Antonio Taranto Goulart

IRON OXIDES IN A SOIL DEVELOPED FROM BASALT

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:

Ilmenite and magnetite of a tholeiitic basalt

\rm{[Fe_{0.92}Al_{0.08}] {Fe_{1.43}Ti_{0.18}\square_{0.39}}O_4}

Quantitative Mössbauer analysis of maghemite-hematite mixtures in external applied field

[Fe0.92Al0.08]Fe1.43Ti0.18◻0.39O4(◻ = 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.

Mecanismos químicos e mineralógicos de transformação da magnesioferrita de solo derivado de tufito, da região do Alto Paranaíba, MG

Mössbauer spectra are presented at 15 and 298 K for magnetically-separated extracts of magnetic soils from a tuffite-derived weathering mantle from Minas Gerais, Brazil. The principal iron-bearing mineral is maghemite in which the degree of Ti or Mg substitution is found to vary with depth. The Mg-maghemite, with lattice parametera0=0.838 nm, is a new natural mineral. It is associated with anatase in the magnetic-separates.

Multiple Iron-Rich Spinel Phases and Hematite in a Magnetic Soil Developing on Tuffite

In a recent paper, Goulart et al (1994) reported the existence of a ferric ilmenite, (0.85)FeTiO3, (0.15)Fe203, in a soil developed from basalt. At that time, it was not clear whether the solid solution was directly inherited from the parent rock, dominantly, a tholeiitic basalt, or was somehow formed during pedogenesis. Some (A1, Ti)-maghemite and hematite were also reportedly detected in this soil (Goulart 1994). In the present work, a magnetic extract of a basalt sample from the same geological domain was studied in detail, and its mineralogy is described. Firstly, a piece of the basalt was collected in a cut just aside the highway BR 452, 37.2 km northwest from Tupaciguara (Minas Gerais State, Brazil), from a rock block 4 m below the surface, underlying a dusky red magnetic Alfisol (saturation magnetization a~ ~ 2 JT -1 kg-l). Then, the rock sample (a, < 1 JT -~ kg -~)

Magnesioferrita e caminho pedogenético de transformação de óxidos de ferro magnéticos em dois perfis de solo derivados de tufito da região do Alto Paranaíba (MG)

A natural ilmenite was extracted from the silt fraction of a soil developed from basalt of Minas Gerais State, Brazil. The sample was characterized by chemical analysis, X-ray diffraction, magnetic measurements and Mössbauer spectroscopy. The Mössbauer spectrum at 293 K shows two doublets: a major subspectrum of Fe2+ (relative area 74%) and a minor one of Fe3+ (26%). An incipient magnetic structure appears at 85 K, with a broadline sextet. These results may be explained by supposing that the separate is a solid solutionxFeTiO3 · (1−x)Fe2O3, withx=0.85, which is magnetically ordered at low temperature. The lattice parameters for this hexagonal structure area=0.5082±0.0001 nm andc=1.398±0.001 nm.

Óxidos de ferro magnéticos de um tufito da região do Alto Paranaíba, MG

Mixtures of synthetic maghemite and hematite have been studied by Mössbauer spectroscopy at room temperature with an applied magnetic field of 0.2 T. The results show that the relative area of each subspectrum reproduces very satisfactorily the weighted quantities, whereas the agreement is poor in absence of the external field, particularly for higher fractions of hematite.

Structural Rietveld refinement of magnetite and hematite of a steatite and its forming soil from Quadrilatero Ferrifero, MG, Brazil

Magnetic soils forming on tuffite of the region of Alto Paranaiba, Minas Gerais, Brazil, usually contain iron-rich spinels exceptionally rich in magnesium and titanium. In this work, samples of the magnetically separated portion from the sand fraction of a Brunizem (Chernossolo) and from its mother-rock material were analyzed with synchrotron X-ray diffraction and 57Fe-Mossbauer spectroscopy. Magnesioferite (MgFe2O4) and maghemite (its pure non-stoichiometric spinel structure, Fe8/3 ⊕ 1/3 O4, where ⊕ = cation vacancy, corresponds to γFe2O3) were the magnetic iron oxides so identified. Basing on these data, a consistent chemical-mineralogical model is proposed for the main transformation steps involving these iron oxides in the pedosystem, starting on magnesioferrite to finally render hematite (αFe2O3), passing through maghemite as an intermediate specie.