Magnetic separation and evaluation of magnetization of Brazilian soils from different parent materials

Ten selected Brazilian soils representative of Oxisols, Ultisols, Alfisols and Inceptisols and derived from gneiss, basalt, sandstone, diabase and itabirite were sampled and examined to provide insight into the behavior of their magnetic minerals. The objective was to measure the magnetization of the sand, silt and clay fractions and to submit the clay fraction to variable magnetic fields to obtain the magnetization curves and to study their magnetic behavior as related to their parent material lithology. The separation procedure using a magnetic separator was done in two different ways: dry separation for the sand and wet separation for the silt and clay fractions. X-ray diffraction patterns of magnetic and nonmagnetic fractions were obtained and the segregation of ferrimagnetic minerals were shown. The magnetization (σs) of the total and the magnetic and nonmagnetic soil fractions was evaluated using a special analytical balance. The σs values obtained varied from 0.03 to 17.72 J T−1 kg−1 for sand; 0.05 to 8.91 J T−1 kg−1 for silt and 0.05 to 2.54 J T−1 kg−1 for the clay fractions. The results from the magnetically concentrated fractions varied in the same fashion but reached higher values. Magnetization curves showed spontaneous magnetization (Ms) values of 0.6 to 3.35 J T−1 kg−1, high field slopes (λ) ranging from 0.17 to 0.77 J T−2 kg−1 and, clearly, the influence of the parent material lithology on the nature of the soil magnetic behavior. Magnetic measurements suggested pathways for ferrimagnetic minerals transformations in soil systems.