Ferralsols and other Soils of the Hot Regions - Major Soil Groups of the World - page 154

Figure 5.2, traced from the original drawings of Eschenbrenner (1988),
shows how these transformations occur.

Quartz is abundant to begin with. A granite contains about 25 per
cent quartz by weight. Counted among the resistant minerals, it tends
to get concentrated because the other minerals disappear first. In such
a context, its partial dissolution can take place unnoticed. In fact,
the process is important in the tropical environment and may easily
represent 50 per cent of the mineral's original mass. There are three
chief ways of demonstrating this: (i) the calculation presented at the
end of § 1.2.4, (ii) study of a thin section, (iii) analysis of drainage water
that, in quartzose regions, contains silica. The solubility of quartz is 3
to 7 mg per litre.

Dissolution of quartz

The concretions are made up of concentric layers. In the wet season, the
Fe ++ ions are extracted from the soil mass (matrix); in the dry season,
they precipitate as Fe 3+ in a layer of oxides and hydroxides deposited on
an initial nucleus. Then the process starts over. Thus, such concretions
cannot form in a climate that is uniformly dry or constantly wet; also an
existing concretion cannot grow unless the soil matrix surrounding it at
a given time withdraws to give place to a new ferruginous layer. Actually
there probably is chemical dissolution of the matrix (i.e. kaolinite) at the
boundary of the concretion. According to some authors, the progressive
desilication by hydrolysis (Chap. 3) results in collapse of the tetrahedral
sheets of kaolinite (Mongelli and Acquafredda 1999). For others, the
iron present in the environment is introduced into the octahedral sheet
of the mineral, which is thus destabilized and destroyed. The silica is
removed. There is a relative concentration of iron and aluminium, that
is, appearance of concentrations of oxides and hydroxyoxides of pure
iron, of aluminous haematite (substitution of up to 20% of the iron by
aluminium) and also of pure aluminium (gibbsite) with maintenance
of a large proportion of residual quartz. The entire process is a form
of epigenesis .
The cuirass gradually builds up. Its mineralogical composition is
variable (Table 5.3).

Formation of ferruginous concretions and then cuirass

Table 5.3 Average composition of ferrallitic cuirasses.

Quartz Kaolinite Haematite Goethite Gibbsite

0-7% 10-30% 10-50% 5-45% 5-12%