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 MiSssbauer 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 A1 substitution; saturation magnetization of 49 J T -* kg '), and certainly a common origin. Lattice parameters of clay-Mh are more difficult to deduce, as magnetic separation was ineffective in removing nonmagnetic phases. A1 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"

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Recebido em 1/10/03; aceito em 13/7/04; publicado na web em 12/11/04 MAGNETIC IRON OXIDES FROM A TUFFITE FROM ALTO PARANAÍBA, STATE OF MINAS GERAIS, BRAZIL. The Mössbauer analysis along with the structural Rietveld refinement based on powder X-ray data for the magnetic fraction (saturation magnetization, σ = 19 J T -1 kg -1) separated from a tuffite material from Alto Paranaíba, state of Minas Gerais, Brazil, reveal that a (Ti, Mg)-rich maghemite (deduced σ = 17 J T -1 kg -1 ) and, for the first time observed in this lithodomain, magnesioferrite (characteristic σ = 21 J T -1 kg -1 ) respond for the magnetization of the rock material. Consistent models for the ionic distribution in these iron-rich spinel structures are proposed.

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