Geoscience Reference
In-Depth Information
In industrial terms, an aluminous layer deserves the name bauxite
(Meyer 1987) only if it is poor in iron and contains more than 40 per
cent alumina in the form of gibbsite [Al(OH) 3 ] or boehmite (AlOOH).
Smectitic 'Ferrallitic' soil
The scheme of progressive desilication (Fig. 3.9, Chap. 3) implies that
we find profiles containing, from the bottom to the top, successive
accumulations of smectites, kaolinite and, finally, gibbsite. This
succession, predicted from geochemical considerations, is also obtained
by modelling (Fritz 1975). But it is rarely observed in nature. It pertains
to relatively dry environments, as in Swaziland for example (Scholten et
al. 1997). It also occurs in the Sudano-sahelian zone, in Burkina-Faso, on
basic rocks (Zeeger and Leprun 1979). In Australia, again on basic rocks,
one sees, from the bottom up: saprolite, smectite (up to 15-m thick) and,
lastly, iron cuirass. There is no horizon dominated by kaolinite (Gaudin
et al. 2004).
In the intertropical environment, the organic matter mineralizes rapidly
because of high moisture content, temperature and biological activity. The
soils are, therefore, scarcely humiferous. But there are two exceptions:
Humiferous tropical soil
￿ on the one hand, the wet depressions where peats can be found,
as we have seen;
￿ on the other hand, the mountains of Africa and India, more than
2000 m in altitude. We can find there a dark organic horizon that
can be 40-cm deep. It contains up to 25 per cent organic matter
(I. Musoni, pers. comm.). Gibbsite diminishes in quantity toward
the top of the profiles, where it is found less crystalline. We
can, therefore, presume that it is being destroyed (Caner 2004).
Aluminium, thus liberated, seems to be the source of the large
quantities of Al-humus complexes found in these soils. Here we
are at the extreme limit of ferrallitic evolution when even gibbsite
disappears; the soil is then close to the non-allophanic Andosols
(Chap. 10).
In the WRB, numerous prefix qualifiers and suffix qualifiers are
applicable to Ferralsols. The principal prefixes are Gibbsic, Plinthic and
Umbric (= humiferous). The major suffixes are Xanthic (yellow) and
Rhodic (red). Also, these soils, like many others, can as well be Eutric ,
Gleyic , etc.
Classification of the Ferralsols
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