Andosols - Major Soil Groups of the World - page 295

There are two stages in the crystallization of mineral substances:
￿ Germination , also called nucleation ,
￿ Crystal growth .
In a dilute medium, for example in a jar with a substance in solution,
seeds rarely appear; they mark the prior joining of some ions. Crystals
will thus be few but develop without difficulty. But in a soil, impurities
capable of initiating crystallization are infinite in number. In these
conditions, germination takes place all over. Crystal growth can produce
only small overlapping crystals that interfere with one another. In short,
the soil is a medium favourable only for structures with short-range
order.
With time and under the influence of alternate phases of wetting and
drying, the amorphous materials (in the broad sense) are transformed
to well-crystallized species. This is Ostwald ripening.

Crystallization

10.2.2 The Minerals of Andosols

Let us now examine crystallization with time by concentration and
desiccation of silica and oxides of iron and aluminium. We shall also
study the mixed pathway involving silica and aluminium together.
Figure 10.3 gives an outline of the system.

Classification

SILICA MIXED PATH ALUMINIUM IRON

Gel Allophane Amorphous Amorphous

Opal Imogolite Gibbsite Al(OH) 3 Ferrihydrite

Quartz
(SiO 2

Halloysite Boehmite Al-O-OH Goethite Fe-O-OH

Metahalloysite Corundum Al 23 Haematite Fe 23

Kaolinite

Fig.10.3 Stages of crystallization and evolution of different compounds containing Si, Al
and Fe. The substances considered as non-crystalline or poorly crystalline are enclosed by
a dashed line.