Understanding the genesis of ultramafic soils and catena dynamics in Niquelândia, Brazil

This study focuses on the fate of chromium during ultramafic weathering under tropical conditions. Three soils were studied along a characteristic catena on the ultramafic outcrop of Niquelândia, Brazil. In these soils, the Cr-bearing minerals are inherited chromites (15.4–26.8 wt.% Cr) and magnetites (0.4–4.8 wt.% Cr) and secondary minerals such as iron oxides (0–2 wt.% Cr) or Ni-smectites (0–6.8 wt.% Cr). Indeed, during weathering, Cr liberated from primary minerals can first be incorporated into Ni-smectites (garnierite) or Fe oxides. As the weathering continues, smectites become unstable and Cr liberated into the soil solution, and incorporates in Fe oxides. In this oxidized environment, Fe and Mn are oxidized and oxides can incorporate elements like Cr, Ni and Al. Two types of chromites were distinguished, i.e. a vein type (VC) and a rock type (RC) according to their composition. Chromites could be used as tracer of pedogenesis processes due to the distinct composition of VC and RC types. They allow us to show that the upper part of some soil profile results exclusively from colluvial processes, and the lower part of the profile results from the ultramafic bedrock weathering. The chromite tracer allows us to propose a model of the catena evolution.