BOEHMITE

AlOOH

Boehmite has the same structure as lepidocrocite (g-FeO(OH)). The structure of boehmite consists of double layers of oxygen octahedra partially filled with Al cations. The stacking arrangement of the three oxygen layers is such that the double octahedral layer is in cubic closed packing. Within the double layer one can discriminate between two different types of oxygen. Each oxygen in the middle of the double layer is shared by four other octahedra, while the oxygens on the outside are only shared by two octahedra. These outer oxygens are hydrogen-bonded to two other similarly coordinated oxygens in the neighbouring double layers above and below. The stacking of the layers is such that the hydroxyl groups of one layer are located over the depression between the hydroxyl groups in the adjacent layer. 

The IR spectrum of boehmite has a characteristic OH stretching band with two equally strong maxima at 3297 and 3090 cm^-1 according to Ryskin ¹. Van der Marel and Beutelspacher ² however reported a very strong maximum at 3280-3287 cm^-1 and a very strong maximum at 3090 cm^-1. The enormous splitting has been ascribed to the presence of a direct bonding between the equivalent hydroxyls and to the high structural regularity of the structure. In the OH bending region boehmite is characterised by two vibrations at 1160 and 1080 cm^-1. The vibration at 755 cm^-1 involves the hydrogen vibrations according to Fripiat et al. ³. Van der Marel and Beutelspacher ² report an additional vibration at 636 cm^-1.

1.     Y.I. Ryskin, in V.C. Farmer (Ed.) The infrared spectra of minerals (Mineralogic. Soc. Monograph 4, 1974) Ch 9, pp 137-181.

2.     H.W. van der Marel and H. Beutelspacher, Atlas of Infrared spectroscopy of clay minerals and their admixtures (Elsevier, Amsterdam, 1976), 194 pp.

3.     J.J. Fripiat, H. Bosmans and P.G. Rouxhet, J Phys. Chem. 71 (1967) 1097.

SELECTED REFERENCES ON SPECTROSCOPY OF BOEHMITE:

1.    Frost, R.L. and Kloprogge, J.T. (1999) Infrared emission spectroscopic study of brucite. Spectrochimica Acta, 55, 2195-2205.

2.    Frost, R.L. and Kloprogge, J.T. (2001) The application of near-infrared spectroscopy to the study of brucite and hydrotalcite structures. Canadian Journal of Analytical Science and Spectroscopy, 45(4), 96-102.

3.    J.A. Gadsden, Infrared Spectra of Minerals and Related Inorganic Compounds. Butterworths & Co. Ltd., London, U.K., 277 pp (1975).  

4.     Y.I. Ryskin, in V.C. Farmer (Ed.) The infrared spectra of minerals (Mineralogic. Soc. Monograph 4, 1974) Ch 9, pp 137-181.

5.     H.W. van der Marel and H. Beutelspacher, Atlas of Infrared spectroscopy of clay minerals and their admixtures (Elsevier, Amsterdam, 1976), 194 pp.

6.     J.J. Fripiat, H. Bosmans and P.G. Rouxhet, J Phys. Chem. 71 (1967) 1097.

Original spectra shown for this mineral can be obtained on request from J.T. Kloprogge (E-mail t.kloprogge@qut.edu.au), or R.L. Frost (E-mail r.frost@qut.edu.au).

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