The Influence of Aluminum on Iron Oxides. Part II. Preparation and Properties of Al-Substituted Hematites

U. Schwertmann, R. W. Fitzpatrick, R. M. Taylor and D. G. Lewis1
1 Institute für Bodenkunde der TU München, 8050 Freising-Weihenstephan, BRD; Dept. of Soil Science and Agrometeorology, Univ. of Natal, P.O. Box 375, Pietermaritzburg 3200, Natal, South Africa; C.S.I.R.O., Division of Soils, Private Bag, No. 2, Glen Osmond, S.A. 5064, Australia and Waite Agricultural Research Institute, Dept. of Soil Science, Univ. of Adelaide, Glen Osmond, S.A. 5064, Australia, respectively.

Abstract: Aluminum-substituted hematites (Fe2−xAlxO3) were synthesized from Fe-Al coprecipitates at pH 5.5, 7.0, and in 10−1, 10−2, and 10−2 M KOH at 70°C. As little as 1 mole % Al suppressed goethite completely at pH 7 whereas in KOH higher Al concentrations were necessary. Al substitution as determined chemically and by XRD line shift was related to Al addition up to a maximum of 16–17 mole %. The relationship between the crystallographic a0 parameter and Al substitution deviated from the Vegard rule. At low substitution crystallinity of the hematites was improved whereas higher substitution impeded crystal growth in the crystallographic z-direction as indicated by differential XRD line broadening. At still higher Al addition crystal growth was strongly retarded. The initial Al-Fe coprecipitate behaved differently from a mechanical mixture of the respective “hydroxides” and was, therefore, considered an aluminous ferrihydrite.

Key Words: Aluminum • Ferrihydrite • Goethite • Hematite • Iron

Clays and Clay Minerals; April 1979 v. 27; no. 2; p. 105-112; DOI: 10.1346/CCMN.1979.0270205
© 1979, The Clay Minerals Society
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