Mineralogy and geochemistry of ferromanganese nodules in semiarid calcareous soils from Northeastern Brazil

Abstract

Fe/Mn nodules are important records of pedogenesis, weathering processes, and pedoenvironmental conditions. However, data regarding the morphology and mineralogy of Fe/Mn nodules are limited due to difficulties concerning the identification of their low crystallinity Fe/Mn oxides. The hypothesis is that nodules in calcareous soils in Northeastern Brazil present crystalline forms of Mn oxides, whose mineralogy and chemical composition are linked to slight variations in relief. This study investigated the mineralogy and geochemistry of Fe/Mn nodules of soils on different relief surfaces (convex, flat, and concave). The nodules were analyzed using sequential selective dissolution procedure (SSDP), X-ray diffraction, elemental analyses, micromorphology scanning electron microscopy, and energy dispersive spectroscopy. Improvements of the SSDP were needed to identify Mn oxide minerals (birnessite and lithiophorite) in nodules. The relief features are determinants for the distribution and formation of minerals in nodules. The occurrence of calcite silt-sized on the convex and straight surfaces indicates pedogenetic genesis. An increase in kaolinite, highly crystalline lithiophorite, and the absence of calcite in the nodules close to the concave surface also indicates synchronism with pedogenesis. Pseudo-hexagonal lithiophorite and birnessite with plate-like habits are important scavengers of Ce, Co, V, and Ba. This research represents the first record of the detailed morphology and identification of Mn minerals in Fe/Mn nodules using SSDP in semiarid calcareous soils from Northeastern Brazil.