Red Soils of the Mediterranean and Dry Tropical Zones - Major Soil Groups of the World - page 273

silicates, goethite grains…) and its transformation to haematite, which
then becomes dominant (Fig. 9.7). When the latter is present in the
rock, it evidently is preserved. All of this involves a dry climate and
an aerated soil. We should keep in mind that brunification corresponds
to extraction of iron and its transformation to goethite, in a humid
environment relatively rich in organic matter (Chap. 8).

Ferriferous mineral of the rock
+ aeolian additions

Individualized
concretions

Soil solution
Fe ++

HAEMATITE
in the soil

Oxidation
drying

Reduction
solubilization

Migrations

Oxidation

Hydration of
solids

Loss of
Fe in
solution

GOETHITE
in the soil

FERRIHYDRITES
in the soil

++

Drying/rubefaction

Fig. 9.7 Mechanisms of rubefaction (fi lled heavy arrows, bold text) and of derubefaction
(dashed arrows, italics); solutes drawn as circles and solid phases as rectangles. From
Michalet (1991); Trolard (1995); Singer et al. (1998); Schwertmann et al . (1999); Trolard and
Bourrié (2004); Chen et al . (2010).

Ferrihydrites and the closely related feroxyhytes are oxyhydroxides of
the approximate formula FeOOH or Fe(OH) 3 , often actually quite variable
because of poor crystallinity, in which the iron is already in the trivalent
form as the names well suggest. These compounds are precursors to
haematite or goethite (see diagram). The transition of ferrihydrite to
haematite corresponds to a desiccation causing dehydroxylation. It does
not occur in total absence of water (Schwertmann et al. 1999). Thus there
is no rubefaction in a totally dry desert environment and many dunes
in the Sahara are yellow…

Throughout one part of pedological development, the clay-haematite
association maintains good structure, good aeration and penetration
of oxygen. But acidification, linked to leaching of bases, gradually
intervenes. The consequence is lowering of the pH. At the geochemical
level, the soil in question comes closer to the Fe 2+ /Fe 3+ line in the Eh/pH
diagrams (Chap. 12, §12.1.3, Fig. 12.1). In other words, the reduction of

Derubefaction (yellowing, xanthization)