Geoscience Reference
In-Depth Information
cited by Arnalds (2004), but there are more complicated formulae
involving
Si
o
,
Al
o
and
Al
p
together.
The specificity and efficacy of the reagents are not fully established
(Kleber
et al
. 2004). In the soil, there is a sort of continuum between
gels and well-crystallized minerals with minerals of short-range order
in between. So research is continuing on new methods of study (García-
Rodeja
et al
. 2004).
By using the reagents mentioned above, it has been shown that the
Andosol is a soil in which a high proportion (~20%) of the aluminium
is in the amorphous form.
10.3
GENESIS OF ANDOSOLS
10.3.1
Andosolization and De-Andosolization
Volcanic materials are easily decomposed. They abruptly yield large
amounts of Si, Al and Fe directly if they are glassy, or after hydrolysis
of minerals that they usually contain, for example (FAO 2006):
CaFeSi
2
O
6
(
augite
) + 2
H
2
O
Æ
Ca
2+
+
Fe
2+
+ 2
SiO
2
+ 4
OH
-
KAlSi
3
O
8
(
microcline
)
+ 2
H
2
O
Æ
K
+
+
Al
3+
+ 3
SiO
2
+ 4
OH
-
Modern methods of investigation have shown that the aluminium
later passes to a coordination state of VI and interacts with oxygen to
give a layer similar to that of gibbsite. Then it associates with silica
tetrahedra (SiO
4
). This gives allophane (Hiradate and Wada 2005).
This is inverse evolution compared to progressive desilication (Chap.
3, Fig. 3.9), in which 1/1-type clay minerals lose their tetrahedra to
give gibbsite. But this evolution toward clay minerals is only initiated
in Andosols. Actually, these soils correspond to cold, humid climates,
where organic matter is abundant and determines the formation of
organo-mineral complexes. In other words, the mineral ions are blocked
by carbon compounds and cannot coprecipitate to form conventional
clay minerals (Siefferman 1969). We have seen earlier that, in allophane,
the tetrahedral sheets are incomplete and the octahedral sheets (circular)
are limited to a small number of aluminium atoms. At the same time,
organic matter is protected by amorphous materials and is accumulated,
whence the black colour of the soil. In Andosols, evolution is thus
facilitated at its start (weathering of rocks) but is slowed down later
(no formation of true clay minerals). The precursors of clay minerals,
Generalities
