Mineralogical approach to parent material characterization of soils from agroecological region 6, lower purnabhaba floodplain, in Bangladesh

The study was conducted to examine the mineralogical composition of deeper layers of soils from the agroecological region (AEZ) 6, Lower Purnabhaba Floodplain, in Bangladesh. Surface layers of soils of the same AEZ were analyzed for the mineralogical composition in a previous study. The parent material of the soils belonging to the AEZ 6 has been supposed to be the Tista Alluvium, however, the mineralogy of the surface layers was rather similar to that of terrace soils developed from the Madhupur Clay. In order to verify the mineralogical suite throughout the profile of soils of the AEZ 6, soil samples of three different depths of 30-45, 60-75 and 75-90cm selected from six profiles were subjected to particle-size and mineralogical analyses. It was found that all soil samples contained the high amount of clay (31.0-64.0%) and that the texture of most soil samples were designated as silty clay. The 2μm clay fraction was commonly dominated by mica (19-44%) followed by interstratified mica-vermiculite-smectite (26-40%) and kaolinite-smectite (19-39%) minerals, accompanied with small amounts of kaolinite, chlorite, interstratified mica-chlorite, vermiculite, quartz, feldspars and lepidocrocite. The results indicated that the Madhupur Clay as the parent material had a much greater impact on the mineralogical composition in the 2μm clay fraction in all profiles with insignificant contribution of the Tista Alluvium, while one soil layer of the Jaonia-6 profile showed a relatively higher contribution of the Tista Alluvium. From the findings of the present study, soils of the Lower Purnabhaba Floodplain (AEZ 6) should be included in the mineralogical suite of mica-mixed layer minerals-kaolinite along with terrace soils in Bangladesh.