Iron-rich spinel phases from sand fraction of three Chilean soils developing on volcanic materials

The magnetic separates from the sand fraction of three Chilean soils formin g on volcanic materials were studied by powder XRD, RT and 80 K Mossbauer s pectroscopy, saturation magnetization measurements, and scanning electron m icroprobe analysis. Soil samples were collected from the B-horizon of pedon s from native and cultivated areas. Results, particularly from powder x-ray diffractometry, Mossbauer spectroscopy, and chemical analysis, reveal rela tively complex magnetic mineral assemblages in all samples. Fe (> 70 mass% Fe2O3), Ti (ranging from similar to2 to similar to 26 mass% TiO2), Al (from 1 to similar to6 mass% Al2O3) and Mg (from 1 to similar to6 mass% MgO) are the main elements found in the iron oxide-spinel structures. In at least o ne sample, namely the magnetically separated portion from the sand fraction of the fertilized Collipulli soil, the 80 K Mossbauer spectrum indicates t hat 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 electro n microprobe data, reveal a large variability not only of chemical composit ion, but also of crystalline structure and magnetic properties of these mag netic minerals. The effect of agricultural practices, particularly the cont inuous chemical fertilization and soil acidity correction over several year s, 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.