Potencial agrogeológico dos xistos do Supergrupo Rio das Velhas, Itabira, Minas Gerais, Brasil

Abstract

In the Quadrilátero Ferrífero region, Brazil, certain rocks currently classified as waste in iron mining operations show potential for use as soil conditioners or rock dust remineralizers (REM). Among them, schists from the Rio das Velhas Supergroup, Nova Lima Group, are noteworthy. This study evaluated the agrogeological potential of these schists as soil remineralizers and as inputs for CO₂ capture. Chemical analysis of a composite sample, comprising three schist samples collected in the Itabira region, revealed a base saturation sum of 16.36%, SiO₂ content of 44.85%, MgO of 7.56%, K₂O below 1%, and a theoretical CO₂ sequestration potential of 150 kg CO₂ per ton of material applied to soil. The main paragenesis includes chlorite (~21%), actinolite (~26%), calcite and dolomite (~15%), quartz (~18%), plagioclase (~14%), and annite (~6%). Quartz volume is below 25%, meeting the requirement established by Brazilian Normative Instruction N°. 05/2016 (Brasil, 2016) for classification as a remineralizer. However, the K₂O content is below the minimum threshold set by the same regulation (<1%). The observed levels of CaO and MgO, together with the mineralogy, suggest a high CO₂ sequestration potential through enhanced weathering. Samples were homogenized and ground to below 0.3 mm, with 84% of the mass composed of particles smaller than 0.075 mm. Different rock powder doses were tested in four soil types, two clayey and two sandy, to assess the pH increase in the soil-rock mixture as a function of application rate. Doses applied to sandy soils ranged from 0 to 150 mg per 10 g of soil (equivalent to 0 to 15 t ha⁻¹, assuming incorporation in the 0–10 cm layer). For clayey soils, doses ranged from 0 to 300 mg per 10 g of soil (equivalent to 0 to 30 t ha⁻¹). Results indicated that doses between 50 and 150 mg per 10 g of soil stabilized pH between 5.5 and 6.5 after 24 and 72 hours of incubation, evidencing the schist’s effectiveness in neutralizing soil acidity. These correspond to agronomic doses of 5 to 15 t ha⁻¹ in the 0–10 cm soil layer. The 90-day incubation experiment showed that shale application significantly improved soil chemical properties by increasing calcium and magnesium levels while reducing acidity. Therefore, despite the K₂O concentration being below 1%, the material shows potential for use as a soil conditioner or as a raw material in blended REM formulations.