an engineering geological characterisation of tropical clays - GBV

65
sericite and chlorite (see next section on scanning electron microscope studies); while those
deficient in the alkali and alkali-earth metals alter into aluminiumsilicates of andalusite,
sillimanite and kyanite.
Theoretical chemical compositions of the minerals kaolinite, illite and montmorillonite are
given in Table 6.5. A comparison of the results in this table with those of chemical analyses in
Table 6.1 suggest the kaolinite in red soils to be depleted in Al2O3. The depletion is most
probably a result of low PH conditions caused by formation of weak carbonic acids in the
soils through organic matter decomposition and/ or hydrothermal alteration at significant
temperatures.
Table 6.5. Theoretical compositions of kaolinite, illite and montmorillonite
(Jasmund & Lagaly, 1993; Moore & Reynolds, 1989; Rösler, 1980).
% content of: Smectites
Il lite Kaolinite
(montmorillonite)
SiO2 48-56 51-54 46,55
Al2O3 11-22 21-29 39,5
H2O 12-24 - 10,0-13,95
Fe2O3 ≥5 0,5-5,3 Trace
FeO - 0,9-1,2 -
BaO - - Trace
MgO 4-9 2,8-3,5 Trace
CaO 0,8-3,3 ,02-,05 Trace
Na2O trace ,08-,10 Trace
K2O trace 2-9 Trace
P2O5 - 0,6-0,82 -
ZrO2 - - -
TiO2 - 0,60-0,82 Trace
MnO - - -
Loss on ignition - - -
Total >80 >79,5 96,0-100,0
800
Impulse
RD 1 - 30cm + Standard
F
F
700
600
Q: Quartz (1%)
H: Haematite
K: Kaolinite
F: Fluorite (CaF2- Standard)
Tr: Specimen holder
500
F
400
214
300
Tr
200
100
K
K
Q
Q
18
K
H
H
K
Q
K
H
Q
H Q
H
H
Q
H
K
H
Q
F
0
0 20 40 60 [ ° 2 ]
Figure 6.7. X-ray diffraction diagram of red soils collected at 0,30m depth.