Geoscience Reference
In-Depth Information
delayed the emergence of
pedology
, because it was thought that the soil
could be properly comprehended through geological study alone.
But, before proceeding from here to rock-soil interactions, it is
necessary to recall some aspects of the structure of clay minerals.
2.2.2 Review of the Structure of Phyllitic Minerals
The clay minerals are formed from three or even four principal
planes of atoms, and their characteristic stacking is repeated several
times (Millot 1964).
Let us consider kaolinite, which comprises three planes of atoms. The
first two are made up of linked atoms forming what conventionally are
called
hexagonal close-packed
assemblages. The two planes together form
the
octahedral sheet
typical of clay minerals. It is denoted by 'O' (Fig. 2.2).
In the diagram, the first plane of atoms is grey; the atoms in the plane
above it are represented by dotted circles.
Filled octahedral cavity
Vacant tetrahedral cavity
Fig. 2.2
Two superposed planes of atoms in hexagonal close-packed arrangement. Togeth-
er they give one octahedral sheet characteristic of clay minerals.
These two planes of atoms are composed of oxygens O and hydroxyls
OH. It is seen that the assemblage of the two planes gives two types of
cavities:
tetrahedral cavities
, which are formed between four atoms (three
below and one above or vice versa) and
octahedral cavities
that are within
six atoms (three at the base and three above). The terms tetrahedral and
octahedral are justified by the fact that the solid bodies having four and
six apices respectively are tetrahedra and octahedra.
In the octahedral sheet of the clay minerals, there are twice as
many tetrahedra as octahedra! But only the octahedral cavities are two-
thirds occupied (sheet called gibbsitic) or all occupied (brucitic sheet)
by various atoms (in particular Al, Fe or Mg).
