Hostname: page-component-848d4c4894-wg55d Total loading time: 0 Render date: 2024-06-02T00:16:05.436Z 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
Get access Rights & Permissions [Opens in a new window]

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.

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

Access options

Get access to the full version of this content by using one of the access options below. (Log in options will check for institutional or personal access. Content may require purchase if you do not have access.)

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