Geoscience Reference
In-Depth Information
The reality is more complex because many intermediate stages occur,
in addition to aluminium silicates. For example:
siliceous ferrihydrites composed of Fe, Si, O and OH,
hisingerite, which is a clay mineral based on silica and iron, with
a molecular Fe
2
O
3
/SiO
2
ratio of ½, or Fe/Si = 1,
aluminous opals with Al/(Si + Al) between 0 and 0.1.
The substances called amorphous are poorly crystalline and
irregular; their structures and compositions are variable. What follows
is not perfectly and finally established. Also, the information given by
mineralogists is related to pure species and does not always correspond
to what is found in soils.
In Andosols we see opals and aluminous opals, which are more or less
dehydrated and organized polymers of silica. They form flat discs of
diameter 0.5 to 5
µ
m. In aluminous opals, an Al locally replaces a Si.
These minerals correspond to environments still rich in silica, which
means pedogenesis of short duration at least in humid climate (a few
thousand years at the very most). But living organisms can recycle silica,
even in Andosols naturally depleted of silica. Diatoms are sometimes
present, also disc-shaped, but with characteristic surface designs.
Phytoliths can also exist, produced by plants. In all these compounds,
silica exists as tetrahedra, as described above.
Compounds of silica
Imogolite was discovered in 1962 by N. Yoshinaga in the weathering
products of an ash bed locally named 'Imogo' (P. Quantin, pers.
comm.).
Everyone of us has seen those broad dog-collars spiked with diamond-
shaped points so that the animal cannot be seized by the throat. Let us
now mentally reverse such a collar so that the pyramids point inwards.
Then we have the structure of imogolite. But we must imagine a very
broad collar because, in this mineral (Wilson
et al.
2001), the tube is a
few
µ
m long, which is considerable compared to its diameter of 3 to
5 nm. This is a nanotube, thus imogolite is fibrous. It is composed of
an octahedral sheet of gibbsite type (the leather of the collar) with a
plane of OH on the outside and one of oxygens on the inside. Silica
tetrahedra (the points of the collar) get fixed to the oxygen plane. Three
oxygen atoms of each tetrahedron are shared with the octahedral sheet.
Imogolite and proto-imogolite
