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The formation of corundum and aluminous hematite by the thermal dehydroxylation of aluminous goethite

Published online by Cambridge University Press:  09 July 2018

M. A. Wells ,
R. J. Gilkes  and
R. R. Anand
M. A. Wells
Affiliation:
Soil Science and Plant Nutrition, School of Agriculture, The University of Western Australia, Nedlands, WA 6009, Australia
R. J. Gilkes
Affiliation:
Soil Science and Plant Nutrition, School of Agriculture, The University of Western Australia, Nedlands, WA 6009, Australia
R. R. Anand
Affiliation:
Soil Science and Plant Nutrition, School of Agriculture, The University of Western Australia, Nedlands, WA 6009, Australia
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Abstract

Dehydroxylation of synthetic and natural goethites with a range of Al-substitution from 0–28 mole% was investigated with a view to predicting the behaviour of soil goethites heated by bush fires. Hematites formed at temperatures ≤ 500°C retain the initial Al-content of the precursor goethite up to a maximum of 28 mole% Al. Loss of Al from the hematite structure occurred at 700°C for synthetic hematites with levels of substitution ≥18 mole% Al, but no crystalline alumina phase was present. Crystallization of corundum occurred for synthetic Al-goethites with levels of substitution ≥18 mole% when heated at 900°C. Formation of corundum reduces the maximum level of Al-substitution in hematites to ∼ 12 mole%. The exsolved corundum occurs as aggregated, platy crystals, 40–45 nm in diameter, containing a maximum of 7 mole% Fe. Al-substituted maghemite (7 mole% Al) formed from high Al-goethites heated at 900°C. Although some corundum in soils may be produced by heating of aluminous goethite by fires, the absence of corundum in natural Al-goethites calcined at 900°C, and the very high temperature (900°C) at which corundum formed from synthetic goethite suggest that other sources of Al may be required as precursors for the corundum formed by heating of soils.

Type
Research Article
Information
Clay Minerals , Volume 24 , Issue 3 , September 1989 , pp. 513 - 530
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1989

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