisation of the laser and the analyser are parallel. Spectroscopically this is the easiest experiment, as we are simply collecting the spectrum at right angles to the flat side of the nacrite crystals. The predominant feature of this spectrum is the lack of intensity in the 3621 cm − 1 peak and the high relative intensity of the 3634 cm − 1 band. Zheng and Bailey suggested all three hydroxyls were at angles of 50–66° to the 001 plane [20]. All three OH– H–O contacts were bent with angles of 132 and 141° and with contacts between 2.94 and 3.12 A, . Not all three inner surface hydroxyls participated in the interlayer hydrogen bonding. The interlayer separation was 2.915 A, , which is slightly larger than for dickite. This was interpreted as a less favourable meshing of the oxygen and hydroxyl
surfaces which is a direct consequence of the layer shifts along the 8.9 A, axis. The band at 3634 cm − 1 is therefore assigned to the inner hydroxyls, which have a significant differential polarisability vector along the C axis. Such a band was reported by Johnson et al. for dickite at 3643 cm − 1 [4]. In their spectrum, significant intensity remained in the 3624 cm − 1 band. In this work, some intensity (1.7%) remains in the 3622 cm − 1 band. Such a small value would indicate that the projection of the polarisability vector of the inner hydroxyl on the C axis is very small, thus confirming the position of the inner hydroxyl as lying in the ab plane. The infrared spectra of nacrite showed peaks at 3700, 3648 and 3622 cm − 1. Whilst there is a no difference in the frequency of the 3700 and 3622 cm − 1 bands between the Raman and the infrared spectra, there is a significant difference between in wavenumbers between the 3648 cm − 1 infrared band and the 3630 cm − 1 Raman band. Such a difference might be attributed to the in-phase vibrations of the inner surface hydroxyls, such that the symmetric vibration is found in the Raman spectrum at 3630 cm − 1 and the antisymmetric vibration predominantly in the infrared at 3648 cm − 1. In crystals which have a centrosymmetric or pseudosymmetric as in the case of nacrite, structure, molecular modes that appear to be both Raman and infrared active, arise from different components in the correlation multiplet. In the nacrite unit cell, four hydroxyl groups exist and may be labelled OH1 to OH4. OH1 is assigned to the inner hydroxyl and the frequency is observed at 3622 cm − 1. OH3 has been attributed to one of the inner surface hydroxyl groups which Johnston et al. have assigned in dickite to the 3700 cm − 1 band [3]. The two remaining hydroxyl groups OH2 and OH4 combine to give the 3630 cm − 1 band. The reason why the frequencies of the inner hydroxyl groups occur at different positions is related to the strength of the hydrogen bond formed between the inner surface hydroxyls and the adjacent oxygens of the next siloxane layer. The OH3 hydroxyl is weakly hydrogen bonded and therefore the stretching frequency occurs at 3710 cm − 1. The OH2 and OH4 hydroxyls are strongly hydrogen bonded and the
R.L. Frost, J.T. Kloprogge / Spectrochimica Acta Part A 56 (2000) 931–939
937
Fig. 3. Raman spectra of the low frequency region of nacrite at 298 K. Spectrum (a) is the experiment A(CC)A, (b) is B(CC)B and (c) is C(AA)C.
frequency of these two hydroxyl groups occurs at 3630 cm − 1. It is interesting to note that in the A(CC)A spectrum the band is observed at 3630 and in the B(CC)B spectrum, the band is observed at 3634 cm − 1. This suggests that there are two overlapping bands at similar frequencies.
3.6. Spectra of the low frequency region The Raman spectra of the low frequency region corresponding to the spectra obtained at 90° to the three crystal faces of nacrite are shown in Fig. 3 and the spectral data reported in Table 2. With the use of the Raman microprobe it is difficult to accurately determine bands which occur below 150 cm − 1. The reason for this is that the Rayleigh line filters cut in and prevent the measurement of spectra in this region. The three spectra shown correspond to the three experiments A(CC)A, C(BB)C and C(AA)C. The data in Table 2 is in excellent agreement with the data originally pub-
Table 2 Raman spectral data for the low frequency region of nacrite Spectrum (a)
Spectrum (b)
Spectrum (c)
179 197.7 239 271 332 366 421.5 432.9 461.3 481.6 543.3
179 198.4 238.9 270.5 332.2 367.4 431.9 461.4 478
179 198.1 239.3 270.2 331.8 367.4 430.9 463 475
538
646.4 706 748.2 799 827 915.4 987.5 1039.9
641.2 747 800.3
542 604 646 705 751 800.5
915.1 986.8 1039
915.6 988 1036
938
R.L. Frost, J.T. Kloprogge / Spectrochimica Acta Part A 56 (2000) 931–939
lished by Wiewiora et al. for polycrystalline nacrite. One method of analysing the vibrations of sheet silicates is to consider the molecular vibrations to be composed of four parts: firstly the vibrations of the distorted octahedron AlO6 with S6 symmetry where Al is the octahedral cation, secondly the H –O –H triangle of C26 symmetry, thirdly the distorted tetrahedron of SiO4 of C36 symmetry and fourthly the vibrations of the OH group. The vibrations of the first molecular unit occur below 210 cm − 1, the second group between 200 and 300 cm − 1 and the third group above 300 cm − 1. If the AlO6 moiety has octahedral symmetry, then the point group is Oh for a perfect octahedron or S6 for the distorted octahedron. In which case one would predict Raman active bands for AlO6 octahedron at 196 cm − 1 for the A1g (n1) vibration, 162 cm − 1 for the Eg (n2) vibration and 107 cm − 1 for the F2g (n5) mode. The other vibrations for this molecular unit are predicted at 162 and 92 cm − 1 for the 2F1u (n3 and n4) modes which are infrared active but not Raman active. The mode predicted for the F2u (n6) vibration at 89 cm − 1 is neither Raman or Infrared active. The observed Raman spectra show a peak at 197 cm − 1 which is attributed to the A1g (n1) of the AlO6 in the octahedral layer. For the O – H – O triangle, the vibrational modes are A1(n1) +A1(n2) +B2(n3) all of which are both Raman and Infrared active. The predicted values for these vibrations are calculated to be at 265, 187 and 240 cm − 1, respectively. Two prominent bands are easily observed at 267 and 239 cm − 1 and these are attributed to the A1(n1) and the B2(n3) of the O – H – O triangle. These bands are the symmetric stretching and asymmetric stretching vibrations. The B2(n3) mode is not theoretically Raman active but because of symmetry reduction, is observed. The third predicted band is observed at 180.6 cm − 1 and is attributed to the B2(n3) vibrational mode. Another weak band is also observed at 256.5 cm − 1. This band may result from distortions of the O – H –O unit. One of the major differences in the spectra from the different crystal faces of the nacrite crystal occurs in this part of the spectrum. The 267 cm − 1 band varies in intensity and is most intense for the
c axis spectrum and less intense for the b axis spectrum. The predicted internal vibrations of the SiO4 tetrahedral units are known and are the n1(a1), n2(e), n3(f2) and n4(f2) modes. These vibrations are all Raman active and are predicted to occur at 678, 363, 1055 and 1020 and 455 and 470 cm − 1, respectively for the point group Td. The splitting of the F2 modes resulting in the doublets at 1055 and 1020 and 455 and 470 cm − 1 occurs when the symmetry is reduced from Td to C36. The band observed at 646 cm − 1 is attributed to the n1(a1) mode of the SiO4 tetrahedral molecular unit. This band is very weak and varies between the spectra from the different crystal faces. The n2(e) mode is observed as an intense peak at 367 cm − 1. The n4(f2) modes are observed at 432 and 463 cm − 1. The band profile in this part of the spectrum is complex with prominent shoulders at 421 and 482 cm − 1. The intensity of these two bands varies with the particular crystal face and are most intense for the a axis spectrum. These bands are also observed in the infrared spectrum at 427 and 471 cm − 1. A further shoulder is observed at 509 cm − 1. The band at 543 cm − 1 is broad and has not been assigned. Prominent peaks are observed at 706, 748 and 799 cm − 1. These bands are attributed to Al–O–Si lattice-flexing vibrations. The 748 and 799 cm − 1 bands are found in the IR absorption spectra but are weak in the IR. In the Raman spectrum of nacrite an intense sharp band is observed at 988 cm − 1 and the intensity of this band varies greatly across the spectra of the three axes and varies from a band of high intensity to zero. A band was not observed at 1055 cm − 1. The band at 915 cm − 1 has a strong shoulder at 932 cm − 1 and these bands are attributed to the OH deformation bands
4. Conclusion Single crystal Raman spectra of nacrite at 298 and 77 K have been obtained. The spectra are orientation dependent and the spectra depend on the physics of the experimental set-up for the collection of the Raman spectra. Bands are observed at 3710, 3646, 3630 and 3623 cm − 1. Upon
R.L. Frost, J.T. Kloprogge / Spectrochimica Acta Part A 56 (2000) 931–939
obtaining the Raman spectra at liquid nitrogen temperature, the band at 3648 cm − 1 was not observed but an additional band at 3603 cm − 1 appeared. This latter band is attributed to the hydroxyl stretching of water in the nacrite layers. The band attributed to the hydroxyl OH3 shifted to lower frequency upon cooling to liquid nitrogen temperature. A slight shift of the bands attributed to OH2 and 4 and also the inner hydroxyl OH1 was observed.
Acknowledgements The financial and infra-structure support of the Queensland University of Technology Centre for Instrumental and Developmental Chemistry is gratefully acknowledged. Normandy Industrial Minerals through the Chief Geologist, L. Barnes, is also acknowledged for his financial assistance.
References [1] A.M. Blount, I.M. Threadgold, S.W. Bailey, Clays Clay Miner. 17 (1969) 185. [2] R. Prost, A.S. Damene, E. Huard, J. Driard, J.P. Leydecker, Clays Clay Miner. 37 (1989) 464.
.
939
[3] C.T. Johnston, S.F. Agnew, D.L. Bish, Clays Clay Miner. 38 (1990) 573. [4] C.T. Johnston, J. Helsen, R.A. Schoonheydt, D.L. Bish, S.F. Agnew, Am. Miner. 83 (1998) 75. [5] S.W. Bailey, Review of the structural relationships of the kaolin minerals, in: H. Murray (Ed.), Kaolin Genesis and Utilisation, Specialist Publication No. 1, The Clay Minerals Society, Boulder, Colorado, USA. 1993. [6] C.T. Johnston, G. Sposito, R.R. Birge, Clays Clay Miner. 33 (1985) 483. [7] K.H. Michaelian, Can. J. Chem. 64 (1986) 285. [8] V. Pajcini, P. Dhamelincourt, Appl. Spectrosc. 48 (1994) 638. [9] A. Wiewiora, T. Wieckowski, A. Sokolowska, Arch. Miner. 135 (1979) 5. [10] R.L. Frost, S.J. van der Gaast, Clay Miner. 32 (1997) 471. [11] R.L. Frost, Clays Clay Miner. 43 (1995) 191 – 195. [12] R.L. Frost, P.M. Fredericks, J.R. Bartlett, Spectrochim. Acta 20 (1994) 667. [13] R.L. Frost, P.M. Fredericks, H.F. Shurvell, Can. J. Appl. Spectrosc. 41 (1996) 10 – 14. [14] R.L. Frost, T.H.T. Tran, L. Rintoul, J. Kristof, Analyst 123 (1998) 611. [15] U. Johansson, R.L. Frost, W. Forsling, J.T. Kloprogge, Appl. Spectrosc. 52 (1998) 1277. [16] S.P.S. Porto, Spex Speaker 13 (1968) 1. [17] T.R. Gilson, P.J. Hendra, Laser Raman Spectroscopy, Wiley – Interscience, London, 1970, pp. 97 – 99. [18] A. De Paepe, P.J. Hendra, Internet J. Vibrational Spectrosc. 3 (1999) 25. [19] B.I. Swanson, Appl. Spectrosc. 27 (1973) 382. [20] H. Zheng, S.W. Bailey, Clays Clay Miner. 42 (1994) 46.