an engineering geological characterisation of tropical clays - GBV
an engineering geological characterisation of tropical clays - GBV
an engineering geological characterisation of tropical clays - GBV
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<strong>an</strong>d prismatic apatite occur as accessories (Saggerson, 1991). Composition <strong>of</strong> the groundmass<br />
is also summarised in Table (3.4).<br />
Table 3.1 Chemical <strong>an</strong>alyses <strong>of</strong> alkali-feldspars from the Kapiti phonolite <strong>an</strong>d similar rocks.<br />
1 2 3 4<br />
SiO2 65,61 62,79 64,33 64,80<br />
Al2O3 20,48 22,12 20,94 21,14<br />
Fe2O3 0,20 0,36 0,2 0,68<br />
FeO 0,20 0,41 0,58 -<br />
MgO 0,00 - - 0,10<br />
CaO 7,03 3,76 2,01 1,18<br />
Na2O 6,26 7,35 7,22 6,94<br />
K2O 0,32 2,98 4,71 5,26<br />
H2O+ 0,03 0,19 0,27 0,20<br />
H2O- 0,00 0,07 0,10 0,20<br />
Totals 99,93 100,03 100,36 100,00<br />
1. Alkali-feldspar phenocryst (<strong>an</strong>orthoclase). From Kapiti phonolite, Stony Athi River (Häkli, 1960).<br />
2. Loose crystals <strong>of</strong> <strong>an</strong>orthoclase (potash-oligoclase). Crater <strong>of</strong> Mount Erebus, Antarctica (Mountain, 1925).<br />
3. Anorthoclase. Crater slopes, Mount Kenya (Mountain, 1925).<br />
4. Anorthoclase phenocrysts. From Kenyte, Mount Kenya (Spencer, 1937).<br />
The thin section descriptions <strong>an</strong>d results <strong>of</strong> chemical <strong>an</strong>alysis <strong>of</strong> Kapiti phonolite from the<br />
study area <strong>an</strong>d its environs, has been shown by various authors to be similar (Baker, 1954;<br />
Bowen, 1937; Smith Campbell,1950; Saggerson, 1991; Prior, 1903). The results <strong>of</strong> chemical<br />
<strong>an</strong>alyses are summarised in Table 3.1, which also shows a comparison <strong>of</strong> the composition <strong>of</strong><br />
alkali-feldspar phenocrysts from the Kapiti phonolite; with those <strong>of</strong> <strong>an</strong>orthoclase crystals from<br />
Mount Kenya <strong>an</strong>d Antarctica. From the table, specimens from the Kapiti phonolite are found<br />
to show a marked rise in the ratio <strong>of</strong> alkalis, Na2O/K2O, with increase in lime (CaO) content.<br />
Specimens from Mount Kenya <strong>an</strong>d Antarctica, however, exhibit a steady increase in CaO with<br />
increasing Na2O/K2O ratio.<br />
3.3.2 Nairobi phonolite<br />
The Nairobi phonolite covers a large part <strong>of</strong> the Athi plains. It extends from Nairobi National<br />
Park located in the western <strong>an</strong>d south-western parts <strong>of</strong> the study area, <strong>an</strong>d northwards across<br />
the Nairobi city centre to Kiambu township, located north <strong>of</strong> /<strong>an</strong>d outside the present study<br />
area. It overlies the Athi Series with a probable disconformity. Outcrops <strong>of</strong> the phonolite are<br />
observable in the southern <strong>an</strong>d south-western parts <strong>of</strong> the study area in a prominent scarp,<br />
which also shows the contact between the rock <strong>an</strong>d the underlying Athi Series (Saggerson,<br />
1991).