Phys Chem Minerals (1988) 15:470-475 PHl§lSli ~W UH|MIb'llff MIHERALS © Springer-Verlag 1988 Magnetic Properties of Iron-Rich Oxisols* Jacqueline E.M. Allan 1, J.M.D. Coey 1, Mauro Resende 2 and J.D. Fabris 3 1 Department of Pure and Applied Physics, Trinity College, Dublin 2, Ireland 2 Department of Soil Science, Federal University of Vigosa, MG, Brazil 3 EMBRAPA/CNPMS, Caixa Postal 151, 35700 Sere Lagoas, Brazil Abstract. Four occurrences of highly magnetic soil in Brazil have been analysed with a view to identifying the magnetic minerals and quantifying the soil magnetization. Tech- niques used include X-ray diffraction, X-ray fluorescence and M6ssbauer spectroscopy. This approach leads us to identify several ways that these soils, which have spontane- ous magnetization in the range 1 < as < 35 J/T/kg, can come about. One soil, which forms on dolerite (19.6 wt% Fe203), is found to contain fully-oxidized titanomaghemite inher- ited from the parent rock. This oxide has a canted ferrimag- netic spin structure with crs= 36 J/T/kg of sample. The three others, formed on very iron-rich rock (50-90 wt% Fe203), contain magnetite or maghemite as the magnetic species and in two cases the soil is more magnetic than the parent rock (largely composed of pure hematite). Introduction In a recent paper, Resende et al. (1986) estimated that as much as 5 percent of the land surface of Brazil may be covered by Oxisols that are sufficiently magnetic to be at- tracted by a hand-magnet. These soils tend to develop from mafic rocks, or from exceptionally iron-rich formations, and the former have greater agricultural potential than Oxisols derived from other rocks. It has therefore been pro- posed that soil magnetization be used as a soil classification parameter (Resende et al. 1986, 1988). Little is known about the iron oxide mineralogy of these magnetic soils. Four pure iron oxides and hydroxides have a spontaneous magnetization (as) at room temperature: he- matite (~- Fe203, as -~ 0.5 J/T/kg), feroxyhyte (fi- FeOOH, as~10 J/T/kg), maghemite (~Fe203, as~-60 J/T/ kg) and magnetite (Fe304, cr~100J/T/kg). However, isomorphous substitution of aluminium, titanium etc, which is to be expected in soil iron oxides (Schwertmann 1988) can greatly alter the magnetization (O'Reilly 1984, Coey 1988). Particle size can also be relevant, not only because of superparamagnetism, but also because antiferro- magnetic oxides in fine particle form (_-< 10 nm diameter) may develop a spontaneous magnetization. * Paper from the NATO Advanced Study Institute on Physical Properties and Thermodynamic Behaviour of Minerals, Cambridge 1987 In order to establish the precise nature of the iron oxide minerals causing the strong magnetic behaviour of the Oxisols, we have examined the following in some detail: (1) a soil column, formed on rock of basaltic composition (dolerite) with 19.6weight percent Fe203, found near Vigosa in the Zona da Mata of Minas Gerais, (2) three sets of highly magnetic soil and rock samples from the Quadrilatero Ferrifero, Minas Gerais, Brazil. The rock con- tains > 50 weight percent Fe/O3. Samples 1. Samples from the B and C horizons of a vertical sequence at the site of an igneous intrusion near Vigosa. These in- cluded some cobble-sized, rounded pieces of doleritic rock which were distributed throughout the soil. These rocks (V1) were covered in a rind (1-10 mm thick) of pale yellow, weathered, soil-like material (V2) which could be easily scraped off. Outside this was a red interface layer (V3) with the surrounding red soil (V4). From each of these three soil samples (V2-V4), it was possible to extract magnetic grains using a hand-magnet. The largest grains were sand- sized (~0.5 ram) and came from sample V2. The fraction extractable from V3 and V4 with the hand-magnet was quite small (<2%) but a larger fraction, >10 percent, could be extracted by high-gradient magnetic separation. For this, the material was dispersed in water and then repea- tedly passed through a tube of diameter 25 mm filled with steel wool, located in a field of 1.5 T produced by an electro- magnet. After breaking down the rock into fine particles, a magnetic component could also be extracted from V1. The samples resulting from these magnetic separations were designated Vlm-V4m. 2. Three sets of samples of rock and soil from the Quad- rilatero Ferrifero. a) a highly magnetic lateritic rock sample (QF2dr), found at the surface, and the yellowish weathered layer of soil around it (QF2ds) b) a rock sample, composed largely of hematite, found at a depth of 80 cm (QF4ar), its attached weathered layer (QF4as) and the surrounding, highly-magnetic soil (QF4b) sampled at a depth of 40 cm in the B2 horizon and c) samples of highly magnetic soil (MI) and itabiritic rock (I), composed of quartz and hematite, from which it is probably derived. All the samples were analysed by X-ray diffraction (XRD) with Co K~ radiation using a powder diffractometer