an engineering geological characterisation of tropical clays - GBV

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6.4.2 Results
6.4.2.1 Nairobi phonolite
Results of scanning electron microscope analysis show Nairobi phonolite to exhibit a
generally massive, homogeneous and compact structure (Plate 6.1). Texture is mainly of large
phenocrysts embedded in a fine-grained matrix (Plate 6.3). The phenocrysts show presence of
O, Al, Si, K implying K-feldspars; and some Fe and Ti probably due to inclusions of iron and
titanium ore. The phenocrysts show partly corroded and/ or leached and altered surfaces
(Plate 6.2). The matrix also shows an increased presence of Si, K and Fe, as well as some O
and Al, implying a predominance of K-feldspars with some iron oxides and/ or ores. The iron
oxides are most likely a result of weathering and/ or chemical decomposition of phonolite to
form tuffs as well as ferricrete and / or iron concretions (Plates 6.4/6.5). Occasional presence
of Ca in the matrix could be attributed to weathering of phonolitic tuff to form secondary
limestone and/or calcite (Plate 6.5).
Plate 6.1. Massive and compact structure
Plate 6.2. Corroded, leached K-feldspar