Geoscience Reference
In-Depth Information
The principal minerals presented form a broad, more or less continuous
assemblage. Let us start from the structure of allophane and postulate,
without prejudging the mechanisms:
if the tube is elongated, it is becoming imogolite,
if it is folded longitudinally and finally opens up completely to
give a plane body, it is becoming a true clay mineral,
if water is expelled from it, it is halloysite becoming kaolinite,
if iron replaces aluminium, it is going from an aluminous clay
mineral of kaolinite type to a ferriferous clay mineral of hisinger-
ite type,
if the proportion of silica is variable, that is, if tetrahedra are
added or lost, it is alternating between 1/1 clay minerals and
2/1 clay minerals.
Evolutions thus depend on the composition of the solutions in relation
to the nature of the materials, climate, and the degree of pedological
development. All this is less complicated than it seems to be. Our
categorizations, being a bit arbitrary, do not make things clear.
Synthesis
10.2.3 Attempts at Laboratory Synthesis
Attempts at synthesis of imogolite and allophane have been successful
in the laboratory (V.C. Farmer 1979, quoted by Herbillon 1983; Abidin
et
al
. 2007). They enable us to emphasize various important results:
It is possible to synthesize these components at room tempera-
ture, while it is impossible under the same conditions to obtain
true clay minerals within the duration inside the length of a
laboratory experiment.
The synthesis of allophane is better done in neutral medium in
presence of basic cations (Ca, Mg) and that of imogolite in acid
medium (pH ~ 4.5). In nature, when allophanes and imogolite
coexist, the latter occupies sites more subject to rapid circulation
of water with very little solute and only imogolite is present in
noticeable quantity in podzolic soils (Chap. 11).
10.2.4 Chemical Methods of Study of Amorphous
Materials
Various kinds of extractants are used, singly or in combination,
chiefly:
