Geoscience Reference
In-Depth Information
kaolinite is found within the weathering rock, which excludes additions
from without. This clay mineral must, therefore, be the product of
weathering of the feldspar. Older geologists had known this and talked of
'kaolinization of feldspar'. As feldspar crystals have a three-dimensional
structure and kaolinite is a layer mineral, the transformation represents
a complete reorganization at the mineralogical level. This is
neoformation
.
It takes place through the solubilization of the ions composing the donor
mineral. But one wonders what if bonds between certain ions do not
survive in the 'soup of ions in solution', that is, microstructures that will
constitute a primer for the crystallization to occur.
Lastly, some minerals are purely and simply dissolved in water,
which carries their ions out of the soil. Such is the case with say, calcium
carbonate.
In fact, the changes that minerals undergo are often more complex
than those outlined above. For example, a neoformed mineral can
incorporate ions coming from the dissolution of many others and,
hence, may have a multiple origin. Also, limited transformations and
neoformations are often associated with loss of substances through
export.
It has been demon st rated t hat m i nerals are more suscept ible to weat heri ng
when the O/Si ratio (only the tetrahedral sheets are considered for the
clay minerals) is wider (Chadwick and Chorover 2001). This enables
construction of the table below (Table 2.2) of minerals in order of
decreasing resistance to weathering.
Susceptibility to weathering
Table 2.2
Scale of decreasing resistance of minerals to weathering (Chadwick
and Chorover 2001, slightly modified).
Mineral
Formula
O/Si
Kaolinite
[Si
2
]Al
2
O
5
(OH)
4
1.5
Smectite
[Si
4
](Al,Fe,Mg)
2
O
10
(OH)
2
1.5-2
Quartz
SiO
2
2
Muscovite
K[AlSi
3
]Al
2
O
10
(OH)
2
2
Microcline
KAlSi
3
O
8
2.7
Albite
NaAlSi
3
O
8
2.7
Enstatite
Mg
2
Si
2
O
6
3
Diopside
(CaMg)
2
Si
2
O
6
3
Forsterite
Mg
2
SiO
4
4
Anorthite
CaAl
2
Si
2
O
8
4
