Chemistry Reference
In-Depth Information
There are hydrolysis reactions: oxygens of the lattice of the silicates
preferentially bind to H
+
and cations are released. If one compares the com-
positions of feldspar, muscovite, illite and kaolinite, there is in this sequence
a progressive leaching of potassium and generally of alkalis:
3KAlSi
3
O
8
+
H
+
+
12H
2
O
KAl
2
AlSi
3
O
10
(OH)
2
+
6Si(OH)
4
+
3K
+
potassium feldspar
illite
3KAlSi
3
O
8
+
2H
+
+
9H
2
O
Al
2
Si
2
O
5
(OH)
4
+
4Si(OH)
4
+
2K
+
potassium feldspar
kaolinite
Such reactions may be written in a different way to highlight the role of
dissolved CO
2
:
CaAl
2
Si
2
O
8
+
2CO
2
(aq.)
+
3H
2
O
Ca
2+
+
2HCO
3
-
+
Al
2
Si
2
O
5
(OH)
4
Anorthite
kaolinite
So feldspars are altered by leaching of the alkalis into illite group sheet
minerals and then by a further leaching of the alkalis of interlayer into kao-
linite. Similarly biotite is transformed into chlorite. The replacement of K
+
ions in interlayer site by Mg
2+
and/or Fe
2+
leads to vermiculites.
At a more advanced stage, the interlayer ions are replaced by ions of
the sheet, and ultimately they are even evacuated. It is thus formed of mixed
layer chlorite - smectite/vermiculite or illite - smectite/vermiculite. An exten-
sive degradation leads to the formation of smectites and even a hydrous
aluminum silicate gel or allophane.
The intensity of this alteration depends, of course, on the climate: tem-
perature and precipitations.
Smectites derive from basic rocks, especially volcanic ash and tuff.
Bentonites are rocks formed of montmorillonite and beidellite. Ver-
miculite comes from biotite/phlogopite in ultramafic or mafic rocks.
Vermiculite from phlogopitites is exploited in the Palabora carbonatite
complex. Plastic and expansion (exfoliation) properties associated with
the hydrated interlayer of smectites and vermiculite are used in indus-
try (drilling muds, ceramic, cement, etc.). This interlayer water may be
replaced by alcohols and other organic compounds in industrial uses of
these minerals.
The clays formed by weathering may be eroded and resedimented with-
out transformation: many more or less pure clay formations are of purely
clastic
origin. When the eroded clay minerals are very degraded (very exten-
sive leaching of the interlayer thorough, open sheets, etc.), these sedimentary
minerals may be modified and transformed into more crystalline materials
using ions encountered in the sedimentary basin, especially Mg
2+
and/or K
+
:
this is the phenomenon of
aggradation recrystallization
.
