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29Si and 27Al NMR study of amorphous and paracrystalline opals from Australia
Journal of Non-Crystalline Solids 332 (2003) 242–248 www.elsevier.com/locate/jnoncrysol
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Si and 27Al NMR study of amorphous and paracrystalline opals from Australia L.D. Brown *, A.S. Ray, P.S. Thomas
Department of Chemistry, Materials and Forensic Sciences, Faculty of Science, University of Technology, Sydney, P.O. Box 123, Broadway, NSW 2007, Australia Received 1 January 2003
Abstract Four opal-AG (amorphous) and two opal-CT (paracrystalline) samples obtained from various regions in Australia were investigated with 29 Si NMR and 27 Al NMR. The proton cross-polarization 29 Si NMR technique was used and the resulting spectra consisted of two main resonances: )102.0 and )111.2 ppm for opal-AG; and )102.5 and )112.2 ppm for opal-CT. These peaks were assigned to the Q3 (1OH) and Q4 resonances, respectively. Using very short contact times, a third, very weak peak at )94 ppm was resolved in an opal-CT specimen, which was assigned to silicon in the Q2 (2OH) arrangement (i.e. a silicon with twin hydroxyl groups). It was found that the opal-CT samples contained a higher proportion of both geminal and vicinal silanol groups (Q2 and Q3 ) than the opal-AG samples. The geminal silanol groups present in opal-AG and opal-CT are not restricted to opal-AN. The full-width at half-maximum (FWHM) values were 9.5 ppm for the opal-AG samples, and 6.5 ppm for both opal-CT samples, a result which confirms that opal-CT has a higher degree of short-range structural order than opal-AG. The 27 Al NMR spectra of the opals all showed a single resonance at +52 ppm, indicating that the aluminium exists in a tetrahedral arrangement incorporated within the opal structure. Ó 2003 Elsevier B.V. All rights reserved.
1. Introduction Opals are naturally occurring hydrous silica minerals, which exhibit various degrees of structural disorder. Jones and Segnit [1] classified opal into three groups according to its X-ray diffraction (XRD) pattern. Opals that produced an XRD
* Corresponding author. Address: 58 Seymour Street, Hurstville Grove, NSW 2220, Australia. Tel.: +61-2 9570 1634; fax: +61-2 9514 1628. E-mail address: [email protected] (L.D. Brown).
pattern similar to a-cristobalite with minor evidence of tridymite were termed opal-C; opals that showed signs of both cristobalite and tridymite were designated opal-CT; while opals that yielded an XRD pattern that resembled amorphous silica were categorised as opal-A. Using infrared spectroscopy, Langer and Fl€ orke [2], further subdivided opal-A into two groups, opal-AG for gellike, and opal-AN for network-or glass-like. The majority of opals found in Australia are opal-AG. Opal-AG is initially formed from a colloidal silica solution, which slowly coagulates into a solid aggregate of silica particles [3]. Water
0022-3093/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2003.09.027
L.D. Brown et al. / Journal of Non-Crystalline Solids 332 (2003) 242–248
is incorporated into the structure as molecular water (H2 O) and silanol (Si–OH) groups. In addition to vicinal (single) hydroxyl groups that are contained within natural opals, it is possible that geminal (double) hydroxyl groups may also arise [4]. They are most likely to occur at lattice imperfections caused by incomplete polycondensation of silica tetrahedra [5], such as at the surfaces of the silica spheres where the continuous silica network is interrupted. However, there seems to be some uncertainty in the literature concerning the existence of these twin hydroxyl groups [2,6–8]. This study seeks to clarify the nature of the silanol bonds contained within opal-AG and opal-CT using nuclear magnetic resonance (NMR) spectroscopy. NMR spectroscopy is an analytical technique which allows the various local environments of certain nuclei to be investigated. In this case, 29 Si NMR is used to study the relative disorder of opal-AG and opal-CT, and also the silanol bonding arrangement that constitutes the chemisorbed water. Previous NMR studies of opal have been restricted to only two or three types from Australia [7,9,10], despite the fact that Australia remains the worldÕs largest supplier of opals. The technique of proton cross-polarization (CP) 29 Si NMR is also used to study the opals more effectively. Essentially, a single radio frequency (RF) pulse is applied to the protons, and then two RF fields are simultaneously applied to the 1 H and 29 Si nuclei, enabling a transfer of polarization to occur. The time during which the two RF fields are applied is called the contact time, and varying this parameter alters the relative intensities of peaks in the NMR spectrum. Details of the technique are described elsewhere [11]. For short contact times, silicon atoms in close proximity to protons will give enhanced signals than those further from protons [12]. Hence, CP measurements will allow the observation of the degree of hydroxyl substitution of the Si atoms. In this discussion, the standard Qn notation is used, where Q represents a given silica tetrahedron, and n represents the number of associated bridging oxygens (Si–O–Si) per tetrahedron. Silicon nuclei that are more shielded result in higher-
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field (more negative) chemical shifts. In this way, silicates in which the tetrahedra are fully polymerised (Q4 ) yield chemical shifts progressively more negative than those that are not (Q3 ). Similarly, increasing the number of substituted Al ions in each of the SiO4 tetrahedra results in a correspondingly less negative chemical shift [13]. 27 Al NMR is also briefly used to reveal the way in which aluminium is incorporated into the silica network within opals.
2. Experimental For this study, four opal-AG and two opal-CT samples were analyzed with magic angle spinning (MAS) 29 Si NMR spectroscopy. The opal-AG samples were obtained from four major opal fields within Australia: Lightning Ridge, Coober Pedy, Andamooka and White Cliffs; while the opal-CT samples were from Tintenbar in Australia, and Mexico. One of the opal samples from Tintenbar was heated to 700 °C, and analyzed to compare the results of the aged and unaged 29 Si NMR spectra. All samples were cut to ensure there was no contamination from the surrounding host rock, before being crushed with an agate mortar and pestle. The larger pieces of crushed opal were selectively packed into the sample rotor. All spectra were recorded on a Bruker 300 MAS NMR spectrometer. The 29 Si NMR single-pulse spectra were acquired using an operating frequency of 56.2 MHz, and a spinning rate of 3 kHz. Chemical shifts were measured relative to the 29 Si resonance in kaolin. A recycle delay time of 20 s was used between each 90° pulse, until 256 scans were accumulated. For the 29 Si {1 H} CP NMR experiments, a contact time of between 6 and 10 ms was used, with a recycle delay time of 1 s. Sample spinning speeds were 7 kHz. A minimum of 10 240 scans were collected for each sample. The 27 Al NMR spectra were obtained with an operating frequency of 78.2 MHz, and a spinning rate of 7 kHz. The chemical shifts were measured relative to the 27 Al resonance in AlCl3 . A total of 1024 FIDs were collected with a recycle delay time of 1 s.
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Fig. 1. Single-pulse (bottom).
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Si NMR spectra of an unaged Tintenbar opal sample (top) and a thermally aged Tintenbar opal sample
3. Results The single-pulse 29 Si MAS NMR spectra showed a single broad asymmetric peak centred at )111.8 ppm for opal-AG samples, and at )111.9 ppm for opal-CT samples. After 256 scans were completed on the opal-CT sample from Tintenbar, the spectrum showed signs of a second peak at around )102 ppm, so additional pulses were acquired to improve the S/N ratio and attempt to resolve the apparent peak. After numerous scans were accumulated, the spectrum revealed a small but distinct second peak at )102.5 ppm. The thermally aged Tintenbar specimen was also analyzed with 29 Si NMR, and the intensity of the resonance at )102.5 ppm had substantially reduced when compared to the original unaged sample. The two spectra of the Tintenbar samples are given in Fig. 1. Full-width at half-maximum (FWHM) values were 9.5 ppm for all four opalAG samples, and 6.5 ppm for the two opal-CT samples. These results are in close agreement with those of other workers published in the literature [9,14]. The 29 Si {1 H} CP MAS NMR spectra of the opal-AG samples revealed a small shoulder at )102.0 ppm in addition to the primary peak, which was centred at about )111.2 ppm. CP spectra of opal-CT samples showed the original
peak at )112.2 ppm and a greatly enhanced peak at )102.5 ppm. Fig. 2 shows the CP spectra of the
Fig. 2. Proton cross-polarization 29 Si NMR spectra of opals. From top to bottom: Mexican, Tintenbar, Coober Pedy, White Cliffs, Andamooka, and Lightning Ridge.
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structural arrangement. The asymmetry in the 29 Si NMR spectra of opal-AG samples is most likely due to a combination of the major Q4 resonance at )111 ppm and the lesser signal from Q3 (1OH), and perhaps Q4 (1Al), both of which occur at approximately )102 ppm [15]. 4.2. Cross-polarization
Fig. 3. A proton cross-polarization 29 Si NMR spectrum of the Tintenbar sample, obtained with a contact time of 6 ms (bottom). A proton cross-polarization 29 Si NMR spectrum of the Tintenbar sample, obtained with a contact time of 6 ms, with a 55 point fast Fourier transform smoothing function applied (top).
opals, using a contact time of 10 ms. A small shoulder was discernible at approximately )94 ppm for these spectra, especially noticeable with the sample from Tintenbar, so the contact time was reduced to 6 ms and another spectrum was recorded from this sample. In that spectrum, an extremely weak peak was resolved at approximately )93.5 ppm (Fig. 3). 27 Al NMR was also performed on all six opal samples. Each of the spectra showed a single, welldefined peak between at +51.6 and +53.2 ppm.
4. Discussion 4.1. Single pulse The single broad peak at )111.8 ppm is characteristic of amorphous silica that is in the Q4
The resonance at )102.5 ppm is assigned to silicon in the Q3 (1OH) configuration, for two reasons. Firstly, the peak is absent for the sample that had been thermally aged. After heating, silanol bonds are expected to be permanently removed from the silica network, and this is reflected in the difference between the two spectra. Secondly, the peak is significantly enhanced when the proton cross-polarization technique is used, indicating that it is not caused by Q3 (1Al). The CP spectra closely resemble those of other researchers [7,10]. The weak resonance at approximately )94.0 ppm is most likely due to silicon in the Q2 (2OH) structural arrangement. Shorter contact times allow silicon atoms that are in close proximity to protons to give enhanced signals with respect to those further from protons [16]. When the contact time was reduced from 10 to 6 ms, both the )102.5 ppm peak and the )94.0 ppm peak were significantly intensified, signifying that it is most likely a Q2 resonance. This result suggests that there are indeed geminal silanol groups contained within opal-AG and opal-CT, and they are not restricted to opal-AN, as suggested elsewhere [7]. The silica structure of opal-AG now appears to be remarkably similar to that of a synthetic silica gel [17]. Although the concentration of these twin hydroxyl groups must be extremely low, this result elucidates some earlier discussions on the thermal characteristics of opals, which postulated the existence of geminal hydroxyl groups within the structure [18]. Some of the spectra were curve-fitted (using Origin 6.1 software) and generally the spectra could be approximated very well with the superposition of three Gaussian peaks at )93.5, )102.5 and )111.2 ppm. Fig. 4 shows the deconvolution of the single-pulse spectrum of a Tintenbar sample; Fig. 5 shows the deconvolution of the CP
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Fig. 4. A single-pulse 29 Si NMR spectrum of unaged Tintenbar opal sample (top). The curve below it is a theoretical curve resulting from the superposition of the two Gaussian peaks (bottom) at )102.5 and )111.9 ppm.
Fig. 5. A proton cross-polarization 29 Si NMR spectrum of Andamooka opal sample (top). The curve below it is a theoretical curve resulting from the superposition of the three Gaussian peaks (bottom) centred at )94.0, )102.5 and )111.5 ppm.
spectrum of an Andamooka sample. From the proportions of these component peaks, it is clearly
evident that opal-CT contains more single hydroxyl bonds than opal-AG, and probably more twin hydroxyl groups as well. This deduction is in contrast to what is reported elsewhere [2,19]. A curve fit was also attempted on the CP NMR spectrum of the Tintenbar sample, where the contact time was 6 ms. A reasonable fit was obtained with the three principal Q2 , Q3 , and Q4 resonances; however a slightly more accurate fit was achieved if a forth peak at approximately )98 ppm was included. This peak may be due to two different levels of shielding of Q3 (1OH) groups, caused by varying degrees of H-bonding such as an internal and a surface silanol, as proposed by Adams [10]. The degree of structural order of the sample can be inferred from the 29 Si NMR signal; the broadness of the signal is due to a distribution in the average Si–O–Si bond angle per tetrahedron, and thus represents the extent of the relative localised disorder of the silica framework [14]. In this respect, opal-CT obviously has more short-range order than opal-AG, because it shows a halfwidth of only 6.5 ppm, compared to 9.5 ppm for opalAG. The chemical shift, d, of silica polymorphs also correlates with the Si–O–Si bond angle [14,20]. The opal-AG spectra show Q4 resonances at )111.2 ppm, while the two opal-CT spectra show their Q4 shifted to an even higher field, at )112.2 ppm. This signifies that opal-CT has a slightly
L.D. Brown et al. / Journal of Non-Crystalline Solids 332 (2003) 242–248
greater mean Si–O–Si angle than opal-AG, a result that may be a consequence of the gradual siloxane rearrangement in the transformation of opal-A to opal-CT. For all the types of opal investigated, the 27 A1 NMR spectra display a single resonance around +52 ppm. This result establishes that all the Al ions are all in a tetrahedral arrangement; i.e. substituting for silicon atoms within the molecular structure, and thus opal is technically an aluminosilicate. This is in contrast to what is reported elsewhere [21], which indicated that there was little or no Al for Si substitution in Australian opals. Since these samples contain as much as 1% Al [22,23], it is not surprising that the Al atoms occasionally substitute for silicon; the opals were originally polymerised from silica-rich solutions, which must have contained sufficient Al to become incorporated within the network structure. A resonance corresponding to an octahedral configuration of Al (d þ5 ppm), was not detected. These results concerning the tetrahedral Al content of opal-AG and opal-CT complement the results of a study of opal-A speleothems [24], in which most of the samples contained allophane and therefore octahedrally coordinated Al. According to the work done by Merino [25] on the tetrahedral-aluminium content of aqueous lowtemperature silicates, and assuming the temperature of formation of opal-AG was 25 °C, this result indicates that the original pH of the sol must have been in excess of 6.5; since this is the higher end of the pH range over which the octahedral to tetrahedral transition can take place (5.5–6.5). This is in accordance with the theory of opal formation from a colloidal silica solution, where coagulation of silica particles is enhanced in an alkaline solution [26].
5. Conclusion The four opal-AG samples exhibited virtually identical 29 Si NMR spectra, and this is not surprising given their common process of formation. The opal-AG samples contained an approximately equal proportion of Q3 (1OH) groups. Proton cross-polarization 29 Si NMR revealed that all the
247
opals analyzed appear to contain Q2 (2OH) (or geminal) silanol groups, with the opal-CT samples containing a higher fraction of both Q3 and Q2 units than the opal-AG samples. Hence, opal-CT contains more single hydroxyl bonds than opal-AG, and also more twin hydroxyl groups. Geminal (twin) silanol groups are present within opal-AG and opal-CT and are thus not restricted to opal-AN. The variation of the halfwidths in the 29 Si NMR spectra of the opal-CT samples showed that these opals contained a significantly higher degree of short-range order than the opal-AG samples. All of the opal samples investigated contained aluminium in a tetrahedral configuration.
References [1] J.B. Jones, J. Geol. Soc. Aust. 18 (1971) 57. [2] K. Langer, O.W. Fl€ orke, Fortschr. Miner. 52 (1974) 17. [3] P.J. Darragh, A.J. Gaskin, B.C. Terrel, J.V. Sanders, Nature 209 (1966) 13. [4] E.R. Segnit, T.J. Stevens, J.B. Jones, J. Geol. Soc. Aust. 12 (1965) 211. [5] J.A. Hockey, B.A. Pethica, Trans. Faraday Soc. 57 (1961) 2247. [6] X.-D. Cong, R.J. Kirkpatrick, S. Diamond, Cement Concrete Res. 23 (1993) 811. [7] H. Graetsch, H. Gies, I. Topalovic, Phys. Chem. Miner. 21 (1994) 166. [8] H. Graetsch, I. Topalovic, Chem. Erde 56 (1996) 387. [9] H. Graetsch, A. Mosset, H. Gies, J. Non-Cryst. Solids 119 (1990) 173. [10] S.J. Adams, G.E. Hawkes, E.H. Curzon, Am. Miner. 76 (1991) 1863. [11] A. Pines, M.G. Gibby, J.S. Waugh, J. Chem. Phys. 59 (1973) 569. [12] I. Farnan, S.C. Kohn, R. Dupree, Geochim. Cosmochim. Acta 51 (1987) 2869. [13] M. M€agi, E. Lippmaa, A. Samoson, G. Engelhardt, A.R. Grimmer, J. Phys. Chem. 88 (1984) 1518. [14] B.H.W.S. de Jong, J. Van Hoek, W.S. Veeman, D.V. Manson, Am. Miner. 72 (1987) 1195. [15] J.B. Murdoch, J.F. Stebbins, I.S.E. Carmichael, Am. Miner. 70 (1985) 332. [16] G.E. Maciel, D.W. Sindorf, J. Am. Chem. Soc. 102 (1980) 7606. [17] D.W. Sindorf, G.E. Maciel, J. Am. Chem. Soc. 105 (1983) 1487. [18] J.B. Jones, E.R. Segnit, Miner. Mag. 37 (1969) 357. [19] O.W. Fl€ orke, H. Graetsch, B. Martin, K. R€ oller, R. Wirth, Neues Jahrbuch Miner. Abh. 163 (1991) 19. [20] E. Dupree, R.F. Pettifer, Nature 308 (1984) 523.
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[21] F. Bartoli, D. Bittencourt Rosa, M. Doirisse, R. Meyer, R. Philippy, J.C. Samama, Eur. J. Miner. 2 (1990) 611. [22] P. Bayliss, P.A. Males, Min. Mag. 35 (1965) 429. [23] H. Harder, J. Jb. Miner. Mh. 3 (1995) 121.
[24] J.A. Webb, B.L. Finlayson, Am. Miner. 72 (1987) 1204. [25] E. Merino, C. Harvey, H.H. Murray, Clays Clay Miner. 37 (1989) 135. [26] R.K. Iler, The Chemistry of Silica, Wiley, New York, 1979.
29
Si and 27Al NMR study of amorphous and paracrystalline opals from Australia L.D. Brown *, A.S. Ray, P.S. Thomas
Department of Chemistry, Materials and Forensic Sciences, Faculty of Science, University of Technology, Sydney, P.O. Box 123, Broadway, NSW 2007, Australia Received 1 January 2003
Abstract Four opal-AG (amorphous) and two opal-CT (paracrystalline) samples obtained from various regions in Australia were investigated with 29 Si NMR and 27 Al NMR. The proton cross-polarization 29 Si NMR technique was used and the resulting spectra consisted of two main resonances: )102.0 and )111.2 ppm for opal-AG; and )102.5 and )112.2 ppm for opal-CT. These peaks were assigned to the Q3 (1OH) and Q4 resonances, respectively. Using very short contact times, a third, very weak peak at )94 ppm was resolved in an opal-CT specimen, which was assigned to silicon in the Q2 (2OH) arrangement (i.e. a silicon with twin hydroxyl groups). It was found that the opal-CT samples contained a higher proportion of both geminal and vicinal silanol groups (Q2 and Q3 ) than the opal-AG samples. The geminal silanol groups present in opal-AG and opal-CT are not restricted to opal-AN. The full-width at half-maximum (FWHM) values were 9.5 ppm for the opal-AG samples, and 6.5 ppm for both opal-CT samples, a result which confirms that opal-CT has a higher degree of short-range structural order than opal-AG. The 27 Al NMR spectra of the opals all showed a single resonance at +52 ppm, indicating that the aluminium exists in a tetrahedral arrangement incorporated within the opal structure. Ó 2003 Elsevier B.V. All rights reserved.
1. Introduction Opals are naturally occurring hydrous silica minerals, which exhibit various degrees of structural disorder. Jones and Segnit [1] classified opal into three groups according to its X-ray diffraction (XRD) pattern. Opals that produced an XRD
* Corresponding author. Address: 58 Seymour Street, Hurstville Grove, NSW 2220, Australia. Tel.: +61-2 9570 1634; fax: +61-2 9514 1628. E-mail address: [email protected] (L.D. Brown).
pattern similar to a-cristobalite with minor evidence of tridymite were termed opal-C; opals that showed signs of both cristobalite and tridymite were designated opal-CT; while opals that yielded an XRD pattern that resembled amorphous silica were categorised as opal-A. Using infrared spectroscopy, Langer and Fl€ orke [2], further subdivided opal-A into two groups, opal-AG for gellike, and opal-AN for network-or glass-like. The majority of opals found in Australia are opal-AG. Opal-AG is initially formed from a colloidal silica solution, which slowly coagulates into a solid aggregate of silica particles [3]. Water
0022-3093/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2003.09.027
L.D. Brown et al. / Journal of Non-Crystalline Solids 332 (2003) 242–248
is incorporated into the structure as molecular water (H2 O) and silanol (Si–OH) groups. In addition to vicinal (single) hydroxyl groups that are contained within natural opals, it is possible that geminal (double) hydroxyl groups may also arise [4]. They are most likely to occur at lattice imperfections caused by incomplete polycondensation of silica tetrahedra [5], such as at the surfaces of the silica spheres where the continuous silica network is interrupted. However, there seems to be some uncertainty in the literature concerning the existence of these twin hydroxyl groups [2,6–8]. This study seeks to clarify the nature of the silanol bonds contained within opal-AG and opal-CT using nuclear magnetic resonance (NMR) spectroscopy. NMR spectroscopy is an analytical technique which allows the various local environments of certain nuclei to be investigated. In this case, 29 Si NMR is used to study the relative disorder of opal-AG and opal-CT, and also the silanol bonding arrangement that constitutes the chemisorbed water. Previous NMR studies of opal have been restricted to only two or three types from Australia [7,9,10], despite the fact that Australia remains the worldÕs largest supplier of opals. The technique of proton cross-polarization (CP) 29 Si NMR is also used to study the opals more effectively. Essentially, a single radio frequency (RF) pulse is applied to the protons, and then two RF fields are simultaneously applied to the 1 H and 29 Si nuclei, enabling a transfer of polarization to occur. The time during which the two RF fields are applied is called the contact time, and varying this parameter alters the relative intensities of peaks in the NMR spectrum. Details of the technique are described elsewhere [11]. For short contact times, silicon atoms in close proximity to protons will give enhanced signals than those further from protons [12]. Hence, CP measurements will allow the observation of the degree of hydroxyl substitution of the Si atoms. In this discussion, the standard Qn notation is used, where Q represents a given silica tetrahedron, and n represents the number of associated bridging oxygens (Si–O–Si) per tetrahedron. Silicon nuclei that are more shielded result in higher-
243
field (more negative) chemical shifts. In this way, silicates in which the tetrahedra are fully polymerised (Q4 ) yield chemical shifts progressively more negative than those that are not (Q3 ). Similarly, increasing the number of substituted Al ions in each of the SiO4 tetrahedra results in a correspondingly less negative chemical shift [13]. 27 Al NMR is also briefly used to reveal the way in which aluminium is incorporated into the silica network within opals.
2. Experimental For this study, four opal-AG and two opal-CT samples were analyzed with magic angle spinning (MAS) 29 Si NMR spectroscopy. The opal-AG samples were obtained from four major opal fields within Australia: Lightning Ridge, Coober Pedy, Andamooka and White Cliffs; while the opal-CT samples were from Tintenbar in Australia, and Mexico. One of the opal samples from Tintenbar was heated to 700 °C, and analyzed to compare the results of the aged and unaged 29 Si NMR spectra. All samples were cut to ensure there was no contamination from the surrounding host rock, before being crushed with an agate mortar and pestle. The larger pieces of crushed opal were selectively packed into the sample rotor. All spectra were recorded on a Bruker 300 MAS NMR spectrometer. The 29 Si NMR single-pulse spectra were acquired using an operating frequency of 56.2 MHz, and a spinning rate of 3 kHz. Chemical shifts were measured relative to the 29 Si resonance in kaolin. A recycle delay time of 20 s was used between each 90° pulse, until 256 scans were accumulated. For the 29 Si {1 H} CP NMR experiments, a contact time of between 6 and 10 ms was used, with a recycle delay time of 1 s. Sample spinning speeds were 7 kHz. A minimum of 10 240 scans were collected for each sample. The 27 Al NMR spectra were obtained with an operating frequency of 78.2 MHz, and a spinning rate of 7 kHz. The chemical shifts were measured relative to the 27 Al resonance in AlCl3 . A total of 1024 FIDs were collected with a recycle delay time of 1 s.
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Fig. 1. Single-pulse (bottom).
L.D. Brown et al. / Journal of Non-Crystalline Solids 332 (2003) 242–248
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Si NMR spectra of an unaged Tintenbar opal sample (top) and a thermally aged Tintenbar opal sample
3. Results The single-pulse 29 Si MAS NMR spectra showed a single broad asymmetric peak centred at )111.8 ppm for opal-AG samples, and at )111.9 ppm for opal-CT samples. After 256 scans were completed on the opal-CT sample from Tintenbar, the spectrum showed signs of a second peak at around )102 ppm, so additional pulses were acquired to improve the S/N ratio and attempt to resolve the apparent peak. After numerous scans were accumulated, the spectrum revealed a small but distinct second peak at )102.5 ppm. The thermally aged Tintenbar specimen was also analyzed with 29 Si NMR, and the intensity of the resonance at )102.5 ppm had substantially reduced when compared to the original unaged sample. The two spectra of the Tintenbar samples are given in Fig. 1. Full-width at half-maximum (FWHM) values were 9.5 ppm for all four opalAG samples, and 6.5 ppm for the two opal-CT samples. These results are in close agreement with those of other workers published in the literature [9,14]. The 29 Si {1 H} CP MAS NMR spectra of the opal-AG samples revealed a small shoulder at )102.0 ppm in addition to the primary peak, which was centred at about )111.2 ppm. CP spectra of opal-CT samples showed the original
peak at )112.2 ppm and a greatly enhanced peak at )102.5 ppm. Fig. 2 shows the CP spectra of the
Fig. 2. Proton cross-polarization 29 Si NMR spectra of opals. From top to bottom: Mexican, Tintenbar, Coober Pedy, White Cliffs, Andamooka, and Lightning Ridge.
L.D. Brown et al. / Journal of Non-Crystalline Solids 332 (2003) 242–248
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structural arrangement. The asymmetry in the 29 Si NMR spectra of opal-AG samples is most likely due to a combination of the major Q4 resonance at )111 ppm and the lesser signal from Q3 (1OH), and perhaps Q4 (1Al), both of which occur at approximately )102 ppm [15]. 4.2. Cross-polarization
Fig. 3. A proton cross-polarization 29 Si NMR spectrum of the Tintenbar sample, obtained with a contact time of 6 ms (bottom). A proton cross-polarization 29 Si NMR spectrum of the Tintenbar sample, obtained with a contact time of 6 ms, with a 55 point fast Fourier transform smoothing function applied (top).
opals, using a contact time of 10 ms. A small shoulder was discernible at approximately )94 ppm for these spectra, especially noticeable with the sample from Tintenbar, so the contact time was reduced to 6 ms and another spectrum was recorded from this sample. In that spectrum, an extremely weak peak was resolved at approximately )93.5 ppm (Fig. 3). 27 Al NMR was also performed on all six opal samples. Each of the spectra showed a single, welldefined peak between at +51.6 and +53.2 ppm.
4. Discussion 4.1. Single pulse The single broad peak at )111.8 ppm is characteristic of amorphous silica that is in the Q4
The resonance at )102.5 ppm is assigned to silicon in the Q3 (1OH) configuration, for two reasons. Firstly, the peak is absent for the sample that had been thermally aged. After heating, silanol bonds are expected to be permanently removed from the silica network, and this is reflected in the difference between the two spectra. Secondly, the peak is significantly enhanced when the proton cross-polarization technique is used, indicating that it is not caused by Q3 (1Al). The CP spectra closely resemble those of other researchers [7,10]. The weak resonance at approximately )94.0 ppm is most likely due to silicon in the Q2 (2OH) structural arrangement. Shorter contact times allow silicon atoms that are in close proximity to protons to give enhanced signals with respect to those further from protons [16]. When the contact time was reduced from 10 to 6 ms, both the )102.5 ppm peak and the )94.0 ppm peak were significantly intensified, signifying that it is most likely a Q2 resonance. This result suggests that there are indeed geminal silanol groups contained within opal-AG and opal-CT, and they are not restricted to opal-AN, as suggested elsewhere [7]. The silica structure of opal-AG now appears to be remarkably similar to that of a synthetic silica gel [17]. Although the concentration of these twin hydroxyl groups must be extremely low, this result elucidates some earlier discussions on the thermal characteristics of opals, which postulated the existence of geminal hydroxyl groups within the structure [18]. Some of the spectra were curve-fitted (using Origin 6.1 software) and generally the spectra could be approximated very well with the superposition of three Gaussian peaks at )93.5, )102.5 and )111.2 ppm. Fig. 4 shows the deconvolution of the single-pulse spectrum of a Tintenbar sample; Fig. 5 shows the deconvolution of the CP
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Fig. 4. A single-pulse 29 Si NMR spectrum of unaged Tintenbar opal sample (top). The curve below it is a theoretical curve resulting from the superposition of the two Gaussian peaks (bottom) at )102.5 and )111.9 ppm.
Fig. 5. A proton cross-polarization 29 Si NMR spectrum of Andamooka opal sample (top). The curve below it is a theoretical curve resulting from the superposition of the three Gaussian peaks (bottom) centred at )94.0, )102.5 and )111.5 ppm.
spectrum of an Andamooka sample. From the proportions of these component peaks, it is clearly
evident that opal-CT contains more single hydroxyl bonds than opal-AG, and probably more twin hydroxyl groups as well. This deduction is in contrast to what is reported elsewhere [2,19]. A curve fit was also attempted on the CP NMR spectrum of the Tintenbar sample, where the contact time was 6 ms. A reasonable fit was obtained with the three principal Q2 , Q3 , and Q4 resonances; however a slightly more accurate fit was achieved if a forth peak at approximately )98 ppm was included. This peak may be due to two different levels of shielding of Q3 (1OH) groups, caused by varying degrees of H-bonding such as an internal and a surface silanol, as proposed by Adams [10]. The degree of structural order of the sample can be inferred from the 29 Si NMR signal; the broadness of the signal is due to a distribution in the average Si–O–Si bond angle per tetrahedron, and thus represents the extent of the relative localised disorder of the silica framework [14]. In this respect, opal-CT obviously has more short-range order than opal-AG, because it shows a halfwidth of only 6.5 ppm, compared to 9.5 ppm for opalAG. The chemical shift, d, of silica polymorphs also correlates with the Si–O–Si bond angle [14,20]. The opal-AG spectra show Q4 resonances at )111.2 ppm, while the two opal-CT spectra show their Q4 shifted to an even higher field, at )112.2 ppm. This signifies that opal-CT has a slightly
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greater mean Si–O–Si angle than opal-AG, a result that may be a consequence of the gradual siloxane rearrangement in the transformation of opal-A to opal-CT. For all the types of opal investigated, the 27 A1 NMR spectra display a single resonance around +52 ppm. This result establishes that all the Al ions are all in a tetrahedral arrangement; i.e. substituting for silicon atoms within the molecular structure, and thus opal is technically an aluminosilicate. This is in contrast to what is reported elsewhere [21], which indicated that there was little or no Al for Si substitution in Australian opals. Since these samples contain as much as 1% Al [22,23], it is not surprising that the Al atoms occasionally substitute for silicon; the opals were originally polymerised from silica-rich solutions, which must have contained sufficient Al to become incorporated within the network structure. A resonance corresponding to an octahedral configuration of Al (d þ5 ppm), was not detected. These results concerning the tetrahedral Al content of opal-AG and opal-CT complement the results of a study of opal-A speleothems [24], in which most of the samples contained allophane and therefore octahedrally coordinated Al. According to the work done by Merino [25] on the tetrahedral-aluminium content of aqueous lowtemperature silicates, and assuming the temperature of formation of opal-AG was 25 °C, this result indicates that the original pH of the sol must have been in excess of 6.5; since this is the higher end of the pH range over which the octahedral to tetrahedral transition can take place (5.5–6.5). This is in accordance with the theory of opal formation from a colloidal silica solution, where coagulation of silica particles is enhanced in an alkaline solution [26].
5. Conclusion The four opal-AG samples exhibited virtually identical 29 Si NMR spectra, and this is not surprising given their common process of formation. The opal-AG samples contained an approximately equal proportion of Q3 (1OH) groups. Proton cross-polarization 29 Si NMR revealed that all the
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opals analyzed appear to contain Q2 (2OH) (or geminal) silanol groups, with the opal-CT samples containing a higher fraction of both Q3 and Q2 units than the opal-AG samples. Hence, opal-CT contains more single hydroxyl bonds than opal-AG, and also more twin hydroxyl groups. Geminal (twin) silanol groups are present within opal-AG and opal-CT and are thus not restricted to opal-AN. The variation of the halfwidths in the 29 Si NMR spectra of the opal-CT samples showed that these opals contained a significantly higher degree of short-range order than the opal-AG samples. All of the opal samples investigated contained aluminium in a tetrahedral configuration.
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