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

ABSTRACT: Dehydroxylation of synthetic and natural goethites with a range of AI-substitution from 0-28 mole~o was investigated with a view to predicting the behaviour of soil goethites heated by bush fires. Hematites formed at temperatures < 500 ~ retain the initial A1-content of the precursor goethite up to a maximum of 28 mole % A1. Loss of A1 from the hematite structure occurred at 700 ~ for synthetic hematites with levels of substitution>18 mole % A1, but no crystalline alumina phase was present. Crystallization fcorundum occurred for synthetic Al-goethites with levels of substitution> 18 mole~o when heated at 900 ~ Formation of corundum reduces the maximum level of Al-substitution i hematites to ~ 12 mole~o. 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 % A1) formed from high AI-goethites heated at 900 ~ 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~ and the very high temperature (900 ~ at which corundum formed from synthetic goethite suggest that other sources of AI may be required as precursors for the corundum formed by heating of soils