Silica-Sesquioxide Ratios as Aids in Characterization of some Temperate Region and Tropical Soil Clays
1982
Tan, K. H. | Troth, P. S.
The usefulness of silica-sesquioxide ratios in the characterization of clays was studied by comparative analysis of primary and secondary minerals, and clays isolated from selected temperate region and tropical soils. The reference minerals and the clay fractions (<2 µm), separated from A and B horizons, were analyzed for total Si, Al, and Fe contents by the lithium metaborate method and atomic absorption spectrophotometry. Characterization of the minerals was conducted by x-ray diffraction, differential thermal analysis (DTA), and infrared analysis. The data indicated that the molar silica-sesquioxide ratios were capable of detecting differences in mineral composition, not revealed by x-ray diffraction or the other methods. However, a considerable overlap in values of the ratios was noted for clays of the temperate region soils. In the absence of x-ray diffraction and/or other analysis, this overlap imposed some restrictions as to the use of the ratios in the identification of mineral species from a mixture of 2:1-type of clays, but was not critical in the distinction of kaolinite from montmorillonite as monominerals. Kaolinite in the temperate region Ultisols and tropical soils had SiO₂/R₂O₃ (1.41 to 2.15) and SiO₂/Al₂O₃ (1.63 to 2.46) ratios in general agreement with those of reference kaolinite. However, the lower values of SiO₂/Fe₂O₃ and Al₂O₃/Fe₂O₃ ratios of the soil clays distinguished them from the reference clay as being higher in iron content. The idea that increased weathering decreased the values of the silica-sesquioxide ratios was not substantiated by the present results, since the clay minerals revealed similar if not higher silica-sesquioxide ratios than the primary minerals.
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