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Iron Oxides in Selected Brazilian Oxisols: II. Mössbauer Studies

Dep. de Solos, Univ. Federal de Vicosa, Vicosa-MG 36570, Brazil

L. H. Bowen

Dep. of Chemistry

S. B. Weed

Dep. of Soil Science, North Carolina State Univ., Raleigh, NC 27695-7619

*Corresponding author.

ABSTRACT

Clay fractions (<2 µm) of Oxisols derived from four different parent materials, all from the Triangulo Mineiro Region, Minas Gerais, Brazil, were analyzed using Mössbauer spectroscopy to provide insight into their Fe mineralogy. As little as 0.5% Fe as hematite in the total clay was quantitatively determined by this technique. Aluminum substitution in the Fe oxides resulted in lowering of the field of maximum probability (H _max) and this Al substitution affected the magnetic ordering of goethite more strongly than did poor crystallinity. However, Al substitution estimated from H_max showed some inconsistencies with varying temperature of Mössbauer analysis as well as with values obtained from chemical analysis. Apparently, the currently available Mössbauer calibration curves, obtained from synthetic samples of hematite and goethite, are not readily applicable to soil samples. Little difference in goethite content was found among the soils developed on parent material from mafic rocks. The data suggest that, in the presence of high levels of Fe in an ustic environment, the amount of goethite formed is not strongly correlated with organic matter. The values of the lower half-width (LHW) of the hyperfine field distribution at room temperature showed some hematites to consist of very fine particles (broad LHW), whereas other hematites were better crystallized (sharper LHW).

Paper no. 12459 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh, NC 27695-7643. This work was supported in part by the National Science Foundation, Grant EAR-850656.

Received for publication February 5, 1990.