Geoscience Reference
In-Depth Information
season, annual rainfall greater than 1000 mm, which may reach
5 m or more; good drainage is essential.
Andosols are particularly abundant in the Pacific Ring of Fire, in
central Indonesia, and in East Africa. They also occur in the volcanic
islands emerged from the sea: the Azores, Iceland, Hawaii, etc. They can
be seen up to an altitude of 4000 m, especially in the Andes (P. Quantin,
pers. comm.).
10.2
THE 'AMORPHOUS MATERIALS' IN ANDOSOLS
10.2.1
The Phases of Matter
Traditionally three phases of mineral matter are recognized:
Crystalline phase
. Concepts of molecule, mole or atomic mass have
no significance for crystalline minerals. Their elementary struc-
ture,
the lattice
, is infinitely repeated and there is theoretically no
limit to the possible macroscopic size for a crystal with perfect
structure. The largest gypsum crystals in the world, seen in the
Naica caves in Mexico, are about 12 m long and thus contain
billions of perfectly arranged atoms.
Noncrystalline material
corresponds to substances not having defi-
nite crystalline arrangement: from one point to another, there
is variation in interatomic distance, coordination and geometry.
These substances have an approximate chemical composition
and can be more or less rich in such or such component. They
do not diffract X-rays, which therefore cannot be used to study
them. They are often rich in water. Glass is a sort of amorphous
phase but is solid.
Poorly crystalline materials
or
minerals with short-range order
con-
stitute a phase intermediate between the two described above.
The mineralogical structure is regular over very short distances;
beyond that atoms are placed more or less in disorder. Only
instruments designed for very minute study at atomic scale can
be used to characterize such substances.
Allophane
and
imogolite
,
present in Andosols, fall in this category.
Even in glasses, there is no absolute disorder; bonds between atoms
do exist. For example, silica tetrahedra are formed and are linked more
or less regularly through their apices in the compounds based on SiO
2
.
Figure 10.2 presents the principal types. The tetrahedra are viewed in
plan and are represented by small, black equilateral triangles.
Classification
