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

Abstract

A dusky red Latosol developing on tuffite was examined in detail in order to characterize its iron oxide mineralogy. Magnetic separation and selective chemical treatment were employed. The mineralogical analyses were carried out by powder X-ray diffraction and Mossbauer spectroscopy, with the electron microprobe technique being used to assess the chemical composition of selected grains. Results reveal that iron-rich spinel-structure phases from this pedosystem are rather variable in chemical composition and lattice parameters, and can be grouped in at least three major populations: the first group consisting of the iron-richest phase, with very low Ti and Cr and virtually no Mg content and high Al; the second group presenting Ti>Mg>>Al>Mn contents but no Cr and the third group related to the Mg-richest phase with Mg>Ti>>Al>Mn. Two major populations of hematite were also identified: one with ∼15 mol% Al and other practically Al-free. The first group is thought to be a product of neoformation in an Al-rich pedogenic medium whereas the second is likely to be transformed directly from maghemite. Though chemical treatment along with physical techniques provide a powerful way to characterize iron oxides of tropical soils, some methodological difficulties still remain, particularly those concerned with (i) the characterization and quantification of maghemite in presence of relatively large amount of hematite in the soil mixture, particularly by Mossbauer spectroscopy, and (ii) the allocation of the isomorphous substituents for iron in the cation sites of the spinel structure. Possible mechanisms of alteration of iron-rich phases in this pedosystem are discussed.