an engineering geological characterisation of tropical clays - GBV

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Plate 6.24. Solution pores/ cavities in weathering smectitic clay matrix with Al, Ca, Fe,
and high Si content; and associated K, Ti crusts.
6.4.2.4 Red soils
The red soils show a relatively loose, porous and friable structure of weakly bound or
cemented lumps of clay matrix (Plate 6.25). Large grains of iron concretions and quartz are
embedded in a fine weathering clay matrix. The iron concretions show an elevated Fe content,
up to 70% by weight (Plate 6.29); and weather through leaching to form solution pores (Plate
6.30). Quartz grains are slightly abraded and rounded to sub-rounded (Plate 6.28), and are
probably detrital, having been derived by erosion and water transport. The relatively high
porosity of the soil structure (Plate 6.31) could therefore be partly attributed to binding and
cementation of clay matrix by iron oxide to form detached lumps; and partly to leaching of
soluble bases of Ca, Mg, K, Na; as well as solution effects caused by Fe going into reaction.
Plate 6.25. Loose, porous and friable structure
of red soils.
Plate 6.26. Lumps of cemented clay
matrix with Fe-rich crusts.
The fine matrix shows presence of Si and Al mainly, suggesting kaolinite; as well as Fe,
implying heamatite. Fine crusts bearing Ti, Mn and K also occur (Plate 6.27) . Kaolinite
generally occurs as fine weathered and altered plates which have lost the original six-sided
form and/ or characteristic crystal structure. The plates are commonly weakly bound
/cemented together and covered by fine Fe-rich crusts of matrix (Plate 6.28). Weathering and