A peculiarity of the consistency of bentonite clay was profiled on the basis of the consistency limits and the parameters derived from the limit data, and the interrelationships between the consistency and other primary characteristics of bentonite were investigated and described for eighteen market samples of Ca-and Na-based bentonite, and two samples of bentonite from high-density polyethylene geomembrane-supported geosynthetic clay liners.
The resulting regression equation between the water content (ω, %) at the consistency limit and the specific volume (f, %) was f=2.55ω+100, where f is percent volume ratio per volume of the soil particles, and a factor of proportionality, 2.55, is the statistical specific gravity of the soil particles. Therefore, the regressionequation consists well with the theoretical equation.
Another regression equation between the plasticity index (Ip) and the liquid limit (ωL, %) was IP=ωL-58. The authors believe the constant, 58, to be the statistical plastic limit, because the regression equation is of same type for the definition of the plasticity index, Ip=ωL-ωp, where wp is the plastic limit.
The magnitude of the liquid limit and the plasticity index is basically dependentupon the content of the colloidal particle size fraction (that is finer than 1 μm). Thereby, the extent of the liquid limit increases with the colloid fraction content. The profile can be divided into classes determined by the polymorphism of the free silica impurity (quartz and α-cristobalite) contained in bentonite. This cristobalite contributes significantly to the amount of the colloid fraction, and it is soluble in a heated sodium hydroxide solution and a measured quantity of the cristobalite may be characterized as an intercept of the soluble silica at the zero alkali-processing time on the bentonite.
Bentonite containing the cristobalite has a tendency to form a soil structure that has relatively low shrinkage ratio and relatively high specific volume, resulting in a direct effect on the shrinkage limit measurements. A direct interrelationship between the liquid limit and the cation exchange capacity (CEC) was not apparent, however, significantly higher results for liquid limits were found at sodium cation/CEC equivalent ratios greater than 50%.
The regression equation, Sp=0.04ωL+2.6, was obtained from the relationshpi between the swelling power (Sp, cm³/2g) and the liquid limit. The equation or the regression line is useful as a nomograph for rapid solution on the liquid limit from the swelling power that the testing is readily. An interrelationship between the swelling power and the liquid limit was described by introducing two dimensionless parameters, specific swell volume-change (fsp/f₀, %) and specific volume-change at the liquid limit (fL/f₀, %) that are volume ratios per volume at the shrinkage limit. The resulting regression equation, fSP/f₀=2.28 (fL/f₀-13), was obtained. The equation describes that quantity of water contained in the swelling power of bentonite is 2.28 times as large as that of the liquid limit, and also, the constant in the parenthesis, 13%, is an additional dead space between domains or aggregates of the clay particles in the swelling power test. This is may be applicable to only the interaction of water with high plastic clay such as bentonite.

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Vivianite (Fe₃ (PO₄) ₂·8H₂O), an iron phosphate mineral has been identified in biomats produced around As polluted tubewell, which is used for drinking water in Hazigonj, Chandpur, Bangladesh. The As polluted groundwater and biomats (brown, green and gray) were analyzed by ED-XRF, XRD and optical microscopy, complimented with SEM-EDX. The biomats are composed mainly of metabolically active microorganisms such as coccus, bacillus and filamentous types of bacteria and algal filaments. A small amount of As accumulation in biomats is evidenced by SEM-EDX. Furthermore, X-ray powder diffraction patterns revealed that the formation of vivianite in different colored biomats (brown, green, brown and gray), which is a secondary mineral composed mainly of P and Fe, in association with quartz, feldspars, illite, chlorite and cordierite. Vivianite possibly contains a small amount of As by isomorphic substitution for P, resulting solid solution of symplesite. The microorganisms may play an important role in mineralization of iron phosphates.

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