Bentonite

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.