IRON-MANGANESE NODULES IN A SEMIARID ENVIRONMENT

Relationships between the chemical, mineralogical, and morphological c haracteristics 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, loc ated 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 vernad ite). However, they contained no siderite or rhodochrosite, both highl y 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 c oatings and/or matrix impregnations.