Journal of African Earth Sciences 35 (2002) 123–134
www.elsevier.com/locate/jafrearsci
Mineralogical, geochemical, and sedimentological characteristics
of clay deposits from central Uganda and their applications
George W.A. Nyakairu
a
a,1
, Hans Kurzweil b, Christian Koeberl
a,*
Institute of Geochemistry, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria
b
Institute of Petrology, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria
Accepted 10 October 2001
Abstract
In Uganda, Precambrian rocks have undergone extensive weathering and erosion, and are locally altered to form considerable
clay deposits. We have studied the geochemical, mineralogical, and sedimentological characteristics of clay deposits from central
Uganda to determine their composition, source rocks, deposition, and possible use in local industry. Samples were collected from
the Kajjansi, Kitiko, Masooli, and Ntawo deposits (near Kampala), all of which are currently used for both industrial and traditional brick, tile, and pottery manufacture. The deposits are widely scattered individual basins, with clays deposited under lacustrine and alluvial environmental conditions, and were all found to belong to the sedimentary group. The clays are composed of
silt–sand fractions and predominantly consist of kaolinite and have a relatively high Fe2 O3 content. The studied deposits are
chemically homogeneous, except for the samples richer in sand fraction, which have higher SiO2 and K2 O values. The chemistry of
the studied samples, compared to European clays, shows that they need elaborate treatment to render them suitable for ceramics
production. An analysis of the chemical and mineralogical composition of the clays has demonstrated that, taken as a whole, they
possess characteristics satisfactory for brick production.
Ó 2002 Elsevier Science Ltd. All rights reserved.
Keywords: Clay deposits; Precambrian rocks; Uganda
1. Introduction
Clays occur widely in many parts of Uganda. Besides
their geological interest, they are of importance for local
industry. They have been used to produce rather poorquality bricks, tiles and pottery by primitive methods for
several years. Scattered clay pits and brick kilns along
the roadsides document the uncontrolled and low-technology exploitation of the Uganda clay occurrences.
Apart from artisan brick producers, there are organized
clay works, such as Uganda Clays and Pan African Clay
Products at Kajjansi, and Allied Clays at Masooli along
the Gayaza road, which supply the construction industry in Kampala and surroundings. Starting in 1986,
there has been an increase in construction activity in
Kampala. The above-mentioned industries cannot meet
*
Corresponding author. Tel.: +43-1-4277-53110; fax: +43-1-42779531.
E-mail address: [email protected] (C. Koeberl).
1
Current address: Department of Chemistry, Makerere University,
P.O. Box 7062, Kampala, Uganda.
the ever-increasing market demand for the construction
materials needed. Traditional methods of production,
which do not take account of the chemical and mineralogical characteristics, are still practiced. In the traditional method of brick production, raw clay material is
mixed with water and covered for about a week. The
paste is placed in a wooden mould as shown in Fig. 2(a)
and (b). The bricks are spread and covered with cut
grass until they are dry. However, during the rainy
season, plastic sheets are used to cover the bricks (Fig.
2(c)). The bricks are fired in field kilns, which consist of
a large pile of unfired bricks with tunnels in the bottom
of the pile (Fig. 2(d)). The pile is cemented with clay and
contains 10,000–15,000 bricks. A wood fire is built in the
tunnel and kept burning for 4–6 days and the tunnels are
then closed with unfired bricks and also cemented with
clay. The hot exhaust from the wood fire flows through
the pile, and heats the center of the pile enough to fire
the bricks in the core of the pile. The pile is then allowed
to cool and dismantled.
Few studies have been made of the clays used in the
brickworks or of raw materials used for pottery in
0899-5362/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved.
PII: S 0 8 9 9 - 5 3 6 2 ( 0 1 ) 0 0 0 7 7 - X
124
G.W.A. Nyakairu et al. / Journal of African Earth Sciences 35 (2002) 123–134
Fig. 1. Generalized geological map of the study and surrounding areas extracted from the geology map of Kampala sheet NA 36-14 (Geological Survey of Uganda, 1962). The inset is a map of
Uganda.
G.W.A. Nyakairu et al. / Journal of African Earth Sciences 35 (2002) 123–134
125
Fig. 2. Photographs showing the traditional brick production methods used in Uganda. (a) After a heap of raw clay material is covered for a week,
the paste is placed into a mould. (b) Mould with clay paste ready for drying. (c) Two moulds used to make bricks, spreading of bricks for drying, cut
grass for covering the bricks and also the plastic sheets used in the rainy season in the background, and finished bricks for kiln construction. (d)
Partly finished typical brick kiln under construction, with tunnels in which wood fires are built to fire the bricks.
Uganda. Harris (1946) and Kagobya (1950) studied the
clay deposit at Ntawo, 25 km from Kampala on the
Jinja road (Fig. 1). It was reported that Ntawo clay
exhibited marked shrinkage and cracked on firing, and
the quality of product was inferior when it was evaluated for pottery production. McGill (1965) studied the
nature and distribution of clays from several occurrences around Kampala, and determined their plasticity
with a view to establish a fine ceramics industry. Tuhumwire et al. (1995) measured the physical properties
and discussed the geology of the Kajjansi and Kitiko
deposits located 13 km from Kampala on the Kampala–
Entebbe road. A study of some clay samples from
various deposits in Uganda indicated that they are medium-quality kaolinitic–illitic clays (Nyakairu and Kaahwa, 1998, and references therein). There is ample
demand for quality bricks and other clay products, and,
thus, the present study evaluates the mineralogical and
chemical characteristics of the raw material used, not
only in individual brick and tile works, but also by the
traditional potters. This will help to give a better understanding of the clay materials, as well as of their
geochemistry and source rocks.
2. Geology
The study areas are indicated on Fig. 1. The areas are
mostly underlain by Precambrian rocks that include
sedimentary and metasedimentary lithologies, which
comprise fine-grained sandstones, slates, phyllites, and
schists. The more highly metamorphosed rocks include
quartzites, muscovite–biotite gneisses, and subordinate
schist, which may locally contain cordierite. The above
rocks, together with amphibolites and epidosites, are
part of the Buganda series, which make up a wider
Buganda–Toro system with the Toro series of Western
Uganda (Schlueter, 1997). In deeply weathered areas,
parts of the basement are exposed in the form of undifferentiated gneisses and late granites, as well as migmatized and remobilized parts of the Buganda series.
These rocks are overlain in places by swamp deposits,
alluvium, and lacustrine deposits. The underlying gneissic and granitoid rocks of the Precambrian basement
have been extensively weathered and transported to
produce clays. Some of these clays are weathering
products of schists and amphibolites, and of basic rocks
of the younger Buganda series. According to McGill
126
G.W.A. Nyakairu et al. / Journal of African Earth Sciences 35 (2002) 123–134
(1965), the clays can generally be classified as sedimentary and transported alluvial clays. It is from these clays
that the samples used in this study were obtained.
Clays derived from gneissic and granitoid rocks of the
basement are leached and enriched in quartz. These
clays are thought to have formed by leaching of the
decomposed bedrock, and are normally separated from
the bedrock by a layer of large quartzite pebbles (Harris,
1946). The clays occur as surficial layers with a general
thickness varying between 2 and 5 m (cf. Kaliisa, 1983).
The main features of the studied clays are their low wetto-dry shrinkage, refractory nature, and an extremely
high plasticity, which is attributed to high kaolinite
content. Due to their high plasticity, these clays can be
classified as ball clays, which makes them suitable for
pottery, earthenware, and binders in refractory material
production. In some areas, other clay occurs together
with the clays derived from gneissic and granitoid rocks
of the basement in the same deposits. The bulk of these
other clays are essentially micaceous schist derivatives,
which gives them a yellowish-brown color, as opposed
to the dull gray appearance of the clays derived from the
gneissic and granitoid rocks of the basement. However,
most of the clay is medium gray to brownish gray. In
some areas this type of clay is more thoroughly weathered and leached than in some locations found in the
Kajjansi clay field (McGill, 1965).
3. Methods and results
3.1. Methods
In this study we analyzed 24 clay samples taken from
deposits that supply the construction industry in Kampala and surrounding trading centers. These samples
belong to the Kajjansi (n ¼ 6), Kitiko (n ¼ 8), Masooli
(n ¼ 4) and Ntawo (n ¼ 6) locations, as indicated in
Fig. 1. Well-mixed large samples, which were reduced to
about 250 g each by coning and quartering, were taken
from pits of at least 5 m depth, dug at each of the deposits. Samples were dried at 60 °C, and divided for
grain size, mineralogical, and chemical analyses. For
grain size analysis, sample aliquots of 100 g were oxidized and disaggregated with 15% H2 O2 and were left to
stand overnight (cf. Dalsgaard et al., 1991). Water (200
ml) was added to each of the samples, and was disaggregated with an ultrasonic probe for 3 min. The samples were wet sieved into fractions of 2, 1, 0.5, 0.25,
0.125, 0.063, and 0.032 mm. These fractions were dried
at 80 °C, and weighed to 0.1 g. The <32 lm fractions (10
g) were mixed with sodium hexametaphosphate (20 ml)
and ultrasonically disaggregated, and grain size analysis
was performed by X-ray monitoring of gravity sedimentation (Micromeritics SediGraph 5100) up to 0:2 lm
(cf. Coakley and Syvitski, 1991).
Aliquots of 20–30 g of each dried sample were powdered in an agate mill. The chemical analyses (major
elements) were performed on powdered samples using
X-ray fluorescence (XRF) spectrometry at the University of the Witwatersrand, Johannesburg, South Africa.
For details on procedures, precision, and accuracy, see
Reimold et al. (1994). Mineralogical analysis was performed at the Institute of Petrology, University of Vienna, Austria, on bulk rock powders using a powder
X-ray diffractometer (Philips PW 3710) operated at
45 kV/35 mA using Ni-filtered CuKa radiation, with
automatic slit and on-line computer control. The samples were scanned from 2° to 40° 2h. Mineral identification on the diffractograms was processed using Philips
PC-APD software, version 3.5B. The quantitative mineralogical composition was evaluated using the normative calculation method of Fabbri et al. (1986). Total
carbon was determined with a LECO elemental analyzer
(Multiphase carbon determinator, RC-412) at the Institute of Petrology, University of Vienna, Austria.
3.2. Grain size analysis results
Table 1 lists the results of the grain size analysis of the
clay raw materials used in brick production for the
construction industry in Kampala, Uganda. The analyzed samples show a large variation in grain sizes. For
example, the sand contents range from 8 to 65 wt%, silt
content ranges from 19 to 56 wt%, and the clay content
ranges from 5 to 42 wt%. In Shepard’s diagram, the
samples generally plot in the clayey silt, sand–silt–clay,
and silty sand fields (Fig. 3) (cf. Shepard, 1954). The
McManus (1988) diagram (Fig. 4) indicates that the
samples are moderately well sorted, with moderately
high porosity and permeability. Thus, according to
McManus (1988) the samples are suitable for the construction industry. The Kajjansi clay samples are not
easy to classify, as they have varying sizes, ranging from
sand to clay-sized particles (Table 1). The samples plot
within the silt clay, clayey silt, and sand–silt–clay fields
(Fig. 3). The Kitiko samples posses modest percentages
of sand and variable contents of silt and clay (Table 3),
and belong to the silty clay, clayey silt, sand–silt–clay,
and silty sand categories (Fig. 3). The Masooli clays are
rather uniform in terms of grain size (Table 1). The sand
fraction (>50 wt%) is more abundant than the silt fraction, with low clay content. The samples can be classified
as silty sand, clayey sand, and sand–silt–clay (Fig. 3).
The Ntawo clay samples contain variable sand, silt, and
clay fractions. Therefore, these raw materials are part of
the field of sand–silt–clay, silty sand, and sandy silt clays
(Fig. 3).
The parameters median, mean, sorting, and skewness
were calculated from the results of the grain size analysis. The positive skewness of the samples is attributed
to the presence of silt- and clay-size fractions. This result
G.W.A. Nyakairu et al. / Journal of African Earth Sciences 35 (2002) 123–134
127
Table 1
Grain size parameters of clays from deposits supplying the construction industry in Kampala, Uganda, obtained by Sedivision 2.0
Sample
Wt%
Clay
Fraction (wt%)
First–third moment
mm
Silt
Sand
>63 lm
4–63 lm
<4 lm
>20 lm
2–20 lm
<2 lm
x
r
a3
C
M
Kajjansi deposit
Kaj-1
41
Kaj-2
28
Kaj-3
27
Kaj-4
38
Kaj-5
22
Kaj-A
26
47
48
35
54
23
36
12
24
38
8
55
38
12.18
24.04
37.91
7.65
46.61
37.73
43.58
46.12
33.79
51.33
29.19
33.47
44.24
29.84
28.3
41.02
24.2
28.8
49.9
65.37
68.34
51.02
66.14
62.5
9.24
6.57
4.83
10.9
11.91
11.08
40.86
28.06
26.83
38.08
21.95
26.42
7.10
6.26
5.71
7.45
4.85
5.86
4.05
4.19
4.41
3.96
4.61
4.31
0.50
0.70
0.73
0.40
0.68
0.66
0.93
1.39
1.57
0.67
3.20
1.67
0.02
0.04
0.05
0.03
0.08
0.04
Kitiko deposit
Ks1-1
25
Ks1-2
39
Ks1-3
25
Ks1-4
31
Ks2-1
14
Ks2-2
42
Ks2-3
28
Ks-A
32
19
48
39
53
25
43
55
36
56
13
36
16
61
15
17
34
36.79
12.91
35.76
15.68
50.02
14.47
17.3
32.47
36.34
45.41
36.34
47.47
34.61
41.22
47.97
33.21
26.87
41.68
27.9
36.85
15.37
44.31
34.73
34.32
68.36
48.66
66.13
40.44
82.37
49.48
38.66
58.37
6.66
12.07
8.34
28.5
3.2
8.23
33.1
9.68
24.98
39.27
25.53
31.06
14.43
42.29
28.24
31.95
5.10
7.56
6.04
7.09
4.03
7.64
6.89
6.34
4.77
4.37
3.99
3.90
3.94
4.64
4.01
4.61
0.56
0.17
0.84
0.23
1.21
0.07
0.21
0.37
3.54
2.13
1.56
1.87
2.67
1.90
1.33
3.01
0.09
0.02
0.04
0.01
0.10
0.02
0.01
0.04
Masooli deposit
Mas-L
21
Mas-M
26
Mas-U
13
Mas-A
14
38
20
30
35
41
54
57
51
41.56
54.04
56.67
51.36
36.64
18.39
28.44
33.85
21.8
27.57
14.89
14.79
76.76
66.46
79.52
82.67
2.14
7.93
7.2
3.44
21.1
25.61
13.28
13.89
5.27
5.37
4.29
4.43
4.09
4.47
3.55
3.59
1.12
0.81
1.41
1.49
1.23
1.33
1.34
1.49
0.05
0.08
0.09
0.07
Ntawo deposit
Ntw-1
24
Ntw-2
6
Ntw-3
5
Ntw-B
13
Ntw-G
27
Ntw-A
34
40
29
39
56
49
37
36
65
56
31
24
29
36.54
64.95
55.94
30.47
23.76
28.81
35.06
28.12
37.84
53.13
45.53
34.66
28.4
6.93
6.22
16.4
30.71
36.53
58.89
91.37
89.51
65.37
56.71
57.89
17.38
2.22
5.07
21.18
16.21
8.07
23.73
6.41
5.42
13.45
27.08
34.04
5.86
2.34
3.26
5.28
6.21
6.76
3.92
3.47
2.85
3.16
4.12
4.39
0.68
1.42
1.35
1.06
0.47
0.48
1.19
3.74
2.45
0.93
1.50
0.90
0.04
0.33
0.10
0.04
0.04
0.04
CM values after Passega (1957, 1964) and Passega and Pyramjee
(1969); grain size fractions are classified
after Shepard (1954) and Winkler (1954).
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ffi
Mean, x ¼ ðq1 x1 þ þ qn xn Þ=100; standard deviation, r ¼ ðq1 ðx1 xÞ2 þ þ qn ðxn xÞ2 Þ=100; skewness, a3 ¼ fq1 ðx1 xÞ3 þ þ qn ðxn xÞ3 g=
100r3 ; where x is the midpoint of the grain size fraction (measured in phi units); q ¼ percentual frequency of the fraction; C ¼ one percentile; and
M ¼ median.
Fig. 3. Classification of studied clay raw materials from Uganda based
on the sand–silt–clay ratios (Shepard, 1954).
Fig. 4. Ternary diagram of studied clay sediments from Uganda following the relation between sand, silt, and clay components and their
controls over porosity and permeability, after McManus (1988). WS,
well sorted; PS, poorly sorted; MWS, moderately well sorted.
128
G.W.A. Nyakairu et al. / Journal of African Earth Sciences 35 (2002) 123–134
can be ascribed either to different sources or the environmental deposition conditions of the clays. The binary
diagrams, Fig. 5(a)–(c), show that the samples fall into
two distinct groups, which indicate different conditions
of environmental deposition. The group containing the
(a)
most samples is of lacustrine origin, whereas the other
group of samples is formed by fluvial deposition. The
sorting indicates that the sediments are moderately to
poorly sorted and the cumulative curve (Fig. 6) shows
that most samples plot on the silt–sand size boundary.
(b)
(c)
Fig. 5. Binary plots of studied Uganda clay sediments after Friedman (1967): (a) mean against sorting, (b) skewness against sorting and (c) skewness
against mean. Samples fall into two distinct groups of environmental or hydraulic conditions that controlled their deposition.
Fig. 6. Smoothed cumulative frequency distribution of sample Kaj-1, a typical sample, obtained by combining sieve and SediGraph data using
computer software Sedivision 2.0. The shape of the curve indicates that the sample is composed of silt to sand sizes. All clay samples give curves with
similar shapes.
G.W.A. Nyakairu et al. / Journal of African Earth Sciences 35 (2002) 123–134
129
posited by rolling as a transport mechanism. Climatic
changes that took place in the Pleistocene and Holocene,
with corresponding changes in water levels of Lake
Victoria (Bishop, 1969), may have led to clay deposition.
3.3. Chemical results
Tables 2 and 3 show the chemical and the normative
mineralogical composition of the samples, respectively.
The chemical data correlate with the mineralogical
composition and the silica and alumina contents agree
with the quartz and kaolinite contents (Fig. 8). The main
oxides are SiO2 , Al2 O3 , Fe2 O3 , and TiO2 , whereas MnO,
MgO, CaO, Na2 O, K2 O, and P2 O5 are present only in
small amounts. The SiO2 content in Kajjansi clay varies
inversely with the Al2 O3 content, and a relatively low
(<1 wt%) content of the alkali and alkali earth oxides is
apparent. The concentrations of Fe2 O3 vary between
3.11 and 12.2 wt%. Compared to the Kajjansi clays,
Kitiko clays have slightly higher SiO2 and Al2 O3 , and
lower Fe2 O3 contents. Considering the chemical composition (Table 2), Masooli clays have higher SiO2 ,
Al2 O3 , Fe2 O3 , and TiO2 contents compared to the other
deposits. The alkali contents are in the same range as the
other deposits. Chemically, there is a significant amount
Fig. 7. CM diagram for clay sample from Uganda, after Passega and
Pyramjee (1969). C ¼ one percentile; M ¼ median; fields I, II,
III ¼ rolled grains; VII ¼ suspension sediments and rolled ones which
are graded and uniform (Passega and Pyramjee, 1969).
According to the Passega and Pyramjee (1969) binary
plot of median, M, and one percentile, C, called a CM
diagram (Fig. 7), the sample data plotted within class
III, which is defined by Passega and Pyramjee as de-
Table 2
Chemical composition (wt%) of clays from deposits supplying the construction industry in Kampala, Uganda
Sample
SiO2
TiO2
Al2 O3
Fe2 O3
MnO
MgO
CaO
Na2 O
K2 O
P2 O5
CO2
LOI
Kajjansi deposit
Kaj-1
51.59
Kaj-2
54.33
Kaj-3
70.39
Kaj-4
55.26
Kaj-5
76.08
Kaj-A
69.07
1.29
1.17
1.67
1.38
1.02
1.32
22.91
20.62
16.41
23.96
12.29
14.88
9.68
12.20
3.11
6.09
3.27
5.19
0.04
0.05
0.04
0.03
0.03
0.05
0.57
0.13
0.05
0.24
b.d.
0.06
0.38
0.17
0.15
0.25
0.11
0.17
0.10
0.06
0.08
0.10
0.07
0.10
0.83
0.74
0.97
0.66
0.62
0.71
0.08
0.09
0.05
0.06
0.04
0.09
0.05
0.05
0.06
0.12
0.11
0.04
12.39
10.58
7.51
12.44
6.88
8.66
Kitiko deposit
Ks1-1
53.43
Ks1-2
58.08
Ks1-3
70.89
Ks1-4
70.88
Ks2-1
71.50
Ks2-2
55.25
Ks2-3
74.98
Ks-A
64.11
1.39
1.20
1.25
0.72
1.07
1.06
0.58
1.11
23.88
25.78
16.78
14.41
14.42
27.08
11.92
20.00
6.84
3.35
2.25
3.62
4.90
3.38
2.70
4.61
0.04
0.04
0.05
0.04
0.03
0.03
0.03
0.03
0.14
b.d.
b.d.
0.05
b.d.
0.04
b.d.
b.d.
0.16
0.09
0.09
0.20
0.07
0.16
0.18
0.13
0.12
0.05
0.05
0.08
0.07
0.14
0.08
0.07
1.01
0.80
1.00
0.70
0.81
0.63
0.56
0.72
0.25
0.10
0.03
0.04
0.14
0.04
0.03
0.13
b.d.
0.10
0.09
0.31
0.10
0.14
0.38
0.05
11.88
10.97
7.38
8.98
7.38
12.78
9.26
9.42
Masooli deposit
Mas-L
64.00
Mas-M
80.51
Mas-U
80.83
Mas-A
77.26
0.91
0.87
0.81
0.84
19.28
9.96
9.49
11.01
4.54
2.80
2.40
2.92
0.03
0.04
0.10
0.07
0.10
b.d.
b.d.
b.d.
0.14
0.05
0.06
0.08
0.13
0.12
0.10
0.11
0.99
1.09
0.98
0.98
0.04
0.03
0.04
0.04
0.02
0.03
b.d.
b.d.
9.72
5.14
5.66
6.71
Ntawo deposit
Ntw-1
64.65
Ntw-2
84.70
Ntw-3
82.80
Ntw-B
78.77
Ntw-G
74.79
Ntw-A
70.31
1.00
0.82
0.96
1.49
1.44
1.31
16.14
8.59
7.61
9.38
13.67
16.59
7.64
1.46
2.36
1.74
1.88
2.57
0.04
0.04
0.10
0.05
0.04
0.04
0.05
b.d.
b.d.
b.d.
b.d.
0.11
0.13
0.04
0.06
0.10
0.12
0.23
0.08
0.04
0.06
0.12
0.11
0.17
0.69
0.65
0.68
0.99
0.75
1.01
0.10
0.02
0.05
0.06
0.04
0.03
b.d.
0.06
b.d.
0.24
0.16
0.06
9.84
4.14
5.42
7.77
7.45
8.24
All Fe reported as Fe2 O3 ; LOI ¼ loss on ignition; b.d. ¼ below detection limit.
130
G.W.A. Nyakairu et al. / Journal of African Earth Sciences 35 (2002) 123–134
Table 3
Calculated normative mineralogical composition (wt%) of clays from deposits supplying the construction industry in Kampala, Uganda, after
method of Fabbri et al. (1986)
Sample
Illite
Chlorite
Quartz
Albite
Calcite
Hematite
Organic matter
Accessories
Kajjansi deposit
Kaj-1
48.9
Kaj-2
44.2
Kaj-3
31.1
Kaj-4
53.4
Kaj-5
24.4
Kaj-A
29.9
Kaolinite
9.8
8.7
11.4
7.8
7.3
8.4
2.5
0.6
0.2
1.0
22.6
29.0
49.8
25.7
60.8
50.4
0.8
0.5
0.7
0.8
0.6
0.8
0.7
0.3
0.3
0.4
0.2
0.3
10.0
13.4
3.4
6.4
3.6
5.7
b.d.
0.1
0.1
0.6
0.5
0.8
4.7
3.2
3.0
3.8
2.7
3.5
Kitiko deposit
Ks1-1
49.4
Ks1-2
56.7
Ks1-3
31.8
Ks1-4
28.8
Ks2-1
27.8
Ks2-2
61.4
Ks2-3
24.0
Ks-A
42.8
11.9
9.4
11.8
8.2
9.5
7.4
6.6
8.5
0.6
23.7
26.8
50.0
52.9
53.5
22.2
60.1
39.6
1.0
0.4
0.4
0.7
0.6
1.2
0.7
0.6
0.3
0.2
0.2
0.4
0.1
0.3
0.3
0.2
7.4
3.7
2.5
4.0
5.5
3.7
3.0
5.1
0.7
0.2
0.2
0.5
0.3
0.5
1.0
0.4
4.9
2.6
3.1
4.3
2.8
3.1
4.3
2.7
Masooli deposit
Mas-L
37.9
Mas-M
13.4
Mas-U
13.4
Mas-A
17.2
11.6
12.8
11.5
11.5
0.4
39.7
67.3
68.4
63.0
1.1
1.0
0.8
0.9
0.3
0.1
0.1
0.1
4.9
3.1
2.7
3.2
0.4
0.5
1.3
1.0
3.6
1.8
1.8
3.0
Ntawo deposit
Ntw-1
Ntw-2
Ntw-3
Ntw-B
Ntw-G
Ntw-A
8.1
7.6
8.0
11.6
8.8
11.9
0.2
44.6
73.8
73.0
66.4
57.6
49.0
0.7
0.3
0.5
1.0
0.9
1.4
0.2
0.1
0.1
0.2
0.2
0.4
8.4
1.6
2.6
1.9
2.1
2.7
0.3
0.3
1.5
2.5
0.6
0.1
4.1
1.4
2.4
3.4
3.4
3.2
33.3
14.8
11.9
12.9
26.3
30.8
0.3
0.2
0.2
0.5
Note: Simple normative calculations were based on the assumptions of: uniform mineralogical composition (i.e., all samples contain the same
minerals); MgO is used entirely to calculate a chlorite-like phase (Mg:Fe ¼ 1:1); the remaining iron oxide is converted to hematite; CaO is all supplied
by calcite; Na2 O is used to quantify albite; illite is calculated using all K2 O with reference to a hydromica composition (8.5 wt% K2 O); the remaining
Al2 O3 is converted to kaolinite; the remaining silica is quartz; organic matter is estimated by subtracting the CO2 contribution of calcite and
converting the remainder to organic carbon by multiplication with a factor of 1.7.
of SiO2 , but lower Al2 O3 in Ntawo clay compared to the
other deposits (Table 2). The alkali contents are in the
same range as for other deposits and these samples are
depleted in MgO.
Because the deposits are located in swamps, plants
extract K, Na, Ca, and Mg, and add organic compounds, such as tannic and humic acid. The relative
abundance of SiO2 indicates a rather high content
of quartz, whereas Al2 O3 can be correlated with clay
minerals and feldspars. Varying amounts of quartz influence the plasticity and drying behavior of the clays. A
relatively high iron oxide ðFe2 O3 Þ content provides
a characteristic reddish-brown color to the fired clay.
However, Fe2 O3 is not the only factor responsible for
the coloring of ceramic wares (Kreimeyer, 1987). Other
constituents such as CaO, MgO, MnO, and TiO2 can
appreciably modify the color of the fired clay. The temperature of firing, relative amounts of Al2 O3 , and the
furnace atmosphere all play an important role in the
development of color in the fired clay products (Fischer,
1984). The main effect of alkalis in clays is to reduce
their refractory temperature and, therefore, they are
fluxes (Bain, 1987). The loss on ignition (LOI) is an
important characteristic of clays. In addition to being a
vital determination in the routine chemical analysis of
ceramic materials, the total weight loss is also used as
a means of identifying some minerals and as a rough
means of estimating their percentage compositions in
various samples. In entirely kaolinitic clay the loss on
ignition (13.9 wt%) gives a good measure of the clay
content (Searle and Grimshaw, 1959).
X-ray diffraction (XRD) shows that the samples
contain mainly kaolinite, chlorite, and quartz (Fig. 8).
No iron oxides were detected by XRD, which means
that iron could be present in the form of amorphous
oxides or hydroxides, or is too low to be detected by
XRD. Kaolinite is more common than illite, whereas
chlorite and smectite are present only in traces. The
Kajjansi clays are characterized by a kaolinite range
from 24.4 to 53.4 wt%, little variation in the illite content (9 2 wt%), and quartz content between 22.6 and
60.8 wt%, with organic matter being present at less than
G.W.A. Nyakairu et al. / Journal of African Earth Sciences 35 (2002) 123–134
131
Fig. 8. X-ray diffraction patterns of bulk clay samples, Ks-A, Ntw-A, Kaj-A, and Mas-A. K ¼ kaolinite; Ch ¼ chlorite; Cl ¼ clay minerals;
Q ¼ quartz; and Fp ¼ feldspar.
1 wt%. All but one of the Kajjansi samples have low
chlorite content. The clay samples from the Kitiko deposit consist mainly of kaolinite (24–61.4 wt%), illite
ð9:5 2%Þ, quartz (22.2–60.1 wt%), and little chlorite
and organic matter. The Masooli clays have lower kaolinite content, but relatively higher illite and albite
contents, and much higher quartz contents compared to
the Kajjansi and Kitiko clays (Table 3). The clays in the
Ntawo deposit contain kaolinite content comparable to
that of the Masooli clays (Table 3). The illite content is
comparable to that of the Kajjansi and Kitiko clays.
These clays are rich in quartz, and have a low Fe2 O3 and
an appreciable amount of organic matter.
The absence of a significant amount of smectitic
minerals will ensure a ceramic body against possible
difficulties during drying. Sedimentary clays can contain
considerable amounts of organic matter, which influences the firing properties of clays (Robbins, 1984). The
abundance of organic matter is less than 1 wt% for all
samples, except for sample Mas-U, which has 1.3 wt%.
Organic matter may change the clay color to light gray,
blue, brown, or black. Colors, non-clay minerals, and
organic materials are similar in all four clay deposits; the
clays differ, however, in the type of the clay minerals.
4. Suitability for industrial applications
The ternary diagram of Fiori et al. (1989) distinguishes two groups of samples (Fig. 9): those with high
Fig. 9. Ternary diagram: quartz/carbonates + Fe-oxides + accessories + feldspars/clay minerals for the studied clays from Uganda, after
Fiori et al. (1989).
quartz contents and those that are rich in clay minerals.
On the other hand, these samples were classified into
two basic types according to their clay mineralogy
(Table 3): those containing kaolinite and chlorite, and
those containing only kaolinite. On the basis of these
results, and the criteria established by Fiori et al. (1989),
samples with the lowest clay fractions are most suitable
for manufacturing porous and white ceramic bodies.
The chemical data were plotted in a ternary diagram
(silica–alumina–other oxides), as used by Fabbri and
Fiori (1985) to classify raw materials and industrial
ceramic bodies (Fig. 10). This diagram shows ceramic
132
G.W.A. Nyakairu et al. / Journal of African Earth Sciences 35 (2002) 123–134
products (Konta, 1995). According to the results of
particle size analysis (Table 1), the raw materials cannot
be distinguished by grain size as they were distributed all
over the fields (Fig. 3). The data were grouped into three
fractions (>20 lm, 20–2 lm, and <2 lm) for plotting on
Winkler’s diagram to evaluate their suitability for different ceramic products (Winkler, 1954) (Fig. 11). The
diagram shows that most of the samples are rich in the
silt fraction, resulting in the need for special treatment
and processing to render them suitable to produce
structural ceramic products. From chemical and mineralogical analysis, the studied clay samples are ball
clays (cf. Robbins, 1984).
Fig. 10. Triangular diagram of Uganda clays: SiO2 =Al2 O3 =total oxides are plotted, where a ¼ red stoneware (Italy); b, b0 , b00 ¼ white
stoneware for German, English, and French industries, respectively
(data from Fabbri and Fiori, 1985).
compositional fields and reflects the overall chemical
composition of the Ugandan raw materials. In this diagram, some samples plot into the white bodies field, but
most of the samples do not. Taking into account the
ideal composition for an optimum white body product
according to the mentioned authors (SiO2 ¼ 72 wt%,
Al2 O3 and total oxides ¼ 8 wt%), samples outside the
white bodies field need processing in order to reduce
their iron oxide and dilute their quartz contents.
Due to their high iron oxide content, these Ugandan
clays cannot be used for the production of fine ceramics.
From their chemical composition, they could be considered as raw material for use in structural ceramic
5. Environmental aspects of brick production
Brick manufacture is associated with a number of
environmental implications, some of which are beneficial, and others which are potentially detrimental. Traditionally, bricks have been and are produced in
factories, which are sited adjacent to the source of the
clay. The life of the production site may extend beyond
the life of an associated quarry, or beyond individual
phases of quarrying, yielding large excavations in the
vicinity of the plant. These are ideal for consideration
as waste disposal sites, as the ‘bedrock’ is clay-rich, resulting in low permeability, and little or no upgrading
may meet the requirements of the waste disposal authorities. The loss on ignition of the chemical analysis
includes not only water and carbon dioxide but also
harmful species such as sulfur dioxide, chlorine, fluorine,
and nitrogen oxides, which escape as acid gases during
firing unless mineralogical reactions retain them in the
brick (Heller-Kallai et al., 1988). This is caused by the
decomposition of clay minerals, micas, organic matter,
and also iron sulfides that occur in subordinate amounts.
These volatiles have a harmful influence on the natural
environment (Eckhardt et al., 1990). In an assessment of
the potential to generate acid gases, it is important to
assess the composition not only of the raw materials but
also of the fired products, to ensure that their fate during
firing is known.
6. Conclusions
Fig. 11. Grain size classification of clay raw materials from Uganda in
Winkler’s diagram (cf. Winkler, 1954). Fields indicate: (I) common
bricks, (II) vertically perforated bricks, (III) roofing tiles and masonry
bricks, and (IV) hollow products.
Uganda offers considerable prospects for the exploitation of raw materials for the ceramic industry. The
studied clay deposits are derived from gneissic and
granitoid rocks of the basement, and from metamorphosed schists, as well as from amphibolites and basic
rocks of the Buganda series. The boundaries between
G.W.A. Nyakairu et al. / Journal of African Earth Sciences 35 (2002) 123–134
one clay type and another are commonly sharp, and it
can be concluded that the clay deposits are of lacustrine
and fluvial origin, respectively. The differences in the
nature of the clays are a function of progressive stages of
leaching or transport activities, as indicated by a CM
diagram (Fig. 7). The clays mainly formed by weathering of basement rocks during the tropical Pliocene and
accumulation of the detrital material in lakes and
swamps.
Overall, the clays studied here exhibit a wide range of
chemical and mineralogical compositions, and grain size
distributions. The clay raw materials fundamentally
consist of kaolinite, illite, and quartz, with rare feldspars
and minor chlorite. The content of iron oxide is generally high. An important role seems to be played by the
minor mineralogical components, which influence the
firing color. However, the characteristics of the samples
analyzed here are quite far from the requirements of
high-quality ceramics production. Even though all these
materials are currently exploited in the production of
various ceramic products, most of them are not comparable to commercially marketed European counterparts (Fig. 10). The Ugandan raw materials need
elaborate treatment to render them suitable for such use.
This may be largely due to these clays being of sedimentary origin, and due to their not having been
properly washed and sorted. A centralized clay mineral
dressing and preparation plant might be the best way to
overcome the raw materials problems.
Nevertheless, the studied samples show some interesting features for application in the ceramic sector if
well treated, especially considering the high iron oxide
content. However, due to the generally high quartz
contents they may possess a refractory behavior. These
clays have chemical and mineralogical compositions
that indicate their usefulness for brick, ceramic, and
earthenware production. Further systematic applied
testing of the clays has yet to be carried out to determine
their physical, mechanical, and technological properties.
Acknowledgements
We thank W.U. Reimold (Univ. Witwatersrand, Johannesburg) for XRF analyses, S. Gier (Inst. of Petrology, Vienna) for assistance with XRD and grain size
analysis, and M. Dondi (CNR-IRTEC, Faenza) for the
normative mineralogical analysis software. The authors
are grateful to the Austrian Academic Exchange Service
€ AD) for a Ph.D. stipend and partial financial assis(O
tance of fieldwork (to N.G.W.A.). Laboratory work was
supported by the Austrian FWF, project Y58-GEO (to
C.K.). We are grateful to D. Brandt and P. Eriksson for
helpful and constructive comments, which led to improvements in the manuscript.
133
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