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5.8 Minerals
Multi-component analysis can be readily applied to the infrared spectra of min-erals. The latter contain non-interacting components and so the spectrum of a mineral can be analysed in terms of a linear combination of the spectra of the individual components. However, the spectra of such solids exhibit a marked particle-size dependency. The particle size should be reduced (to 325 mesh) prior to preparation of an alkali halide disc. The pellet preparation involves sep-arate grinding and dispersion steps because minerals tend not to be effectively ground in the presence of an excess of KBr. Figure 5.8 illustrates the analysis of a mineral containing several components. The sample spectrum (a) is shown, as well as the calculated spectrum (b) based on the reference spectra of a variety of standard mineral components. The residual difference spectrum (c) shows that the error between the two spectra is small.
1600 1400 1200 1000 800 600 400 (c)
(b) (a)
Absorbance
Wavenumber (cm−1)
Figure 5.8 Analysis of the infrared spectrum of a mineral. (a) Sample spectrum: 50%
albite (k-feldspar); 23% siderite; 17% illite; 10% chlorite. (b) Calculated spectrum: 49.7%
feldspar (8.0% albite, 13.2% orthoclase and 28.5% microcline); 25.2% siderite; 19.0%
illite; 6.8% chlorite. (c) Residual difference spectrum. From Brown, J. M. and Elliot, J. J.,
‘The Quantitative Analysis of Complex, Multicomponent Mixtures by FTIR; the Analysis of Minerals and of Interacting Organic Blends’, in Chemical, Biological and Industrial Applications of Infrared Spectroscopy, Durig, J. R. (Ed.), pp. 111 – 125. Copyright 1985.
John Wiley & Sons Limited. Reproduced with permission.
Infrared spectroscopy has been frequently used to investigate the structural properties of clay minerals [14–18]. These materials are hydrated aluminium sil-icates with a layered structure formed by tetrahedral sheets (containing Si(IV)) via shared oxygen atoms. Clay minerals are usually examined by using transmission infrared methods with a KBr disc. Clay minerals may be differentiated by their infrared spectra through a study of the bands due to the O–H and Si–O groups.
In the O–H stretching region, 3800–3400 cm−1 for clay minerals, there are a number of bands observed. The inner hydroxyl groups between the tetrahedral and octahedral sheets result in a band near 3620 cm−1. The other three O–H groups at the octahedral surface form weak hydrogen bonds with the oxygens of the Si–O–Si bonds in the next layer and this results in stretching bands at 3669 and 3653 cm−1. Where clays have most of their octahedral sites occupied by divalent central atoms such as Mg(II) or Fe(II), a single band in the O–H stretching region is often observed.
In the 1300–400 cm−1 region, clay minerals show Si–O stretching and bend-ing and O–H bendbend-ing bands. The shape and position of the bands depend very much on the arrangement within the layers. For example, for kaolinite or dickite, which mainly have Al(III) in the octahedral position, several well-resolved strong
Inorganic Molecules 109 bands in the 1120–1000 cm−1region are observed. In comparison, for crysotile, which mainly contains Mg(II) in the octahedral sites, the main Si–O band is observed at 960 cm−1. The O–H bending bands are strongly influenced by the layering in clays. Where the octahedral sheets are mainly occupied by triva-lent central atoms, the O–H bending bands occur in the 950–800 cm−1 region.
Where most of the octahedral sites are occupied by divalent central atoms, the O–H bending bands are shifted to lower wavenumbers in the 700–600 cm−1 range.
SAQ 5.5
The mid-infrared spectrum of the clay kaolinite in KBr is illustrated below in Figure 5.9. Assign the infrared bands for kaolinite. What information does this spectrum provide about the structure of this clay?
1.3
0.8
0.3
−0.2 3400 2400 1400 400
Wavenumber (cm−1)
Absorbance
Figure 5.9 Infrared spectrum of kaolinite (cf. SAQ 5.5).
Zeolites are a class of aluminosilicate minerals which are widely used as catalysts and have been well characterized by using infrared spectroscopy [19].
Zeolites possess porous and crystalline structures that are sensitive to excessive pressure, and so a non-destructive sampling technique, such as the use of a diamond ATR cell, is suitable for studying such structures.
Summary
In this chapter, the fundamental features of the infrared spectra of inorganic compounds were introduced. Effects, such as the degree of hydration, on the appearance of infrared spectra were described. The infrared spectra of inor-ganic molecules are determined by the normal modes of vibration exhibited by such molecules and these were summarized and common examples provided.
Infrared spectroscopy is widely used to characterize coordination compounds and the important effects of coordination were introduced. Isomerism in coordi-nation compounds may also be characterized by using infrared techniques and examples were provided. Infrared spectroscopy is extensively used to charac-terize metal carbonyl compounds and examples of how the bonding in such compounds may be understood were provided. The main infrared bands asso-ciated with organometallic compounds were also introduced. Finally, examples of how infrared spectroscopy may be employed to understand the structures of mineral compounds were included.
References
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2. Nakamoto, K., Infrared and Raman Spectra of Inorganic and Coordination Compounds, Part B, Applications in Coordination, Organometallic and Bioinorganic Chemistry, Wiley, New York, 1997.
3. Nyquist, R. A., Putzig, C. L. and Leugers, M. A., Handbook of Infrared and Raman Spectra of Inorganic Compounds and Organic Salts, Academic Press, San Diego, CA, USA, 1997.
4. Clark, R. J. H. and Hester, R. E. (Eds), Spectroscopy of Inorganic Based Materials, Wiley, New York, 1987.
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8. Gunzler, H. and Gremlich, H.-U., IR Spectroscopy: An Introduction, Wiley-VCH, Weinheim, Germany, 2002.
9. Nakamoto, K., ‘Infrared and Raman Spectra of Inorganic and Coordination Compounds’, in Handbook of Vibrational Spectroscopy, Vol. 3, Chalmers, J. M. and Griffiths, P. R. (Eds), Wiley, Chichester, UK, 2002, pp. 1872–1892.
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11. Tudela, D., J. Chem. Edu., 71, 1083–1084 (1994).
12. Ebsworth, E. A. V., Rankin, D. W. H. and Cradock, S., Structural Methods in Inorganic Chem-istry, Blackwell, Oxford, 1987.
13. Cotton, F. A., Wilkinson, G., Murillo, C. A. and Bochmann, M., Advanced Inorganic Chemistry, 6th Edn, Wiley, New York, 1999.
14. Madejova, J., Vibr. Spectrosc., 31, 1–10 (2003).
15. Farmer, V. C. (Ed.), Infrared Spectra of Minerals, Mineralogical Society, London, 1974.
16. Wilson, M. J. (Ed.), Clay Mineralogy: Spectroscopic and Chemical Determinative Methods, Chapman and Hall, London, 1994.
Inorganic Molecules 111
17. Gadsden, J. A., Infrared Spectra of Minerals and Related Inorganic Compounds, Butterworths, London, 1975.
18. Busca, G. and Resini, C., ‘Vibrational Spectroscopy for the Analysis of Geological and Inor-ganic Materials’, in Encyclopedia of Analytical Chemistry, Vol. 12, Meyers, R. A. (Ed.), Wiley, Chichester, UK, 2000, pp. 10 984–11 020.
19. Zecchina, A., Spoto, G. and Bordiga, S., ‘Vibrational Spectroscopy of Zeolites’, in Handbook of Vibrational Spectroscopy, Vol. 4, Chalmers, J. M. and Griffiths, P. R. (Eds), Wiley, Chichester, UK, 2002, pp. 3042–3071.