The association of maghemite and corundum in Darling Range laterites, Western Australia

Anand, R. R.; Gilkes, R. J.

doi:10.1071/sr9870303

Cited by 54 publications

(38 citation statements)

References 19 publications

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“…The dehydroxylation of an AI(OH)~ monomer might therefore be less complete, as more surface-attached A1 is surrounded by other A1 cations, due to the polarizing effects of its neighbours. Extrapolated to a pure AI hydroxide, no A1203 should form under ambient conditions, which agrees with the fact that corundum has been found in soils only after burning events (Anand and Gilkes, 1987;Stanjek, 1987).…”

Section: Discussionsupporting

confidence: 79%

The Influence of Aluminum on Iron Oxides. Part XVI: Hydroxyl and Aluminum Substitution in Synthetic Hematites

Stanjek

Schwertmann

1992

Clays and clay miner.

102

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--Synthetic hematites with A1 substitutions between 0 and 18 mol % were synthesized at different temperatures and water activities. The cell-edge lengths a for different synthesis conditions decreased linearly with increasing A1 substitution. The regression lines, however, had different slopes and intercepts: the series with the highest synthesis temperature (1270 K) had the most negative slope. With increasing AI substitution, the hematites contained increasing amounts of non-surface water. Significant correlations were found between these chemically determined water contents and the deviations of the unit-cell parameters a, c, and V relative to the corresponding 1270 K regression lines. To explain the measured X-ray peak intensities, structural OH had to be included into the theoretical calculations. From intensity ratios normalized to Il ~ 3, it is possible to determine the structural OH separately from the AI substitution, which can be assessed by the shift of the cell-edge lengths relative to the 1270 K regression lines. The incorporation of A1 and OH into the hematite structure induces strain, which was quantified by X-ray diffraction.

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Section: Discussionsupporting

confidence: 79%

The Influence of Aluminum on Iron Oxides. Part XVI: Hydroxyl and Aluminum Substitution in Synthetic Hematites

Stanjek

Schwertmann

1992

Clays and clay miner.

102

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“…The origin of maghemite in soils is controversial, Maghemite may form by burning other Fe oxides in the presence of organic matter (Schwertmann and Fechter, 1984;Anand and Gilkes, 1987), by neoformation from soil solution (Fine and Singer, 1989), or by oxidation of magnetite (Mullins, 1977). Maghemites in the magnetic soils of Brazil may be derived by rapid weathering of magnetite or by direct inheritance from the bedrock (Resende et aL, 1986;Coey et al,, 1991).…”

Section: Introductionmentioning

confidence: 99%

Quantification and Characterization of Maghemite in Soils Derived from Volcanic Rocks in Southern Brazil

Costa

1999

Clays and clay miner.

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Abstract--Many soils developed from volcanic rocks in southern Brazil exhibit spontaneous magnetization caused by the presence of fine-grained maghemite (3J-Fe203), but few attempts were made to quantify or characterize this important soil component. To that end, clays were separated from freely drained soils derived from acid (>~63% Sit2), intermediate (54-62% SiO~), and basic (--<53% Sit;) igneous rocks produced by the Paranfi flood volcanism. The sample set included soils with a wide range of pedogenic development on different landscape positions. The Fe oxide mineralogy of these samples was examined by using a combination of selective dissolution, magnetic susceptibility, and X-ray diffraction (XRD) techniques. Hematite and maghemite were the primary Fe oxides in mature soils (Oxisols, Ultisols, and Alfisols) developed from basic rocks; whereas goethite was dominant in all other soils, especially those formed from acid-intermediate rocks. The association of maghemite with basic rock materials suggests that it was primarily formed by oxidation of lithogenic magnetite. A strong, positive correlation (R 2 = 0.89) was obtained between mass specific magnetic susceptibility (X) of the clay fractions and maghemite contents estimated by XRD. Either method could be used for quantitative analyses, but X was more sensitive than XRD at low maghemite concentrations (<2 wt. %). The clay-sized maghemites were superparamagnetic with an estimated value for the mass specific magnetic susceptibility (X~f) value of 91,000 • 10 -8 m 3 kg ~ and frequency dependent variations of 10-15%. The maghemites also had low unit cell constants, which, if attributed entirely to replacement of Fe by AI, would correlate with AI substitutions in the range of 5-16 mole %. Selective dissolution of the soil maghemites was achieved by treatment of Fe oxide concentrates with 1.8 M I-I2SO 4 at 75~ for 2 h.

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“…The excess A1 may, however, have been exsolved and been present as an undetectable amorphous phase. The absence of corundum in the calcined natural soil goethites may suggest that in soils, other sources of A1, such as A1 tri-hydroxides and oxyhydroxides, kaolinite and organically bound AI (Norrish & Rosser, 1983;Anand & Gilkes, 1987c), may need to be considered as precursors of corundum formed by heating of soils during bush fires. However, this interpretation is based on results for only one natural sample and is consequently speculative.…”

Section: Natural Hematitesmentioning

confidence: 99%

“…Anand & Gilkes (1987c) demonstrated a strong association between corundum and maghemite in these horizons and suggested a mechanism for the formation of these minerals involving a single parent mineral. Grubb (1971) proposed the dehydration of aluminous goethite as a mechanism for the formation of 9 1989 The Mineralogical Society corundum in these materials.…”

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confidence: 99%

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

1989

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Dehydroxylation of synthetic and natural goethites with a range of AIsubstitution 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 of corundum 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 in 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 AIgoethites 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.Corundum is a common accessory mineral of silica-undersaturated, high-grade igneous and metamorphic rocks and thermally metamorphosed sediments (Deer et al., 1962), but it would not be expected to crystallize during low-temperature metamorphism. Corundum can be formed in the laboratory by the high temperature (~ 1200~ calcination of the aluminium tri-hydroxides (Rooksby, 1961), and by the topotactic transformation of diaspore (ct-AIOOH) at 450-580~ (Brown, 1980

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The association of maghemite and corundum in Darling Range laterites, Western Australia

Cited by 54 publications

References 19 publications

The Influence of Aluminum on Iron Oxides. Part XVI: Hydroxyl and Aluminum Substitution in Synthetic Hematites

The Influence of Aluminum on Iron Oxides. Part XVI: Hydroxyl and Aluminum Substitution in Synthetic Hematites

Quantification and Characterization of Maghemite in Soils Derived from Volcanic Rocks in Southern Brazil

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

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