The occurrence of iron-manganese concretions, which vary in diameter and shape, are common in limestone-derived soils from northern Minas Gerais, Brazil. Normally, they tend to increase their size with depth in Cambisols (Inceptisols) and Vertisols, and to remain small in Latosols (Oxisols), where they are smoother and more rounded. Aiming to study them, concretions in the B horizons of Red Latosols, B and BC horizons of Cambisols and C horizons of Vertisols were collected and grouped according to their average diameter (C1-f = 0.18 cm; C2-f = 0.53 cm; C3-f = 0.90 cm; C4-f = 1.75 cm). C1 are concretions of Latosols and C2, C3, C4 groups of concretions of Cambisol and Vertisol; after being grinded, they were chemically (Fe and Mn extracted by oxalate and dithionite (CBD); sulfuric acid digestion and triacid acid digestion) and mineralogically (X-ray diffraction) analyzed. Undisturbed samples of soils that had concretions were collected and impregnated for micromorphological analyses. The chemical characterization of concretions revealed that Fe, Si, and Mn are the main components. There was a significant and negative correlation between the concretions' diameter and iron amount (r = -0.88), which agrees with the literature and indicates higher Fe accumulation in smaller diameter concretions. Also, there was a significant and positive correlation between the concretions' diameter and silica content (r = 0.96), explained by different silica contents of soils and horizons where the concretions occurred. For Mn, no correlation was found, which is not in agreement with the literature, which commonly postulates that, as the diameter increases, there is an increment of Mn amount. The quantification of trace elements showed, - besides the elevated Mn amounts, with average values 40 times higher than those found in soils - elevated Ba values (in average 20 times higher than those found in soils) which tend to follow that of Mn (r = 0.99), which was also verified for Co (r = 0.99), Ni (r = 0.94) and Pb (r = 0.99). X-ray diffraction revealed goethite as the sole iron oxide mineral. Quartz, caulinite, lithiphiorite and trace of 2:1 minerals (probably illite/smectite interstratified) were also detected. Micromorphological analyses revealed the presence of true concretions (having concentric internal organization) and iron-manganese nodules. From the center to the borders, the concentric layers of concretions varied from black to reddish-yellow colors, suggesting Mn contribution and possible transformation of Fe-oxides, possibly hematite to goethite. Some fragments of concretions present in soils are cloaked by new Fe additions, indicating that the process of Fe diffusion and concentration is still active.
Fe, Mn nodules and concretions chemistry; mineralogy and micromorphology; redox potential; trace elements; Bambuí limestone