an engineering geological characterisation of tropical clays - GBV

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The trachytes generally show a relatively less compact and more porous structure (Plates 6.6/
6.7), and are characterised by plates of tabular K-feldspar phenocrysts showing patched
overgrowths of Fe and Ca-rich crusts (Plate 6.8). The phenocrysts show a composition of O,
Al, Si, K and occasionally Na, and this is suggestive of sanidine. The feldspars show effects
of weathering and alteration through leached surfaces and solution pores (Plate 6.9). The
matrix is predominated by K-feldspars showing presence of O, Al, Si, K suggestive of
sanidine and/or orthoclase; while crusts rich in Fe and Ca also occur as weathering and
alteration products of the rock.
6.4.2.3 Black clays
The black clays exhibit a generally heavy, dense and massive appearance with solution pores,
cavities and / or cracks found within them (Plates 6.10, 6.11 & 6.24). Quartz grains and K-
feldspar crystals show overgrowths of Fe, Ca and Mg-rich crusts (Plates 6.12, 6.15); and
occur in a fine, weathered and generally smectitic clay matrix (Plate 6.23). The generally
smooth sub-rounded quartz grains (Plates 6.19, 6.20) indicate water transport, and this serves
to support the hypothesis that part of the materials forming black clays have been contributed
by erosional processes from the surrounding high areas. The feldspars show alteration effects
through corroded and leached solution surfaces (Plate 6.18). Apparent detrital components
showing presence of O, Ca, Fe, K, Si, Ti also occur and suggest biotite (Plate 6.14), a
probable result of weathered phonolitic and/ or metamorphic rock inclusions. Iron concretions
(Plates 6.17, 6.21) and blocks of ankerite (Plate 6.22) showing elevated Fe content are also
found embedded in the clay matrix. Traces of organic material (Plate 6.16) are also found.
Occasional weathered plates of kaolinite also form part of the clay matrix (Plate 6.13). The
presence of Fe and Ca-rich crusts could be attributed to underlying phonolitic rocks
weathering through tuff and ferricrete thereby forming secondary limestone and iron
concretions (murram); and partly to weathering of the smectites in the clay matrix. The
solution pores are a probable result of the iron, limestone and/ or other carbonate components
going into reaction/ solution, while the partings and cracks are most likely due to shrinkage
effects in the smectitic clay matrix in a dry condition. Presence of Ti and Zn in the matrix
could be attributed to heavy mineral inclusions transported from surrounding areas.
Plate 6.10. Massive, dense, heavy clays with
solution pores.
Plate 6.11. Partings, cracks due to shrinkage;
and solution pores.