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and 8. In these conditions, different possibilities can be distinguished
according to variation in climate/parent material pairs:
￿ On massive limestone . Decarbonatation leaves a small ferriferous
residue that is later concentrated and rubefied. This phase of
decarbonatation therefore involves a minimal amount of rainfall
to remove the calcium. When the annual rainfall drops below 300
mm, in southern Maghreb for example, the soils are no longer
decarbonated and argillization is very subdued (Boulaine 1978).
The soils then remain brown-coloured. Conversely, an excess of
rain may well flood the profile and provoke reduction of iron.
In sum, red soils are seen in climates with 400-1400 mm annual
rainfall (Bottner 1982), or 1700 mm (Jouaffre et al . 1991) or even
2000 mm (Lamouroux 1983). According to A. Ruellan (Pedology
course, 1984, mimeo), there is no upper limit to rainfall if the
environment stays well drained.
￿ On schist , the altérite is rich in iron and inherited clay minerals
(transformation of micas). It can directly rubefy. But it is
necessary that the small quantity of bases present be retained
to avoid acidification and fall in pH. In fact, this would facilitate
reduction of iron at constant partial pressure of oxygen (Chap. 12,
Fig. 12.1). This is why the red soils on schist occur in dry regions:
Mediterranean (Spain, Greece, Corsica, etc) but not Atlantic. They
are also found in the Albères massif, near Perpignan in the
extreme south of France, but only on southern exposures.
￿ On sandstone . On the northern margin of the Sahara, red soils on
sandstone appear when the annual rainfall exceeds 100 or 150
mm.
￿ On basic igneous rocks (basalt flows, for example), rubefaction is
facilitated by the wealth of bases and by maintenance of a suit-
able pH.
￿ On gravel deposits , as in the Rhône valley for example, decarbo-
natation precedes rubefaction, but this does not prove that the
phenomena could not coexist. In fact, because of their lower resis-
tance, limestone gravels are solubilized even before the igneous
rocks have barely started to weather.
￿ On marls (CaCO 3 + clay). Rubefaction is often absent for lack of
sufficient drainage. But on certain facies with palygorskite, the
porosity is high, approaching 40 per cent (Durnerin et al . 1980).
Also, this fibrous mineral is labile and easily decomposes to give
stabler clay minerals. The medium thus remains permeable and
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