an engineering geological characterisation of tropical clays - GBV

60
2µm size) filtered both by means of a programmable centrifuge as well as through a filter
membrane. The original soil sample had been previously and moderately ground to reduce
strong textural effects usually caused by presence of quartz and feldspars. Treatment with
hydrogen peroxide (3-10% H2O2) to oxidise any organic matter found had also been done,
followed by treatment with ammonia solution (0,01 N NH3-water) to remove the carbonate
components and prevent Ca ions from flocculating the clay particles in prepared suspensions.
Calcium fluoride standard was adopted where upon preparations of 200mg CaF2 mixed with
1g of soil sample were used.
The presence of swelling clay minerals (smectites) was investigated by measuring and
comparing diffractograms from filtered air-dried specimens with those from filtered and
glycolised specimens. Glycolisation involved heating specimens to 40 °C to collapse the
layers of any montmorillonite present, followed by heating in a chamber of glycol
atmosphere. Usually glycol has the same effect as water and, in the presence of smectites,
would go in-between the layers and move them apart, thereby causing pronounced expansion.
The difference in the amount of impulse (reflex intensity) between the resulting
diffractograms and / or smectite peaks and those from air-dried specimens would therefore be
significant (Reynolds and Moore, 1989; Sattler, 2000).
The clay minerals were identified from basal reflections obtained from their sheet-like
structure and by matching the obtained diffraction patterns with calculated one – dimensional
diffraction profiles ( Reynolds and Hower, 1970; Reynolds and Moore, 1989; Sattler, 2000).
Primary non-clay minerals were also identified by scanning the preparations at selected
ranges; and include quartz, calcite, dolomite and iron oxides. The minerals were identified by
matching their characteristic diffraction peaks with known standards.
6.3.2 Results
The results of percentage composition of clay minerals in the black clays are summarised in
Table 6.3, while the corresponding diffractograms are given in Figures 6.1 to 6.6. The
diffractograms obtained for the red soils are presented in Figures 6.7 to 6.10. However, it was
not possible to determine the percentage clay mineral composition of the red soils. This is
most probably due to the high contents of iron in these soils which caused formation of very
thin layers of specimens on glass slides, thus making it difficult to prepare oriented samples
necessary for drop analysis and/ or pipette analysis methods.
The results of the analyses show the black clays to be composed of over 90% smectites
(montmorillonite: CaNaMgFeAlSiOOH.HO) and less than 10% kaolinite (Al2Si2O5(OH)4).
Illite (K-Na-Mg-Fe-Al-Si-O-H2O) occasionally occurs in trace form, and is probably an
alteration product of the feldspars and / or kaolinite. Also found contained are quartz (4-9%)
and accessories of K-feldspars [sanidine (K,Na)AlSi3O8), orthoclase (Na, K)Si3O8),
microcline (KAlSi3O8)], haematite (Fe2O3) and carbonates, i.e.calcite (CaCO3), dolomite
(CaMg(CO3)2, ankerite (Ca(Fe,Mg)(CO3)2, siderite (FeCO3)].
According to Coduto (1994), montmorillonite often results from weathering of
ferromagnesian minerals, calcic feldspars and volcanic materials. In addition, sodium
montmorillonite is often formed from weathering of volcanic ash, while other
montmorillonites form in environments of alkaline conditions characterised by a supply of
magnesium ions and a lack of leaching. The montmorillonite in black clays of the present
study must have resulted from weathering and alteration of volcanic ash previously deposited
in a basin-like lake environment. Additional contribution in montmorillonite formation