Geoscience Reference
In-Depth Information
It represents the dominant form of iron in Andosols. The mineral is
reddish brown, rhombohedral and not very stable.
There are siliceous ferrihydrites with controversial mineralogical
structure. The presence of silica (Si-O bonds found) seems to slow down
the transformation to very well crystallized forms of goethite (Herbillon
1983).
In the last 25 years, all sorts of different minerals containing iron have
been grouped under the term hisingerite: poorly crystallized smectites
close to saponites (trioctahedral) or nontronites (dioctahedral) and also
mixed oxides of Fe and Si organized into hollow spheres (reviewed by
Robert and Herbillon 1990).
For mineralogists studying pure species, hisingerite, named after the
Swedish mineralogist Wilhelm Hisinger, is a clay mineral of formula
Fe
2
Si
2
O
5
(OH)
4
.2H
2
O. Kaolinite corresponds to Al
2
Si
2
O
5
(OH)
4
. Thus
hisingerite is a clay mineral similar to kaolinite, but it is hydrated and
has iron replacing aluminium in the structure.
Hisingerite
Gibbsite is a colourless mineral of formula Al(OH)
3
and characterizes
many Ferralsols (Chap. 5). It may occur in Andosols, often in small
quantity.
Gibbsite (recap.)
Goethite, mineral of formula Fe-O-OH or Fe
2
O
3
.H
2
O is also discussed
in Chapter 5 (§ 5.1.2) and Chapter 12 (§ 12.2).
Goethite (recap.)
Before reaching the surface, magmatic materials pass through all sorts
of rocks in volcanic vents. These vents, therefore, eject various minerals
into the atmosphere. It is also necessary to allow for hydrothermal
alteration. All said, one may find in soils: micas, 1/1 or 2/1 clay minerals,
talc, pyrophyllite, quartz, amphiboles and pyroxenes… The 2/1 clay
minerals deserve special mention. They are hardly or not formed at all in
Andosols (Dahlgren
et al
. 2004). If present, they could be destroyed. Also,
the strong desilication that takes place in these humid environments
releases aluminium that may be trapped in interlayer position and give
Al-interlayered 2/1 minerals or, in other words, 2:1:1 clay minerals (van
Ranst
et al
. 2008).
Other minerals
