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Silica tetrahedra
Regular
nucleus
Crystalline structure
of quartz (cristobalite)
Paracrystalline structure
of a silica gel
Silica glass (amorphous
solid phase)
Fig. 10.2
Structure of compounds based on silica (Pédro 1983).
A misuse of language, related to insufficient knowledge for long, has
passed into current language: the totality of truly amorphous substances
and paracrystalline substances in soils is incorrectly termed 'amorphous'.
It is true that there are common features:
1. Small size of 5-500 Å (Å = angström = 10 -4 µ m), thus not neces-
sarily greater than the thickness of a clay-mineral layer.
2. High reactivity : in keeping with the great external surface area,
up to 1000 m 2 g -1 (against 10-100 m 2 g -1 for a clay particle) and
presence of broken bonds at the edges of the nucleus, which
favour capture or exchange of ions; instability of bonds.
3. Protective effect of organic matter with which they are firmly
associated to form organo-mineral complexes. The organic matter
thus bound is very stable, which partly explains its accumulation.
In an experiment, after 40 days of incubation, the mineralization
of carbon had not reached even 5 parts per thousand (Aran et
al. 2001). The protective effect seems to be related to several
parameters: quantity of metallic elements (Al, Fe) present, their
relative proportion (Al/Fe), type of organic anion involved
[humic acids versus fulvic acids that are more susceptible to
decomposition (Bel Hadj Brahim 1987)].
In addition to the strength of the bonds with mineral material and
the direct toxicity of Al and Fe in the biomass, the protection of the
organic matter may have yet other causes (Shoji et al . 1996): deficiency in
phosphorus for the microbes because it is fixed in the soil, disturbance
of the normal functioning of microorganisms by strong adsorption
of enzymes on mineral substrates, fineness of the micropores locally
preventing access to bacteria (Kilbertus 1980).
General properties of 'amorphous materials'
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