Geoscience Reference
In-Depth Information
positions were fi lled by K
+
, the crystal lattice would be close to mica, i.e., to mus-
covite and biotite. Compared to mica, all of these positions in illite are not fi lled by
K
+
- some positions are free and available to exchangeable cations from the outside
solution of mineral salts. The specifi c surface accessible to water molecules and the
value of CEC of illite are both smaller than they are in smectites. In illite, although
cations K
+
form an inseparable part of the crystal lattice, they do not occupy all
potentially fi tting positions between triple layers as they do in mica. Clay minerals
in the illite group differ from mica in still another manner. There are more frequent
substitutions of central Si
4+
by Al
3+
in their tetrahedral sheet as well as more fre-
quent substitutions of Al
3+
by Fe
2+
in their octahedral sheets. Therefore, the T-O-T
confi gurations contain an internal unbalanced negative charge that is balanced by
exchangeable cations residing on the external sides of the tetrahedrons. As a result,
illites have a medium value of CEC. As a permanent part of the crystal lattice, some
K
+
cations serve as an obstacle against water molecules entering the entire interlayer
space. They must enter only at places where K
+
is missing. Thus, these K
+
cations
curtail and restrict entrances of water molecules and exchangeable cations from the
outside solution. But this behavior caused by the mutual bonding of triple layers is
not always realized. Whenever K
+
does not occupy these potential positions as part
of the lattice, the physical behavior of illites begins to approach that of smectites
(Fig.
5.9
).
It is of interest to also know that the K
+
ion in the lattice of some illites is occa-
sionally substituted by Mg
2+
. Owing to the difference in size of these cations, the
crystal lattice is suffi ciently altered to make it easier for both water molecules and
exchangeable cations to enter into the triple interlayer space. Although illites
slightly swell and shrink, such processes are rather restricted. Their internal specifi c
surface is below 50 % of their total specifi c surface that ranges from about 40 to
90 m
2
/g with CEC values being roughly 10-40 meq/100 g.
When we demonstrate the basic physical characteristics using our slats model,
we deal again with a black slat representing the octahedral sheet to which are nailed
on both of its sides white slats representing tetrahedral sheets. Up to this compari-
son the model is identical to that of smectites. But now a principle difference occurs.
A small ring representing cation K
+
nails both triple layers together on some posi-
tions and balances at least partly the negative charge of each white slat.
The origin of many clay minerals is demonstrated in the simplest way when the
weathering of mica is studied. The crystal lattice of mica, formed by triple layers
T-O-T bound together by K
+
cations occupying all available positions between two
neighboring tetrahedral sheets, is written in an abbreviated form as repetitive triple
layers T-O-T-K
+
-T-O-T-K
+
-T-O-T-K
+
and so on, where the symbol K
+
represents
the cation linking the neighboring triple layers T-O-T together. One of the important
steps in weathering is the extraction of some K
+
cations from their fi xed positions in
the interlayers between T-O-T confi gurations. In this way illites are formed. This
extraction is accompanied sometimes with the substitution of the central cation in
the tetrahedrals or octahedrals. The more intensive the weathering, the more K
+
are
pulled out until all K
+
disappear from their originally fi xed positions in the inter-
layer. In this way smectites are formed. When very strong weathering lasts for a
