Clay Mineralogy of Black Soils of Rajmahal Trap

Black soils (Vertic Ustochrept, Typic Chromustert, Typic Ustorthent and Typic Haplustalf) developed on dolerite and basalt in low lying areas of Rajmahal trap (Bihar) contain clay samples of multicomponent interstratified systems involving smectite-vermiculite-mica and possibly kaolinite layers. The chemical composition together with CEC suggests the occurrence of minerals of expanding type. Low content of K₂O and the weak and diffused d-spacings at 1.0 nm (001) and 0.5 nm (002) regions for mica indicate that the mica occurring in the system might have poor crystallinity possibly due to removal of K and involvement in the interstratification tending towards the formation of expanding type of minerals. Fairly high values for Fe₂O₃ and MgO suggest the occurrence of nontronite together with montmorillonite and/or vermiculite. Under the prevailing pedochemical environment, kaolinite may be present only as a transported product through the washings from upland and may have further undergone weathering and transformation leading possibly to the formation of interstratified minerals with expanding layer. The overall scenario depicted here on the basis of chemical. XRD and DTA data revealed that these clays are at unstable state tending towards the synthesis of expanding layers with multicomponent interstratification.