an engineering geological characterisation of tropical clays - GBV
an engineering geological characterisation of tropical clays - GBV
an engineering geological characterisation of tropical clays - GBV
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64<br />
800<br />
Impulse<br />
700<br />
600<br />
500<br />
SC 33 -50cm + St<strong>an</strong>dard<br />
Q: Quartz Td: Tridymite<br />
Sm: Smectites Cr: Cristobalite<br />
Or: Orthoclase<br />
Sn: S<strong>an</strong>idine<br />
Mc: Microcline<br />
H: Haematite<br />
Sd: Siderite<br />
Ak: Ankerite<br />
D: Dolomite<br />
Q<br />
F: Fluorite (CaF2-St<strong>an</strong>dard)<br />
Tr: Specimen holder<br />
F<br />
254<br />
F<br />
F<br />
400<br />
Quartz: 7%<br />
Accessories: K -Feldspars, Haematite,<br />
Carbonates<br />
Rest: Clay minerals<br />
(S. Texture preparation)<br />
300<br />
200<br />
100<br />
Sm<br />
Or Or<br />
Sn Sn<br />
Sm<br />
Q<br />
Td<br />
Cr<br />
Or<br />
98 Sn<br />
Sn Mc<br />
Mc<br />
Sm<br />
Ak Q Sd<br />
D<br />
Sd Q<br />
Or<br />
Sn<br />
H Q Sn<br />
Mc<br />
Tr<br />
Mc<br />
Q<br />
Q<br />
Sd<br />
Ak<br />
Q<br />
H<br />
Sm<br />
Q<br />
D<br />
H<br />
Q<br />
0.0<br />
0 20 40 60 [ ° 2 ]<br />
Figure 6.6. X-ray diffraction diagram <strong>of</strong> black <strong>clays</strong> collected at 0,50m depth.<br />
The diffractograms for the red soils show them as containing mainly kaolinite <strong>an</strong>d haematite<br />
with accessories <strong>of</strong> quartz. According to Day (2001) <strong>an</strong>d Holtz & Kovacs (1981), kaolinite<br />
belongs to kaolin minerals, i.e. a group <strong>of</strong> clay minerals consisting <strong>of</strong> hydrous aluminium<br />
silicates. Approximate mineralogical compositions <strong>of</strong> red soils could therefore be estimated<br />
from results <strong>of</strong> chemical <strong>an</strong>alyses (Table 6.1) obtained for the soils. This was done by<br />
assuming that the kaolinite portion is constituted mainly by SiO2 <strong>an</strong>d Al2O3; <strong>an</strong>d haematite<br />
by Fe2O3. The resulting estimated mineralogical compositions for the red soils are presented<br />
in Table 6.4, <strong>an</strong>d show the red soils as being mainly kaolinite (80-81%) <strong>an</strong>d haematite (15-<br />
16%) sediments; with accessories <strong>of</strong> quartz (1-3%) as well as minerals <strong>of</strong> tit<strong>an</strong>ium (about 2%)<br />
<strong>an</strong>d m<strong>an</strong>g<strong>an</strong>ese (about 1%). The kaolinite must have resulted from weathering <strong>an</strong>d alteration<br />
<strong>of</strong> aluminiumsilicate-rich feldspars in the underlying Nairobi trachytes <strong>an</strong>d other volc<strong>an</strong>ic<br />
materials, under humid conditions (rainfall) <strong>an</strong>d in the presence <strong>of</strong> carbon dioxide gas.<br />
Table 6.4. Estimated mineralogical composition <strong>of</strong> red soils.<br />
Sample No. Kaolinite<br />
(%)<br />
Haematite<br />
(%)<br />
Quartz<br />
(%)<br />
Tit<strong>an</strong>ium<br />
(TiO2)<br />
M<strong>an</strong>g<strong>an</strong>ese<br />
(MnO2)<br />
(%)<br />
Rd1-30cm 80 15 1 2 1<br />
Rd1-100cm 81 16 3 2 1<br />
Rd1-200cm 80 16 2 2 1<br />
Rd1-400cm 81 15 1 2 1<br />
In addition, hydrothermal alteration <strong>of</strong> the aluminiumsilicates could have formed<br />
pseudomorphs <strong>of</strong> kaolinite from feldspars <strong>an</strong>d/or muscovite, topas, leucite, <strong>an</strong>dalusite, <strong>an</strong>d<br />
pyrophyllite contained in the rocks. Kaolinite-rich red soils containing alkali <strong>an</strong>d alkali-earth<br />
metals (Na, K, Mg, Ca) usually alter during favourable conditions into secondary feldspars,
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