Geoscience Reference
In-Depth Information
Table 12.6 Ionic radii in nm.
Fe 3+
Fe 2+
Ca 2+
According to C.E. Marshall
0.067
0.083
0.106
According to Pauling
0.064
0.076
0.099
On its part, ferrous iron is liable to move, which is why deferrification
zones appear in the soil. Complete bleaching, though localized, is a very
typical feature of excess water (Evans and Franzmeier 1988; Tassinari
et al . 2002). Colours with hue (Munsell Color Chart) of N, 10Y, 5GY, 5G,
10G, 5BG, 10BG, 5B, 10B and 5GB result. But this is true only for acid
or neutral soils. Calcareous soils, unless they are very strongly reduced,
are in the domain of stable ferric forms. Then acidification is needed to
start derubefaction (Chap. 9).
Migration of ferrous iron
Ferrolysis is described as an alternation of oxidation and reduction of
iron interacting with clay (Brinkman 1970, 1977). In a period of saturation
with water and, consequently, reducing conditions, iron passes to the
ferrous form and replaces the cations retained on the clay, including
Al 3+ . Vizier (1987) showed that the cation exchange capacity could be
saturated to 94 per cent by Fe 2+ . In the dry season, iron is oxidized,
resulting in production of hydrogen ions, perhaps according to the
reaction:
Fe 2+ + 2 H 2 O Æ FeOOH + 3 H + + e -
The protons will attack the octahedral sheet of the mineral, which
will be decomposed and will release its aluminium. Many explanations
are given in the literature regarding the mechanism: by involvement of
Al at the edge of the sheet or caused by the overall electrical charge of
the layer.
Aluminium will partly be evacuated and partly be held in the
interlayer position (transformed clay minerals of aluminous chlorite
type progressively replacing smectites).
Ferrolysis
As Fe 3+ is not soluble, it means that bacteria attack the oxides, that is
the solid phase, for reducing the iron. These microorganisms seem to be
able even to utilize the structural iron of smectites, perhaps not directly
but by means of the organic compounds they secrete. But we do not
know whether dissolution of the clay is provoked to enable recovery of
Biological attack of clays
 
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