Aluminum-Bearing Goethite in Venezuelan Laterites

E. Mendelovici, Sh. Yariv1 and R. Villalba
Laboratorio Fisico-Química de Materiales, Centro de Ingeniería y Computación Instituto Venezolano de Investigaciones Cientificas, Apartado 1827, Caracas, Venezuela
1 On sabbatical leave from the Hebrew University of Jerusalem, Israel.

Abstract: The presence of Al-bearing goethites has been unequivocally established in Venezuelan laterite sediments by means of infrared spectroscopy (IR), chemical dissolution, and X-ray powder diffraction (XRD) methods. The composition of these samples ranges from [Fe0.89Al0.11]O(OH) to [Fe0.76Al0.24]O(OH). The data of dissolution experiments using a modified dithionite (CDB) treatment suggest a parallel behavior between Fe and Al; the gradual dissolution of Al is associated with the destruction of the Al-containing goethite. The interpretation of the CDB dissolution results for SiO2/Fe2O3 is different; silica was only slightly extracted from phases other than goethite. Substitution of Fe3+ by Al3+ in these goethites was represented on the XRD patterns by a lowering of the (110) and (111) reflections corresponding to a reduction in size of the unit cell of goethite. IR spectroscopy showed the formation of such solid solutions by a shift of the 405 cm−1 absorption band, assigned to v (Fe-O) in synthetic goethite, to >460 cm−1 in the spectrum of Al-bearing natural goethite. Moreover, this spectrum shows a shift of the 3140 cm−1 absorption, due to v (OH), to higher frequencies, indicating a H-bond weakening in [FexAl(1−x)]O(OH) compared to FeO(OH).

Key Words: Al-goethite • Dithionite dissolution • Goethite • Laterite • Venezuela

Clays and Clay Minerals; October 1979 v. 27; no. 5; p. 368-372; DOI: 10.1346/CCMN.1979.0270507
© 1979, The Clay Minerals Society
Clay Minerals Society (www.clays.org)