Characterization and distribution of mineral phases in an Amazonian humiluvic Spodosol profile by XRPD, HR-TEM, SAED and SEM-EDX

The Amazonian biome has a fundamental role in the global climate change scenario and the understanding the physicochemical aspect in Amazonian soils is of great importance. The nine horizons constituting a typical Amazonian Spodosol profile were studied by different complementary techniques: X-ray powder diffraction (XRPD), high-resolution transmission electron microscopy (HR-TEM), selected area electron diffraction (SAED) and scanning electron microscopy-energy-dispersive X-ray spectrometry (SEM-EDX). The XRPD analysis revealed the presence of different minerals along the profile, i.e. quartz from 0 to 214 cm (horizons from A1 to Bhs) and kaolinite from 215 to 290 cm (horizons from Tr to K2). The HR-TEM images and SAED patterns revealed the presence of kaolinite in the horizon A1, which was not detected by XRPD. Application of the Rietveld refinement enabled the quantification of the minerals in each horizon. The SEM-EDX analyses allowed to identifying changes in the horizon surface morphology as a consequence of ongoing deformation. A significant decrease of particle sizes and a marked variations of their surface features with increasing depth were measured by SEM. In conclusion, the results of this work provide new insights into the relationship between chemical properties and mineral changes along the soil profile.