Geoscience Reference
In-Depth Information
consequence, the oxides that are left in the undissolved residue
have an Al content that increases with successive extractions
(Peterschmitt et al . 1996; Muggler et al . 2001).
￿ Haematite is responsible for the red colour. When it disappears,
the sample becomes yellow (goethite) or even white when
there are no further iron oxides. Generally, the soils of dry
environments are red; those of humid regions are yellow.
￿ Besides the well-crystallized minerals, there is a poorly
crystallized fraction that is extractable by ammonium oxalate
and is conventionally denoted by Fe o . But it is not abundant in
the red soils and pertains more to Luvisols. In the latter soils it
plays a key role in binding with clay and therefore in structural
stability (Duiker et al . 2003).
Figure 9.6 summarizes the situation, if we add that the total iron can
also be extracted by acids ( Fe t ).
During soil development
the circumference of the
circles is modified
Interpretation
Total iron
Oxalate iron
Iron of silicates
Well-crystallized
oxides
Amorphous or poorly
crystallized iron
Dithionite iron
Analyses
Acid-
extractable iron
'Free' iron
Fig. 9.6 The iron in soils: analysis and interpretation.
The iron of silicates is estimated by Fe t - Fe d . Some authors (Bech
et al . 1997) are of the opinion that the degree of development of the soil
is given by the ( Fe d - Fe o )/ Fe t ratio. This, in relative terms, is the iron
that has been extracted from the rock and precipitated in the form of
oxides/hydroxides. It must be high in the red soils (> 0.5) and lower in
brown soils (Souri et al . 2006).
But the solubilization phenomena presented were studied in the
laboratory. In nature, the iron oxides could be coated by mono- or
bimolecular layers of organic compounds that, by changing the surface
properties and as a function of pH, speed up or limit dissolution (Eick et
al . 1999). Thus we are far from having examined the question thoroughly,
and investigations are continuing.
To summarize, in the current state of knowledge, rubefaction can
be viewed as extraction of the iron contained in the rock (ferriferous
 
Search WWH ::




Custom Search