The Natural Occurrence of Eta-Alumina (η-Al2O3) in Bauxite

David B. Tilley and Richard A. Eggleton
Centre for Australian Regolith Studies, Australian National University, Canberra, ACT, 0200, Australia

Abstract: Approximately 20 wt.% of the bauxite from Andoom in northern Queensland, Australia is composed of material that cannot be accounted for by identifiable well-crystallized phases. This poorly- diffracting material (PDM), found within the core of bauxitic pisoliths, has similar characteristics to that of eta-alumina (η-Al2O3); a cubic form of alumina. A differential XRD pattern of the PDM displayed a series of broad diffraction maxima attributed to eta-alumina with a mean crystal size of 9 nm. Unit cell refinement, on the basis of a cubic cell, gave a lattice parameter of a = 7.98 Å for Andoom eta-alumina. TEM and selected-area electron diffraction revealed the PDM to be composed of minute (10 nm wide), randomly oriented crystals of eta-alumina in close association with Al-hematite. Chemical analysis using a nanoprobe showed Andoom eta-alumina to be almost pure alumina with <2 M% Fe, < 1 M% Si and < 1 M% Ti. The closely associated Al-hematite may contain as much as 22 M% Al, however a value closer to the theoretical limit of 17 M% is more likely. A broad absorption band at 3450 cm−1 and 1630 cm−1 in the infra-red spectrum of the PDM indicates the presence of a substantial quantity of H2O, strongly adsorbed onto the surface of the crystals. This is presumably due to η-Al2O3's large surface area of approximately 2200 m2/g. The natural occurrence of η-Al2O3 in bauxite may be the result of low H2O activities within the micro-environment of pores at the time of crystallization. The epigenetic replacement of kaolinite with η-Al2O3 and Al-hematite is put forward as an explanation for the formation of bauxitic pisoliths at Andoom.

Key Words: Akdalaite • Bauxite • Chi-alumina • Eta-alumina • Gamma-alumina • Pisoliths • Poorly-diffracting material • Tohdite

Clays and Clay Minerals; October 1996 v. 44; no. 5; p. 658-664; DOI: 10.1346/CCMN.1996.0440508
© 1996, The Clay Minerals Society
Clay Minerals Society (www.clays.org)