Geoscience Reference
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transformation. But some authors consider this scheme a simplification
(Van Rompaey
et al
. 2007).
In cold temperate regions, the evolution of layer minerals in
podzolizing environment is illustrated in Fig. 11.7.
Chlorites
(mostly
trioctahedral)
HIS and HIV in
closed system (B)
Cl/V
Partial
amorphi-
zation:
imogolite,
allophane
Vermiculite
Micas
(mostly
trioctahedral)
M/
V
V/S
Transformed
smectite
(E horizons)
M/S (E horizons)
Solubilization with or without complexation
Fig. 11.7
Evolution of layer minerals in acid and complexing environment (Robert 1970;
Robin
et al.
1981; Egli
et al
. 2003a). The boxes drawn with dashed lines represent the inter-
mediate stages, that is, the various kinds of interstratifi ed minerals. HIS =
hydroxy- interlay-
ered smectite
and HIV =
hydroxy-interlayered vermiculite.
The transformations are accompanied, at all stages, by losses in
solution. Thus the entire sequence is not necessarily gone through and
all the micas and chlorites can disappear before the smectite stage
is reached. In particular, below pH 3, there is direct and preferential
dissolution of the micas. In other words, smectites are rare or absent
in podzolic soils. Their occurrence in noticeable quantities in the clay
fraction (> 20%) will be indication of maturity of the profile and will
require many thousands of years (Egli
et al
. 2003a).
Recapitulation of definitions
Illite
is hardly different from muscovite mica, a dioctahedral mineral.
Illite has less K but corresponds to substitution of Al by Fe or Mg in
the octahedral sheet. Both minerals are very stable.
Ver m ic u l ite
is not much different from biotite, a trioctahedral.
Vermiculite is slightly hydrated. It has more substitution in the
tetrahedral sheets. Iron is partly lost from the octahedra and is
replaced by Mg. These two minerals are not very stable.
Muscovite mica is resistant and sometimes survives at the top of the
profiles, even in the tropical zone.
