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BENTONITES
Clays
produced by the diversification of volcanic ash are
geologically termed "Bentonites". Such clays are
characterized by their power to absorb water greatly in
the process, and remaining in suspension in thin water
dispersions and are characterized by rapid slaking
and slight swelling when placed in water. Usually, such Bentonites
exhibit high decolorizing ability after acid
treatment.
The relative proportion of lime
and soda is leading factor in determining their physical
properties. A high ratio of soda to lime indicates a
swelling bentonite while a low ratio characterizes the
non-swelling type.
Bentonites
vary in colour. It can be grey, blue,
yellow, red and brown. The pH varies from 4 to 10.
Bentonites
are usually composed mainly of montmorillonite, although
some may consist of the rarer clay minerals beidellite,
saponite, hectorite and nontronite. Mineralogically,
bentonites are 75 percent or more of montmorillonite
with fragments of kaolinite, lattite.
felspar, gypsum, unweathered volcanic ash, calcium
carbonate, quartz and traces of other minterals. The mineral
glauconite (green sand), nontronite, beidellite and a few
other ores of sedimentary origin have been successfully
acid-activated but the relative efficiency of the product
was not equal to that obtained from high grade sub-bentonites
in which montmorillonite predominates.
MONTMORILLONITE
The
structure of montmorillonite is a gibbsite layer sandwiched between two
silica sheets to form the structural unit. Such units are
loosely held together in the e-direction with water between
them; depending on the amount of water, the e-dimension
varies from 9.6 to 21.4
Ao .
The
substitutions are mainly within the octahedral layer (Mg2+,
Fe2+, etc, for Al3+) and to a much
less extent within the silicate layer Al3+ for Al4+).
Extensive replacements give rise to a number of
modifications.
Nontronite is an iron-rich member in which Al3+
is largely replaced by Fe2+. Saponite has a large
replacement of 2 Al3+ by Mg2+, and a
little Al3+ for Sl4+. Hectorite
results in the total replacement of 3 Mg2+ for 2
Al3+ and shows Ll-for-Mg substitution.
Replacement of Al by Cr and Zn yields the mineral
Yolkonskite and Sauconite respectively.
Beidellite (the name retained after Weir and
Greene-Kelly, 1962), is an Al-rich variety resulting from
the partial replacement of silicon by aluminium in the
montmorillonite lattice.
ADSORPTION
PROPERTIES OF CLAY MINERALS
Certain
natural clays such as montmorillonite possess adsorption
properties mainly because of their colloidal nature which
stems from their very small particle size. Other important
properties are base exchange capacity and hydrophily. The
relationship between these properties with respect to a
particular clay mineral may best be explained through its
crystalline structure.
In
montmorillonite, isomorphous replacement in the octahedral
layer provides an
excess charge on the lattice. The electrostatic force
binding the layers originates at the centre of a unit
at a distance of about 4.5 A0 from the surface.
The force is sufficiently strong to hold exchange ions at
the surface of the units but not to hold the units
themselves together tightly. Therefore, water may enter the
interlayer space forcing units apart. The lattice expansion
may also vary with the degree of hydration of the cations.
With swelling, the units are held more and more loosely and
readily cleave into extremely thin flakes on addiction in
water exposing a large surface area to the suspending
agents.
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