Fine-grained Mica

Fine-grained Mica weatherable minerals in the soils of Bangladesh include sand and silt particles of feldspars, micas (both black and white), dark coloured ferromagnesian minerals, and 2:1 lattice clays. Fine-grained mica includes fine sandy and silty deposits of recent alluvium in bangladesh soils. Its dimension varies from 0.25 to 0.002 mm. Fine-grained Mica (muscovite and biotite) occurs abundantly in alluvial deposits derived from the Brahmaputra, Ganges and Tista river systems. But sediments derived from northern and eastern hills are generally less micaceous (especially in the case of biotite). madhupur clay also contains much less mica. Deposits from the rivers tista, the brahmaputra, and the ganges are different from those derived from hills and Terrace areas. The former deposits usually contain 5-30 percent micas. Its amount varies with size fraction, some silty deposits containing as much as 80 percent micas in associated sand fraction.

The non-capillary porosity of poorly drained soils is caused by flakes of mica along with other factors. Micas are associated with gleyans in floodplains where mica flakes are oriented parallel to the surface of coating. But on the homogenisation of floodplain soils, mica flakes are arranged in an irregular fashion. New alluviums are generally stratified in Bangladesh. The individual layers may be thin or thick. In uniform silty and clayey deposits, the stratification is mainly caused by thin layers of mica-flakes, which have been deposited with their flat surfaces parallel to the surface of the deposit. [Md Mizanur Rahman Bhuiyan]