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Dehydroxylation of Aluminous Goethite: Unit Cell Dimensions, Crystal Size and Surface Area

Published online by Cambridge University Press:  28 February 2024

H. D. Ruan  and
R. J. Gilkes
H. D. Ruan
Affiliation:
Soil Science and Plant Nutrition, Faculty of Agriculture, University of Western Australia, Nedlands, WA 6907, Australia
R. J. Gilkes
Affiliation:
Soil Science and Plant Nutrition, Faculty of Agriculture, University of Western Australia, Nedlands, WA 6907, Australia
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Abstract

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This work investigates unit cell dimensions, crystal size and specific surface area of aluminous goethite that was progressively dehydroxylated to form hematite. Goethite synthesized from the ferrous system altered to hematite with DTGA maximum increasing from 236° to 273°C for 0 to 30.1 mole % Al-substitution. Unit cell dimensions of goethite and hematite decreased as Al-substitution increased and increased as excess OH increased. The crystallographically equivalent a axis of goethite and c axis of hematite were more sensitive than other axes to the presence of excess structural OH associated with Al-substitution. Specific surface area increased from 147 to 288 m2/g for goethite and from 171 to 230 m2/g for hematite as Al-substitution increased. An increase in specific surface area on heating goethite at temperatures between 200° and 240°C is related to a decrease in the size of coherently diffracting domains of goethite crystals and to the development of pore and structural defects associated with the formation of hematite. The decrease in specific surface area for heating temperatures above 240°C is attributed to the growth of hematite crystals by diffusion.

Keywords

Al-substitutionCrystal sizeDehydroxylationGoethiteHematiteSpecific surface areaUnit cell
Type
Research Article
Information
Clays and Clay Minerals , Volume 43 , Issue 2 , April 1995 , pp. 196 - 211
Copyright
Copyright © 1995, The Clay Minerals Society

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