Weathering and pedogenesis of mafic rock in the Brazilian Atlantic Forest

Genesis and transformation of basic rock-derived soils with shiny ped faces under tropical conditions

“…Clayey texture is typical of soils derived from mafic rocks, of which the weathering of primary minerals, such as amphiboles and pyroxenes, favors the formation of clay minerals and iron oxides and hydroxides (Asio and Jahn, 2007;Buol et al, 2011;Dortzbach et al, 2016;Camêlo et al, 2017;Oliveira et al, 2020;Silva et al, 2020;Santos et al, 2021). The concentrations of oxides and kaolinite were also favored by advanced weathering verified by the indices RA% and Δ4Si% (Table 5).…”

“…The concentrations of oxides and kaolinite were also favored by advanced weathering verified by the indices RA% and Δ4Si% (Table 5). Santos et al (2021) evaluated weathering and pedogenesis in profiles from gabbro-derived soil located near the study area in Pinheiral. They found that the progressive weathering of pyroxenes and plagioclases resulted in the neoformation of clay minerals such as kaolinite and iron oxyhydroxides, namely goethite and hematite.…”

“…The weathering product of ilmenite results from iron dissolution with relative titanium enrichment that results in leucoxene formation (Figure 5) (Deer et al, 1966;Santos et al, 2021). In turn, magnetite weathering through the gradual oxidation of structural Fe 2+ to Fe 3+ might lead to hematite formation during pedogenesis in a mineralogical pathway that differs from that of mafic systems, wherein magnetite can be transformed into hematite by oxidation (Santana et al, 2001;Camêlo et al, 2017).…”

“…However, the contribution to the increase in pedogenic iron oxyhydroxides was not significant compared with pyroxene and amphibole weathering. Santos et al (2021) evaluated soil from pedogenesis gabbro weathering. They found that some of the iron released from weathered primary minerals such as pyroxenes, amphiboles, and ilmenite under alternating wet and dry conditions (redox potential) might precipitate as ferruginous nodules with a moderate to strong degree of impregnation and sharp, crystalline, and/or slightly crystalline edges.…”