M. F. de Jesus Filho

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:

Mineralogical analysis of a weathering mantle derived from tuffite

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

Mg-rich iron oxide spinels from tuffite

[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.

Characterization of a soil ilmenite developed from basalt

Tropical soils often contain high amounts of iron oxides. Hematite (αFe2O3) and goethite (αFeOOH) are the most widespread iron oxides, but magnetite (Fe3O4) and maghemite (γFe2O3) occur in magnetic pedons. A wide range of spinel compositions in the Fe3O4-γFe2O3 series has been identified in magnetic Brazilian soils. Isomorphic substitution of mainly Ti4+, Al3+ and Mg2+, but also of Cr3+ and Mn2+ and other minor elements for iron are related to changes in their structural stability and magnetic properties. Magnetic iron oxides of selected Brazilian pedodomains are discussed, distinguishing those produced from mafic rocks (tuffite, basalt), where primary magnetite transforms to maghemite, from those produced in non-mafic lithologies (such as steatite), where inherited magnetite may be exceptionally stable in the soil.

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

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.

X-ray differential line broadening on tabular haematites

Several synthetic hematites have been analysed by X-ray diffraction and Mössbauer spectroscopy. They have tabular shape and their Morin transition at 85 K show a dependence on the ratio (R) of the sizes inc anda directions. The Morin transition has been observed for samples withR=0.43±0.05.

Mössbauer study of Brazilian soapstone

Powdered crystals of an iron-rich spinel separated from tuffite were examined by chemical analysis, X-ray diffractometry (XRD), magnetometry, scanning electron microscopy and Mössbauer spectroscopy. They are found to contain mainly 48 mass% Fe2O3; 16.9% FeO; 14.3% TiO2; 9% MgO and 3% Al2O3. The magnetization is 29 J T-1 kg-1 at B0 ~ 0.9 T. XRD pattern reveals two cubic phases with α0 = 0.8382(5) and 0.8412(5) nm, respectively. Mössbauer spectra are very complex, but the hyperfine field distribution patterns can be decomposed in two relatively well-resolved Gaussian distributions for the Fe3+ block (averaged isomer shift relative to α-Fe, δ = 0.27 mm s-1) with Bhf = 43.6 T (14.6% of the whole spectral area, WSA) and Bhf = 47.3 (27.7% WSA). A second less resolved distribution block (δ = 0.63 mm s-1) shows two main maxima in the P(Bhf) curve at 35.3 T (25.7% WSA) and 45.2 T (19.7% WSA), respectively. The Mg-rich maghemite (lower α0) appears to be a direct alteration product of a Mg-rich magnetite (higher α0), via an oxidation process of structural Fe2+ to Fe3+ proceeding from the outer to the inner part of the crystal.

Mössbauer study of rock paintings from minas gerais (Brazil)

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.