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The Identification of Gibbsite and Bayerite by Laser Raman Spectroscopy¹

ABSTRACT

Two forms of aluminum hydroxide, Al(OH)₃, are well known as the minerals gibbsite and bayerite. Although bayerite is readily prepared synthetically, it is rarely found in nature. Using conventional X-ray powder-diffraction techniques we have identified two samples of Al(OH)₃ as gibbsite and bayerite and have examined the laser Raman spectra of both forms. The 3000 to 4000 cm^–1 region of the laser Raman spectrum is distinctly different for the gibbsite and bayerite materials. Four strong, sharp bands at 3615, 3520, 3431, and 3361 cm^–1 are observed for gibbsite, while only three strong, sharp bands at 3651, 3542, and 3421 cm^–1 are present for bayerite. These results are shown to be in good agreement with the infrared spectrum of Al(OH)₃ which generally exhibits very broad bands in this region. This technique is suggested as an advantageous method of identification of the two polymorphs. The differences in the vibrational spectra of gibbsite and bayerite are attributed to differences in the crystal structures of the two forms. A correction is presented between the energies of the absorption bands in the hydroxyl stretching region [v(O-H)] and the hydroxide separation distances calculated from crystal structure determinations of gibbsite and bayerite.

¹ Joint contribution from the Departments of Chemistry and Agronomy and Soil Science, University of Hawaii, Journal Series no. 2184 of the Hawaii Agric. Exp. Stn.

² Graduate Assistant and Associate Professor, Dep. of Chemistry. The current address of the senior author is Western Electric, Princeton, NJ 08540.

³ Research Affiliate and Professor of Soils, Dep. of Agronomy and Soil Science, respectively.

Received for publication April 17, 1978. Accepted for publication September 21, 1979.