Bentonite: Public Research Project
Bentonite & Montmorillonite: Smectite Clay Minerals
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What is Bentonite? And Montmorillonite?
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Bentonite Hills in Capitol Reef
National Park
Photo used with Permission, copyright Q.-T.
Luong
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Bentonite Hills - Capitol Reef
National Park
Photo used with Permission, copyright Q.-T.
Luong
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Green Bentonite - Nebraska
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Bentonite is a name given to a particular
clay that was originally found in Fort Benton,
Eastern Wyoming. The name was given by W.C. Knight in 1898.
Previously, it was called Taylorite,
which was named after William Taylor, who first began to
draw attention to the clay deposits.
Geologists often describe bentonite as
a clay mineral containing Montmorillonite, which poses
a rather curious problem, as Montmorillonite is a name
of
a particular
clay
that was discovered in Montmorillon, France, named by
Mauduyt in 1847.
"The name montmorillonite is used currently
both as a group name for all clay minerals with
an expanding lattice, except vermiculite, and also as specific mineral
name. Specifically it indicates a high-aluminia end member
of the montmorillonite group with some slight replacement
of Al3+ by Mg++ and substantially no replacement of Si4+
by Al3+. MacEwan suggested the term montmorillonoid for
the group name to avoid confusion with montmorillonite
as a specific mineral name, and Correns suggested Montmorin
as the group name. Neither of these names has found
favor.
The name smectite suggested as a group name by
the Clay Minerals Group of the Mineralogical Society of
Great Britain at the outset met strong opposition, particulary by many
American mineralogists, but it is becoming widely accepted"
(R.
E. Grim: Clay Mineralogy 2nd edition, McGraw-Hill Book
Company, New York (1968) 41).
Bentonite Chemical Equation: Al2O34SiO2H2O (
Reade Advanced Materials )
Montmorillonite Chemical Equation: Na0.2Ca0.1Al2Si4O10(OH)2(H2O)10
"Sodium bentonite is
the name for the ore whose major constituent is the clay
mineral, sodium montmorillonite.
Montmorillonites are three-layer minerals consisting of
two tetrahedral layers sandwiched around a central octahedral
layer (Figure 1). Oxide anions at the apices of the tetrahedral
subunits are directed inward where they surround interior
aluminum, iron and magnesium cations, thereby forming the
octahedral subunits of the octahedral layer. Bonding, between
the shared interior oxide anions and the cations in both
the tetrahedral and the octahedral layers, links the layers
together and yields the unique sheet structure characteristic
of clay minerals. For montmorillonite, the total negative
charge contributed to the structure by the sum of all the
oxide anions (O=) is somewhat in excess of the total positive
charge contributed by the sum of all the structural cations
(Si+4, Al+3, Fe+2, Fe+3, Mg+2) and imparts a slight overall
negative charge to the surfaces of the clay sheets. This
slight excess negative charge on the sheets is counterbalanced
by free-moving (exchangeable) cations which exist between
them. These three layers in each sheet comprise individual
bentonite platelets which are typically 1 nm in thickness
and 0,2-2 microns in diameter. Dry platelets of sodium
bentonite are most commonly grouped together in a face-to-face
arrangement, with exchangeable cations and small amounts
of adsorbed wares in an interlayer region between each
platelet. The thickness of the interlayer region is variable
depending on the amount of water adsorbed between the platelets."
If the terms bentonite and montmorillonite had both been
coined late in the 20th Century, then it's possible that
one would be saying that " Sodium montmorillonite is the
name
for the ore whose major constituent is the clay mineral,
sodium bentonite."
To fully illustrate this, consider the following classification
made by American Colloid, one of the premiere
Bentonite mining organizations
in the
United
States:
Bentonite - HPM20:
"High-purity, air-classified sodium bentonite, selectively-mined,
consisting of micronized particles and supplied
as a free-flowing powder. This high-purity montmorillonite
is typically used where small particle size is required
in pesticide and fertilizer applications... A Hydrous
aluminum silicate, air purified to concentrate the finest
montmorillonite fraction from the bentonite ore. Contains
traces of feldspar, quartz, calcite, and gypsum."
http://www.colloid.com/AGP/Tech/Volclay%20HPM-20.pdf
We wanted to add the above paragraphs due to the
common misconception that, somehow, bentonites and montmorillonites
are two different types of mineral classes. In other words,
we've talked to people who didn't want bentonite, but instead
montmorillonite, and those who have rejected montmorillonite
because it wasn't labelled as bentonite.
Bentonites ( and montmorillonites ) are further classified
by their dominant anion ( the element in the clay that
has the highest potential for ion exchange ).
Some sources state that there are only two primary types
( sodium and calcium ) of bentonite. However, magnesium
bentonite ( http://www.gsaresources.com/bentonite.htm ),
calcium
magnesium bentonite, and potassium bentonite
( http://vulcan.wr.usgs.gov/LivingWith/VolcanicPast/Places/volcanic_past_indiana.html
), while less common, are unique forms of bentonite, just
like the calcium and sodium bentonites.
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