an engineering geological characterisation of tropical clays - GBV

29
caused by partial segregation of the dark and light-coloured constituents present, during
crystallisation (Saggerson, 1991).
Thin sections show small tabular sanidine phenocrysts (exhibiting Carlsbad twinning) set in a
fine-grained groundmass. The groundmass shows a marked trachytic texture and consists of
orthoclase associated with soda-amphiboles and pyroxenes, as well as rare iron ore. The sodarich
minerals consist of deep brown cossyrite and green aegirine-augite, and occur as
prominent moss-like aggregates of prismatic crystals. The alternating brown and green
patches serve to impart a mottled appearance to the rock in hand specimens.
Table 3.3 gives results of chemical analyses of Nairobi trachyte, as well as other rocks for
comparison purposes. Results of mineralogical analyses of Nairobi trachyte, and a number of
other rocks underlying Nairobi and the present study area, are summarised in Table 3.4
(Saggerson, 1991).
Table 3.4 Mineral compositions of trachytes and phonolites from Nairobi and the study area
(after Saggerson, 1991).
Rock type:
Nairobi Kiambu Nairobi Kandizi Mbagathi Kapiti
trachyte trachyte phonolite phonolite phonolitic phonolite
trachyte
Sanidine X X R Ac
Anorthoclase R X X
Nepheline
X
Titanaugite R X
Olivine
R
Phenocrysts
Matrix
Biotite
X
Alkali feldspar X X X X X X
Nepheline X X X X
Quartz
Aegirine
Aegirine-augite
Ok X X On X X
Cossyrite X X X X X
α-kataphorite X X I X X
β-kataphorite
X
Riebeckite
X
Arfvedsonite
Biotite
X
Barkevikite
R
α-kataphorite – colourless to pale brownish yellow, smoky-brown to rose-red and/ or
reddish yellow
- cleavage 124°
β-kataphorite – pale smoky-brown to greenish yellow, deep purplish brown to opaque
X - present; R - rare; A - altered; Ac – accessory
Ok – overgrowths on kataphorite
On – overgrowths on nepheline
I – including large crystals