About Bentonite

Bentonite is a commercial term for the montmorillonite group of clay mineral, also known as smectite. The term specifically applies to highly colloidal, plastic clay. It has the unique characteristic of swelling to several times its original volume when placed in water and of forming thixotropic gels with water even though the amount of clay is relatively less. It is formed by alteration of volcanic ash in situ. Since alteration of ash may also lead to other clay minerals, depending on the original composition of the ash, mechanism and duration of alterantion. Raw Bentonite may consist of other clay minerals like halloysite, kaolinite and illite. The physical properties of pure montmorillonitic Bentonite and admixture of the clays vary. Bentonite composed of montmorillonite, having different structural, chemical and exchangeable cat-ions exhibits different physical properties. For example, Na-Bentonite has greater swelling property and plasticity than Ca-Bentonite. While Ca-Bentonite is most common, Na-Bentonite is found in few localities.

One way to classify Bentonite is based on its swelling capacity. Bentonite with sodium as the dominant or abundant exchangeable ion has high swelling capacity and form gel like mass. Bentonite with calcium ion is more abundant. Its swelling capacity is less but shows higher green strength. Soda ash treatment increases swelling capacity of such Bentonite. The mixed type with Na-Ca ions has intermediate swelling capacity. Some Bentonites have Mg or H as abundant exchangeable ions and some, like K-Bentonite (known as metaBentonite) are of little industrial use. When Li is found as the exchangeable ion, Bentonite exhibits high swelling capacity.

Bentonite varies in colour from white to grey, yellow, green, blue and black but more often yellow or yellowish green. The weathered outcrops develop a characteristic cellular, popcorn or frothy structure because of swelling and shrinking on wetting and drying. The Bentonite in field has a waxy appearance. Bentonite formed from volcanic ash gives hard structure and contains non-clay materials such as Feldspars, biotite, hornblnde and characteristics of igneous materials. Cristobalite, zeolite and gypsum are commonly associated.

The process of formation of smectite (Bentonite) is essentially a devitrification of natural glass of the ash and crystallization of montmorillonite. The ash, probably contained excessive silica and alkalies and moderate amount of MgO. The silica gave rise to cristobalite. In order for Bentonite to form, it is probably necessary for ash to fall in water. Since much Bentonite is associated with marine formation, it seems certain tha alteration has taken place in sea water, soon after or contemporaneous to ash accumulation. Thus, later alteration of ash by process of weathering is eliminated from Bentonite formation.

Indian Bentonite

Among the sporadic occurrences of Bentonite and Attapulgite in India, Gujarat has almost all the Bentonite resources. Three fourth of the deposits in Gujarat are in Bhavnagar and Kutch districts only. As many as 162 leases are in Kutch and 86 are in Bhavnagar. Bentonite here is associated with Deccan Trap rocks, confined to alternation of ash beds. The original material of the ash is represented by fine-grained semicrystalline basaltic mineral like Feldspars and pyroxenes, which have given rise to montomorillionite. Bentonite occurs in lenses, pockets or in oval shaped bodies 3 to 10 m tick, covered by a residual soil layer 1.5 to 3 m. The soil is sticky, appears frothy and gives a soapy feel. Reddish brown hard laterite or red soil, at times pinkish due to presence of manganese, underlain by grey white aluminous clay or bauxite are observed in higher grounds. These are extreme states of alteration of the volcanic rocks including Bentonite and clays and probably resulted with further leaching of the soft Bentonite. Thus geomorphic features like topographic highs and lows, slope of the ground etc, have certain control on leaching of constituents and occurrence of Bentonite.

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