Iron-manganese nodules in a semi-arid environment

Abstract

Relationships between the chemical, mineralogical, and morphological characteristics and the formation mechanisms of some iron-manganese (Fe–Mn) accumulations were investigated. The nodules studied were from a poorly drained soil profile with a high calcium carbonate content, located in a semi-arid environment. Three types of nodules, Fe-rich (typic or annular morphology) Mn-rich (aggregate morphology), and Fe–Mn-rich (compound morphology), were encountered, all of which were formed in situ. The nodules were found to contain goethite, and a low proportion of poorly crystallised Fe oxides and Mn oxides (birnessite and vernadite). However, they contained no siderite or rhodochrosite, both highly frequent occurrences in carbonatic systems with redox fluctuations. In the formation of compound nodules, Fe- and Mn-rich nodules may act as nucleating structures for subsequent accumulations in the form of coatings and/or matrix impregnations.