an engineering geological characterisation of tropical clays - GBV

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Chapter 6
Chemical and mineralogical analyses
6.1 Introduction
A number of soil samples previously collected from the field were subjected to laboratory
chemical and mineralogical analyses and/ or studies using the X-ray fluorescence (XRF), X-
ray diffraction (XRD) and scanning electron microscope (SEM) techniques.
6.2 X-ray fluorescence (XRF) studies
6.2.1 Scope and method
Chemical analyses were carried out using X-ray fluorescence technique to determine the
percentage composition of major oxides in the clay soils, and include SiO2, Al2O3, Fe2O3,
FeO, MgO, CaO, MnO, Na2O, K2O, TiO2, P2O5 and SO4. Percentage content of F, Cl, S
and Cr, as well as losses on ignition were also determined. The analysis results have been
useful in computing mineral contents in the tested soils (according to Olphen and Fripiat,
1979), especially where direct measurement of mineralogical compositions, such as kaolinite
in red soils, has proved difficult.
A PW-1480 (Rh 100kV LiF220 Ge111 T1AP) X-ray spectrometer and a shale (claystone)
standard were employed in the XRF analysis. Preparation of test specimens involved mixing
of Hoechst wax (as diluent) with soil sample at a ratio of 1:5 to make a homogenous mixture.
6.2.2 Results
Results of chemical analyses obtained for the soils in this study are presented in Table 6.1 for
red soils and Table 6.2 for black clays.
Table 6.1. Results of chemical analyses of red soils obtained in this study.
% content of: Rd1-30cm Rd1-100cm Rd1-200cm Rd1-400cm
SiO2 47,50 47,10 47,20 47,6
Al2O3 32,20 33,40 33,20 33,30
Fe2O3 15,30 15,50 15,50 15,20
FeO - - - -
BaO 0,062 0,061 0,071 0,064
MgO 0,12 0,097 0,14 0,13
CaO 0,26 0,24 0,16 0,26
Na2O ‹ ‹ ‹ ‹
K2O 0,96 0,66 0,68 0,60
P2O5 0,19 0,086 0,096 0,062
ZrO2 0,37 0,37 0,37 0,36
TiO2 1,57 1,51 1,51 1,46
MnO 0,96 0,53 0,61 0,52
Loss on ignition - - - -
Total 99,49 99,55 99,54 99,56
Rd1-30cm, Rd1-100cm, Rd1-200cm & Rd1-400cm are red soil samples.