Mineralogical Changes in Acid Sulfate Soils With Thionic Horizons: Insights From the Delta Doce River, Brazil

ABSTRACT Acid sulfate soils (ASS) with sulfide minerals (i.e., contain sulfidic materials) are widely distributed in both coastal and inland regions, and if exposed to air, the oxidation of sulfide minerals results in the production of sulfuric acid with the formation of thionic horizons (pH < 4) along with profound mineralogical changes. This study, conducted in the Delta of the Doce River, Espírito Santo, Brazil, investigated the mineralogical transformations of ASS with thionic horizons, focusing on the crystallinity of kaolinites under sulfurization conditions. Soil samples from six profiles with thionic horizons were analysed using X‐ray diffraction (XRD), selective extractions and magnetic susceptibility measurements. Mineralogical analysis revealed the predominance of quartz, kaolinite, illite and gibbsite, along with secondary Fe(III) minerals such as jarosite, goethite and hematite, accompanied by a reduction in sulfide content, indicating the advanced stages of sulfide oxidation. In strongly sulfurized soils, an increase in kaolinite crystallinity was observed, suggesting that sulfurization intensifies the destruction of less crystalline phases in the soil. The formation of magnetic minerals was associated with burning processes, highlighting the role of anthropogenic interventions in the mineral transformation of these soils.