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) concre tions, calcretes (lime nodules), and soil matrix in sodic soils (Typic Hala quepts) from Mirzapur, Faizabad, Ghazipur, and Ballia districts of Uttar Pr adesh have been investigated. The data show enrichment of iron and manganes e oxides in Fe-Mn concretions, CaCO3 in calcretes, and insoluble constituen ts 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 c oncretions had haematite and limonite in high proportion, while calcretes p redominantly contained calcite and lesser amounts of quartz, mica, and fiel dspars. The thin section study indicated that staining of calcretes was cau sed by gradual replacement by iron oxides. The calcretes and Fe-Mn concreti ons were formed by pedogenic processes and modified by subsequent environme ntal changes.