Distribution of Clay Minerals – Kaolinite, Illite and Montmorillonite in the Coarse and the Fine Clay Fractions in Some Alluvial Soils.

Abstract

Seven selected soil profiles of alluvial soils from various parts of Thailand were used in the study of the distribution of clay minerals – kaolinite, illite and montmorillonite in coarse and the find clay fractions. The clay fraction of soils was first separated into coarse and fine fractions between 2.0 to 0.2 micron and smaller than 0.2 micron, respectively. The mineralogical composition of both fractions was determined by X-ray diffraction technique. The results reveal that the clay mineralogy of the alluvial soils consists mainly of kaolinite, montmorillonite and illite with kaolinite yielding the biggest proportion in most of the soils studied. Interstratified clay minerals, vermiculite, chlorite, goethite and quartz are also present in small quantities in some soil series. The distribution of kaolinite in the two fractions are nearly the same, slightly more in the coarse clay fraction (2.0 to 0.2 micron) in some soils, however slightly more in the fine clay fraction (<0.2 micron) is also observed. Montmorillonite is found in both fractions of most soils, more in the fine clay fraction than in the coarse clay fraction. Illite segregates largely in th coarse clay fraction and is found in the fine clay fraction in minor amount or does not present in some soils. Other minerals from these alluvial soils such as vermiculite, quartz and chlorite are found only in the coarse clay fraction and the interstratified clay minerals and goethite are presented in both clay fractions. From the reasons that many minerals do not segregate largely in either coarse or fine clay fraction, in general practice, it is not necessary to separate the clay fraction into coarse and fine fractions prior to X-ray diffraction examination of minerals. Separation of the clay fraction < 2 micron is satisfactory.