Synthesis and Properties of Poorly Crystalline Hydrated Aluminous Goethites

Martin Venn Fey1 and Joe Boris Dixon
Department of Soil and Crop Sciences, Texas Agricultural Experiment Station Texas A&M University, College Station, Texas 77843
1 Present address: Department of Soil Science and Agrometerology, University of Natal, P.O. Box 375, Pietermaritzburg 3200, Republic of South Africa.

Abstract: Al-substituted goethites were prepared by rapid oxidation of mixed FeCl2-AlCl3 solutions at pH 6.8 in the presence of CO2 at 25°C. A combination of Al substitution and adsorption of CO2 reduced crystal size (except for an increase at small additions of Al) and produced unusual thin, porous particles. Product goethites had surface areas up to 283 m2/g and unit-cell expansions induced by hydration. Substitution of Al for Fe reduced the 111 spacing and increased infrared OH-bending vibrational frequencies. Al substitution split the goethite dehydroxylation endotherm during differential thermal analysis into a doublet and increased the temperature of all reactions. Both cold and hot alkali solutions dissolved Al from the goethite structure.

After drying the product in vacuo at 110°C. X-ray powder diffraction data indicated minimal deviation from Vegard's law for the goethite-diaspore solid solution up to about 30 mole % Al substitution. Goethite prepared in the presence of 40 mole % Al had a 111 spacing of 2.403 Å corresponding to 36 mole % structural Al if Vegard's law was obeyed. Rapid oxidation of mixed FeCl2-AlCl3 solutions appears to be conducive to a higher degree of Al substitution in goethite than alkaline aging of hydroxy-Fe(III)-Al coprecipitates.

Key Words: Aluminum • Diaspore • Goethite • Infrared • Synthesis • Vegard's law • X-ray powder diffraction

Clays and Clay Minerals; April 1981 v. 29; no. 2; p. 91-100; DOI: 10.1346/CCMN.1981.0290202
© 1981, The Clay Minerals Society
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