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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.
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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