Iron Oxides in a Soil Developed from Basalt

A. T. Goulart1, 2, J. D. Fabris1, M. F. de Jesus Filho1, †, J. M. D. Coey3, G. M. da Costa4 and E. De Grave5, ‡
1 Departamento de Química, UFMG-Pampulha, 31270-901 Belt Horizonte, MG, Brazil
2 On leave from Departamento de Química, UFV, 36571-000 Viçcosa, MG, Brazil
3 Department of Pure and Applied Physics, Trinity College, University of Dublin, Dublin 2, Ireland
4 Departamento de Química, UFOP 35400-000 Ouro Preto, MG, Brazil
5 Department of Subatomic and Radiation Physics, University of Gent, B-9000 Gent, Belgium
Professor Milton Francisco de Jesus Filho died on January 2, 1996.
Research Director, National Fund for Scientific Research, Belgium.

Abstract: A dusky red Oxisol forming on a tholeiitic basalt is found to contain varying proportion of aluminous hematite (Hm) and titanoaluminous maghemite (Mh) in the different size fractions. Maghemite is the main iron oxide in the sand and silt fractions whereas Hm is dominant in the clay fraction, together with gibbsite (Gb), kaolinite (Ka), rutile (Rt) (and probably anatase, An) and Mh. Maghemite is also the major oxide mineral in the magnetic separates of soil fractions (sand, about 65% of the relative Mössbauer spectral area; silt, 60%). Hematite (sand, 30%; silt, 15%) and ilmenite (Im) (sand, 5%; silt, 16%) are also significantly present in the magnetic extract. Accessory minerals are Rt and An. No magnetite (Mt) was detected in any soil fraction. Sand- and silt-size Mh have similar nature (a0 = 0.8319 ± 0.0005 nm; about 8 mol% of Al substitution; saturation magnetization of 49 J T−1 kg−1), and certainly a common origin. Lattice parameters of clay-Mh are more difficult to deduce, as magnetic separation was ineffective in removing nonmagnetic phases. Al content in Hm varies from 14 mol% (clay and silt) to 20 mol% (sand). The proposed cation distribution on the spinel sites of the sand-size Mh is: [Fe0.92Al0.08]{Fe1.43Ti0.180.39}O4 (□ = vacancy, [ ] = tetrahedral sites and { } = octahedral sites), with a corresponding molar mass of 208.8 g mol−1. The predicted magnetization based on this formula is σ ≅ 68 J T−1 kg−1, assuming collinear spin arrangement. The large discrepancy with the experimentally determined magnetization is discussed.

Key Words: Hematite • Ilmenite • Maghemite • Magnetic Fraction • Mössbauer • Spinel

Clays and Clay Minerals; August 1998 v. 46; no. 4; p. 369-378; DOI: 10.1346/CCMN.1998.0460402
© 1998, The Clay Minerals Society
Clay Minerals Society (www.clays.org)