Geoscience Reference
In-Depth Information
called '2/1'. The '2/1/1' or '2/2' clay minerals are minerals of '2/1' type
separated by gibbsitic sheets (such as chlorites).
The smectites are expanding 2/1 clay minerals chiefly of the
montmorillonite, beidellite and nontronite types. These minerals are
distinguished amongst themselves by the abundance and nature
of the ions that can replace, partly or totally, the aluminium in the
octahedral sheets and, partially, the silicon in the tetrahedral sheets.
This substitution alters the cation exchange capacity. Non-expanding
2/1 minerals also exist, e.g. illite.
Gibbsite
Pl. 3
Tetrahedral
sheet
Plane 2
Octahedral
sheet
Plane 1
Kaolinite
K
Mica
K
OH
-
O
Si
4+
Al
3+
Smectite
Fig. 2.4
Principal types of clay minerals. The structure of kaolinite is shown in the manner
we have described above with atom planes 1, 2 and 3 and their sheets O and T. Smectites
have two silica tetrahedral sheets, one placed above and the other below the octahedral
sheet. In the micas, the different layers are bound together by potassium atoms that fi nd room
in the hexagonal cavities facing each other.
Furthermore, soils most often contain
interstratified
minerals, clay
minerals in which the successive layers are of different types, regularly
alternating or stacked irregularly. For example, interstratified kaolinite-
smectite.
2.2.3
Intervention of the Rock
With time, the minerals originally contained in rocks will weather in
different ways as indicated in Figure 2.5.
Types of crystallochemical transformations
