Mineralogical, geochemical, and sedimentological characteristics of ...

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


(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 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


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

20 lm

2–20 lm