Chemical and mineralogical composition of Fe-Mn concretions and calcretes occurring in sodic soils of Eastern Uttar Pradesh, India

The chemical and mineralogical composition of iron-manganese (Fe-Mn) concretions, calcretes (lime nodules), and soil matrix in sodic soils (Typic Halaquepts) from Mirzapur, Faizabad, Ghazipur, and Ballia districts of Uttar Pradesh have been investigated. The data show enrichment of iron and manganese oxides in Fe-Mn concretions, CaCO3 in calcretes, and insoluble constituents in the soil matrix. Hydrochloric acid-insoluble fractions were higher in soil than in concretions and calcretes. Sodium content was also higher in soils than in concretions and calcretes, and it increased with depth. The concretions had haematite and limonite in high proportion, while calcretes predominantly contained calcite and lesser amounts of quartz, mica, and fieldspars. The thin section study indicated that staining of calcretes was caused by gradual replacement by iron oxides. The calcretes and Fe-Mn concretions were formed by pedogenic processes and modified by subsequent environmental changes.