Bentonites are clays composed of very fine particles derived usually from volcanic ash. It is chiefly composed of the hydrous magnesium-calcium-aluminium silicate called montmorillonite.
BENTONITE
Synonyms
Whilkinite.
Source
Bentonites are clays composed of very fine particles derived
usually from volcanic ash. It is chiefly composed of the hydrous
magnesium-calcium-aluminium silicate called montmorillonite.
History
Bentonite, whose name derives from its type locality (San
Benito County, California), is a blue plicate mineral, found in hydro thermally
altered serpentinite. Bentonite fluoresces under ultraviolet light, appearing
light blue in colour.
Geographical Source
It is found in Brazil, France, Britain, Germany, India,
Australia, Japan, China and in the United States (California, Georgia, Florida
etc.). Bentonite is the official state gem of California.
Description
Bentonite occurs slightly greenish grey or blue in colour.
When observed under ultraviolet light it shows light blue colour. It is
insoluble in water, HCl and H2SO4, it occurs as Tubular
dipyramidal crystals and the hardness ranges from 6 to 6.5. Bentonite has a
specific gravity of 3.6 and refractive index of 1.757–1.759; 1.802–1.804.
Microscopy
Small quantity of bentonite when mounted in glycerin and
observed under microscope shows minute Hexagonal crystals with 1 μ to 2.5 μ in size.
Chemical Constituents
The chemical formula of bentonite is BaTiSi3O9
(barium titanium silicate). Generally Bentonite occurs along with some unique
set of minerals and the frequently associated minerals are natrolite (Na2Al2Si3O10
2H2O), albite (NaAl-Si3O8), neptunite [KNa2Li
(Fe, Mn)2Ti2Si8O24], serpentine
[(Mg, Fe)3Si2O5(OH)4] and
joaquinite [NaBa2FeCe2(Ti, Nb)2(SiO3)8(OH,
F) 1H2O].
Identification Test
1. Bentonite is mounted in cresol and
observed on dark field polarized light, it shines brightly.
2. Bentonite acquires permanent red
stain when treated with 1% solution of safranin in 70% alcohol.
3. When bentonite treated with 0.1%
solution of methylene blue in absolute alcohol it takes deep blue colour.
4. Bentonite is first mounted in alcohol
and then water is applied through its sides, the fragments of bentonite swells,
disintegrates into small particles and forms a jelly like matrix.
Uses
The hardness of bentonite makes it suitable for its use as a
gemstone. It is also used as gel in ointment, in creams as a base, in
lipsticks, depilatories and rouges. Highly absorbent bentonite is used for
facing the moulds and preparing the moulding sands for casting metals. The less
absorbent bentonite is used chiefly in the oil industry. Bentonite is also
used as suspending and emulsifying agent and base for plasters.
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