Petrography, mineral chemistry and FTIR spectral study of lateritic iron oxide concretions from the Damodar River basin, India

Iron oxide concretions are present in the lateritic regolith of the Damodar River basin. The concretions are of ellipsoidal, elongated, cylindrical and irregular shapes. The cross-section of the concretions show concentric rims of iron-oxide minerals. Petrography of lateritic iron oxide concretions shows quartz and mica grains within the iron oxide minerals groundmass. The alternate dark and light gray concentric rims of iron oxide minerals are visible in the concretions. Electron microprobe study and FTIR spectroscopy of lateritic concretions shows presence of goethite, hematite, quartz, clay and mica minerals. The etch pattern in quartz grains of the concretions, high substitution of Al for Fe³⁺ in the hematite and goethite structure, and high substitution of iron in kaolinite structure of concretions suggest intense chemical weathering, mobilization of iron-bearing fluid and Fe redox reactions during the formation of concretions. These lateritic iron oxide concretions are of ex-situ or derived origin. Due to fluvial erosion and transportation, the high-level ferruginous materials or primary laterites transfer to the downslope region as low-level secondary laterite. These secondary ferruginous materials deposited at a low-level have preserved iron oxide concretions.