Geoscience Reference
In-Depth Information
(0.001
m) or even less. These techniques supplemented by chemical and thermal
analyses provide quantitative information about the structure of a crystal lattice
(Fig.
5.8
).
As mentioned above, the crystal lattice of montmorillonites was described fi rst.
Only later on was the whole family of minerals similar to montmorillonite discov-
ered. They were named smectites derived from the Greek
smectos
which means
blurred or painted, because their image in electron microscopy lacked sharp con-
tours and because the clay was smoothly and easily spread between the fi ngers.
Smectite
crystal lattices consist of three sheets: tetrahedral-octahedral-tetrahedral,
or briefl y T-O-T confi guration. Let us call this formation a triple layer. We can say
that the octahedral sheet is closed from the top as well as from the bottom side by a
tetrahedral sheet. Because of this type of imprisonment, the octahedral sheet has no
chance to demonstrate the majority of its own characteristic features. At fi rst it looks
as if we merely added a tetrahedral sheet to the top corners of the octahedrals of
kaolinite. However, it is not so simple as it initially appears since there is a signifi -
cant change “in the guts” of smectites, and especially inside montmorillonite. When
we go back to slats in our oversimplifi ed model of kaolinite to describe smectites,
we have one black slat to which the white slacks are nailed, one at the bottom, one
on the top. And although this is the form of the triple layer, there is no glue on the
outer side of triple layer. With the glue being absent, the distance between neighbor-
ing triple layers in smectites is bigger than the distance between double layers of
kaolinite. Glue represents hydrogen bonds in our visual aid of kaolinites. In smec-
tites with no hydrogen bonds between its triple layers, water molecules as well as
ions of solutions can enter the space between triple layers that can mutually move
and slip a little bit. The more water we add to smectite clay, the more the distance
between its triple layers grows, and as a result, we observe a swelling of the clay.
When the clay is slowly drying, water molecules disappear from the space between
triple layers and the sheet of water molecules becomes thinner. We observe the
change as the clay shrinks. Both processes result in minor slipping of the triple lay-
ers, one on the top of the lower one. Hence, the image of smectites and especially
that of montmorillonites in an electron microscope looks hazy without distinct
boundaries. We have to explain now why it is so and why and how smectites differ
so much from kaolinites.
In octahedral sheets of smectites, the three-valent aluminum is in some instances
substituted permanently by the two-valent cation Fe
2+
or Mg
2+
. With their sizes also
not being the same, the confi guration is rendered into a stage of slight instability that
allows the unbalanced negative charge of the sheet to appear on the surface of the
triple layer T-O-T. In a similar way the Si
4+
in the center of some tetrahedral sheets
is substituted by three-valent cations like Al
3+
and less frequently by other lower
valent cations. Both types of permanent substitution result in the appearance of the
negative charge on the outside plane of each triple layer. The result is not only the
absence of hydrogen bonds or their analogy but also an opposite effect. When one
plane of a triple layer T-O-T with an excess of negative charge faces another plane
of T-O-T confi guration with an excess of negative charge, the two triple layers are
μ
