Geoscience Reference
In-Depth Information
Table 5.2
Common oxides and hydroxides of iron and aluminium in tropical
soils.
Fe-O-OH
Æ
Fe
2
O
3
Goethite
(yellow-ochre)
Haematite
(dark red)
[-10.4; -5.83]
[-10.8; -6.18]
Al(OH)
3
Al-O-OH
Al
2
O
3
Gibbsite
(white)
Æ
Boehmite
(white)
Æ
Corundum
[-6.46; -5.81] [-6.53; -5.88]
The numerals represent the solubility, expressed as logarithm of the molar concentrations, in oxidizing
medium (first value) and in reducing medium (second value).
Goethite and gibbsite are seen in the most humid equatorial
environments. In tropical regions with a dry season, they tend to be
replaced by haematite and boehmite, respectively. In the subtropical
region of China, haematite is found at altitudes lower than 600 m and
goethite above this (Felix-Henningen
et al
. 1989).
Aluminium often partly substitutes for iron in the corresponding
oxides. Thus Al-goethite or Al-haematite are obtained. In detail, the
forms of haematite and goethite are many according to the level of
substitution of Al for Fe and according to the crystallinity [
mean coherence
length
or
MCL
(Muggler
et al
. 2001)].
Structure
The structure of the oxides corresponds to superposed sheets of contiguous
O
2-
ions and that of hydroxides to sheets of O
2-
ions superposed on
sheets of OH
-
ions. In both cases, the arrangement defines
hexagonal
closed-packed
assemblages. Between the successive sheets of anions, two-
thirds of the cavities are occupied by Fe
+++
or Al
+++
ions. For further
information, see § 12.2 in Chapter 12.
5.2 TYPICAL PROFILE AND DIFFERENTIATION
5.2.1 Typical Profile of Cuirassed Soil (Plinthic Ferralsol)
Plinthic Ferralsols will be minutely described below. The other types
of tropical soils will then be considered by highlighting their special
features.
The profile studied (Fig. 5.1) is the most complete of those present
in the intertropical zone. It is seen in its entirety only in quarries in the
zone protected from erosion. The depth of the profile is 20 m or greater;
horizon thickness is variable and is given only as an indication.
According to Bourman and Ollier (2002), this is the 'Walther profile',
named after the first author to have given in 1915 a complete description
