Abstract

The magnetic separates from the sand fraction of three Chilean soils forming on volcanic materials were studied by powder XRD, RT and 80 K Mössbauer spectroscopy, saturation magnetization measurements, and scanning electron microprobe analysis. Soil samples were collected from the B-horizon of pedons from native and cultivated areas. Results, particularly from powder x-ray diffractometry, Mössbauer spectroscopy, and chemical analysis, reveal relatively complex magnetic mineral assemblages in all samples. Fe (>70 mass% Fe2O3), Ti (ranging from ?2 to ?26 mass% TiO2), Al (from 1 to ?6 mass% Al2O3) and Mg (from 1 to ?6 mass% MgO) are the main elements found in the iron oxidespinel structures. In at least one sample, namely the magnetically separated portion from the sand fraction of the fertilized Collipulli soil, the 80 K Mössbauer spectrum indicates that magnetite undergoes the Verwey transition. This sample, with saturation magnetization being of order of 40 J T-1 kg-1, is also the most magnetic of all magnetic separates. Results on the iron oxide spinels, from electron microprobe data, reveal a large variability not only of chemical composition, but also of crystalline structure and magnetic properties of these magnetic minerals. The effect of agricultural practices, particularly the continuous chemical fertilization and soil acidity correction over several years, on the iron oxide mineralogy could not be unequivocally established, but the present data provide a relatively detailed description of the magnetic iron oxide assemblages of these Chilean volcanic soils.