INFLUENCE OF ALUMINUM ON IRON-OXIDES .2. PREPARATION AND PROPERTIES OF A1-SUBSTITUTED HEMATITES

Aluminum-substituted hematites (Fe//2// minus //xAl//xO//3) were synthesized from Fe-Al coprecipitates at pH 5. 5, 7. 0, and in 10** minus **1, 10** minus **2, and 10** minus **3 M KOH at 70 degree 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 a//0 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. Refs.