Hostname: page-component-788cddb947-pt5lt Total loading time: 0 Render date: 2024-10-14T22:02:03.710Z Has data issue: false hasContentIssue false
  • English
  • Français

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

Published online by Cambridge University Press:  28 February 2024

David B. Tilley  and
Richard A. Eggleton
David B. Tilley
Affiliation:
Centre for Australian Regolith Studies, Australian National University, Canberra, ACT, 0200, Australia
Richard A. Eggleton
Affiliation:
Centre for Australian Regolith Studies, Australian National University, Canberra, ACT, 0200, Australia
Rights & Permissions [Opens in a new window]

Abstract

Core share and HTML view are not available for this content. However, as you have access to this content, a full PDF is available via the ‘Save PDF’ action button.

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.

Keywords

AkdalaiteBauxiteChi-aluminaEta-aluminaGamma-aluminaPisolithsPoorly-diffracting materialTohdite
Type
Research Article
Information
Clays and Clay Minerals , Volume 44 , Issue 5 , October 1996 , pp. 658 - 664
Copyright
Copyright © 1996, The Clay Minerals Society

References

Brown, J.F., Clark, D. and Elliott, W.W.. 1953. The thermal decomposition of the alumina trihydrate, gibbsite. J Chem Soc 13: 84–88.CrossRefGoogle Scholar
Lippens, B.C. and de Boer, J.H.. 1964. Study of phase transformations during calcination of aluminium hydroxides by selected-area electron diffraction. Acta Crystall 17: 1312–1321.CrossRefGoogle Scholar
Schwertmann, U.. 1988a. Some properties of soil and synthetic iron oxides. In: Stucki, J.W., Goodman, B.A., Schwertmann, U., editors. Iron in soils and clay minerals. Dordrecht, Holland: D. Reidel Publishing Company. p 203–250.CrossRefGoogle Scholar
Schwertmann, U.. 1988b. Occurrence and formation of iron oxides in various pedoenvironments. In: Stucki, J.W., Goodman, B.A., Schwertmann, U., editors. Iron in soils and clay minerals. Dordrecht, Holland: D. Reidel Publishing Company. p 267–308.CrossRefGoogle Scholar
Shirasuka, K., Yanagida, H. and Yamaguchi, G.. 1976. The preparation of η-alumina and its structure. Yogyo Kyokai Shi 84: 610–613.CrossRefGoogle Scholar
Singh, B. and Gilkes, R.J.. 1995. The natural occurrence of χ-alumina in lateritic pisolites. Clay Miner 30: 39–44.CrossRefGoogle Scholar
Spitler, C.A. and Pollack, S.S.. 1981. On the X-ray diffraction patterns of η- and γ-alumina. J Catal 69: 241.CrossRefGoogle Scholar
Stanjek, H.. 1991. Aluminium- und Hydroxylsubstitution in synthetischen und natürlichen Hämatiten. Buch am Erlbach, Germany: Verlag Marie L. Leidorf. 194 p.Google Scholar
Stumpf, H.C., Russell, A.S., Newsome, J.W. and Tucker, C.M.. 1950. Thermal transformations of aluminas and alumina hydrates. Ind Eng Chem 42: 1398–1403.CrossRefGoogle Scholar
Tardy, Y. and Nahon, D.. 1985. Geochemistry of laterites, stability of Al-goethite, Al-hematite, and Fe3+-kaolinite in bauxites and ferricretes: an approach to the mechanism of concretion formation. Am J Sci 285: 865–903.CrossRefGoogle Scholar
Taylor, J.C. and Clapp, R.A.. 1992. New features and advanced applications of SIROQUANT: A personal computer XRD full profile quantitative analysis software package. Adv X-ray Anal 35: 49–55.Google Scholar
Tilley, D.B. and Eggleton, R.A.. 1994. Tohdite (5Al2O3H2O) in bauxites from northern Australia. Clays Clay Miner 42: 485–488.CrossRefGoogle Scholar
Troland, F. and Tardy, Y.. 1987. The stabilities of gibbsite, boehmite, amorphous goethites and aluminous hematites in bauxites, ferricretes and laterites as a function of water activity, temperature and particle size. Geochim Cosmochim Acta 51: 945–957.CrossRefGoogle Scholar
Zhou, R. and Snyder, R.L.. 1991. Structures and transformation mechanisms of the η, γ and θ transition aluminas. Acta Crystall B47: 617–630.Google Scholar