Geochemical Journal, Vol. 35, pp. 245 to 256, 2001
NOTE
The Buwambo kaolin deposit in central Uganda:
Mineralogical and chemical composition
GEORGE W. A. NYAKAIRU,1 C HRISTIAN KOEBERL1* and H ANS K URZWEIL2
1
Institute of Geochemistry, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria
2
Institute of Petrology, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria
(Received February 12, 2001; Accepted June 20, 2001)
Kaolin from the Buwambo deposit, located about 27 km north of Kampala (Uganda), has been analyzed
for its mineralogical and chemical composition. The kaolin is derived from granite of the basement, which
is exposed due to deeply weathered Buganda-Toro cover rocks. Kaolinite is the dominant mineral, with
quartz and muscovite/illite as accessory minerals. Chemical data show that the kaolin is composed mainly
of SiO2 and Al 2O3, with the other oxides being present in trace amounts. The depletion in Ti, Fe, Mn, Mg,
Ca, Na, and K not only shows the extent of the kaolinization, but also gives Buwambo kaolin its almost
white color. The kaolinization and weathering processes have enriched Ni and depleted other trace element contents in the Buwambo kaolin. The chondrite-normalized rare earth element (REE) patterns show
enrichment in the light REEs, with a negative Ce anomaly. The REE pattern and the content of the other
trace elements, show evidence of alteration and weathering processes related to kaolinization. The mineralogical and chemical compositions indicate that the kaolin is suitable for industrial use.
its occur, their industrial use is restricted to manufacturing of bricks and pottery (Nyakairu and
Kaahwa, 1998, and references therein). However,
small quantities of kaolin are locally used as white
wash or “enoni” on the mud and wattle dwellings.
Apart from reconnaissance work by Harris (1946)
and Pekkala and Katto (1994), no research has
been conducted on the Buwambo and similar kaolin deposits. Kaolin from the Migade deposit was
selected for comparison purposes, as its kaolin is
also derived from weathering of the granite. The
Migade hill, on which the Migade kaolin deposit
is located, is about 2 km west of Buwambo. The
primary focus of this study is to describe the occurrence, and to present data for chemistry and
mineralogy of the Buwambo kaolin deposit in order to evaluate the viability of the deposit. Possible applications in industry are suggested.
I NTRODUCTION
In the last decade, the kaolin industry has encountered difficult times due to over-production
in Europe and the USA (Roskill, 1996). The emergence of major new high-quality kaolin sources
in Brazil and Australia, and the constantly increasing supply of low-grade materials, has also affected the market. However, with increasing and
new industrial applications of kaolin, any strategically located promising deposit is worth exploiting, especially so in the developing world.
Kaolin is one of the most valuable, versatile,
and widely used industrial minerals. Although
most important in papermaking, kaolin is used
extensively in the ceramics, rubber, paint, plastics, and pharmaceutical industries (Murray, 1991;
Bundy, 1993). In Uganda, although kaolin depos-
*Corresponding author (e-mail: [email protected])
245
246
G. W. A. Nyakairu et al.
G EOLOGY
The location of the Buwambo kaolin deposit
is shown in Fig. 1. The Buganda-Toro System
comprises a thin cover of overburden above this
deposit. Rocks of the Buganda-Toro System, composed dominantly of cleaved, fine-grained sandstone, slates, phyllites and schists, quartzites,
muscovite-biotite gneisses, and subordinate schist
that locally contain cordierite, overlie the
Buwambo area. A large proportion of the rocks of
the Buganda-Toro System, around the Buwambo
area, have been eroded, exposing Basement rocks,
which consist of undifferentiated gneisses with
some later granites associated with some
migmatized Buganda-Toro rocks. The Buwambo
Fig. 1. Generalized geological map, showing the location of Buwambo and Migade kaolin deposits, after the
geological map of Uganda, Kampala sheet NA 36-14 (Geological Survey, Uganda, 1962). The inset is a map of
Uganda with the general location of the clay deposits.
The Buwambo kaolin deposit in central Uganda
kaolin was, therefore, derived from weathered
granites of the Basement that were exposed when
the Buganda-Toro rocks were removed by erosion.
Granites weather readily to kaolinite and quartz
under favorable conditions, which include high
rainfall, rapid drainage, tropical climate, a low
water table, and an adequate water supply to leach
the soluble components (Murray and Keller,
1993). Kaolin is, in general, derived from altered
feldspars and muscovite. Harris (1946) showed
that the rock from which the feldspar, which altered to the Buwambo kaolin, was a mediumgrained granite, with 60 vol% feldspar. The
kaolinized rock has been discolored by ground
water and a relatively small proportion of it remains white.
Description of Buwambo kaolin deposit
The Buwambo kaolin deposit is located about
27 km north of Kampala along the Kampala-Gulu
road, less than 1 km to the east of the main road
within the Bombo area (Fig. 1). The deposit lies
on top of the Buwambo hill Namakonkomi vil-
247
lage, at an average altitude of 1290 m. The kaolin
deposit, an open cast trench, is situated some 12
m below the summit on the slope west of the hill.
The deposit is at longitude 32°31′54″ east and latitude 0°31′23″ north on map Series Y 732 Sheet
61/III (Publ. by the Uganda Lands and Survey
Department, 1959). Presently, open pit-mining
activities are carried out below a 0.5–1.5-m-thick
soil cover. Kaolinitized rock is visible in an area
of at least 50 m by 100 m across, but it may cover
a much larger area. The kaolin reserve at
Buwambo was estimated to be at least 50000 m3
by Pekkala and Katto (1994), but could be much
larger.
Field observation
The Buwambo hill as a whole is a granite,
weathered and partly kaolinitized on top. As a
whole, the occurrence is composed of quartz, kaolin, feldspar, and muscovite, with secondary iron
hydroxide staining in fissures and cavities, which
are more frequent within the topmost 2 m. Muscovite mica is scattered through the kaolin in tiny
Fig. 2. Representative X-ray diffraction patterns of bulk Buwambo kaolin sample (BW-1) in comparison with
kaolin from the Migade (MG-1) deposit. K = kaolinite; Q = quartz; I/M = muscovite/illite and F = feldspar. The
samples are mainly composed of kaolinite, quartz, and muscovite/illite.
Major element data in wt.%; mineralogical composition in wt.%; all Fe reported as Fe2 O3; LOI = loss on ignition; data for U.K. (England) kaolin from Murray and Keller
(1993); pure kaolin data from Newman and Brown (1987).
Table 1. Chemical and semi-quantitative mineralogical compositions of kaolins from the Buwambo and Migade deposits, central Uganda. The
mineralogical composition calculations were performed from XRD spectra using a modified method after Schultz (1964).
248
G. W. A. Nyakairu et al.
249
The Buwambo kaolin deposit in central Uganda
flakes. Quartz also occurs in veins, 2–10 cm wide
and as lenses up to 0.5 m across, it is mostly
grayish in color. The kaolin body is intimately
penetrated by reddish veins, which probably mark
groundwater passages.
SAMPLES AND METHODS
For this study, 12 kaolin samples from
Buwambo and 6 from Migade were analyzed. Six
Buwambo and three Migade samples were selected
for X-ray fluorescence (XRF) spectrometry analysis. The samples were collected at an average
depth of 4 m from the top of the open pit and at a
regular spacing of a few meters horizontally. Sample preparation, as well as major and trace element analysis, by X-ray fluorescence spectrometry
and instrumental neutron activation analysis, and
mineralogical studies by X-ray diffraction, were
done as described by Nyakairu and Koeberl
(2001).
RESULTS
Mineralogy
The main minerals identified by XRD in the
kaolin deposits are kaolinite [Al 2Si 2 O 5(OH) 4 ],
quartz and illite/muscovite, with minor amounts
of feldspars (Fig. 2; Table 1). Kaolinite alone comprises about 74–93 wt.% of all analyzed samples.
The similarity in mineralogical composition of the
Buwambo and Migade kaolins shows that these
two deposits were probably derived from the same
source rocks, and also may indicate a similar extent of kaolinization. The low feldspar content
shows the weathering extent source rocks have
undergone (cf., Mongelli and Moresi, 1990).
Geochemistry
Major elements Table 1 shows the chemical composition of selected kaolin samples from the
Buwambo and Migade deposits. For comparison
compositional data of kaolin from the U.K.
Fig. 3. Plots of: (a), (b) major element oxides; (c), (d) trace elements of Uganda kaolin samples normalized to
Singo granite (data from Nagudi et al., 2000). The kaolins all show alumina enrichment and depletion in the other
oxides. Note the Ni enrichment in all kaolins.
250
G. W. A. Nyakairu et al.
(Murray and Keller, 1993) are included. The Singo
granite (Nagudi et al., 2000) occurs in western
central Uganda and was derived from the Basement rocks, hence, is assumed to be similar in
composition to the granite rocks from which the
kaolins studied were derived, and is the only such
rock formation in Uganda for which detailed major and trace element data are available. The major element distribution reflects the mineralogy of
the kaolin samples. The kaolin samples, are characterized by low SiO2 (values ranging between
46.4 to 52.4 wt.%) and high Al 2O3 (values ranging from 32.7 to 38.3 wt.%). The Fe2O3 , MnO,
MgO, and K2O contents of the Buwambo kaolin
are similar to those of the Migade and U.K.
kaolins. The presence of minor amounts of CaO
and Na2 O in the U.K. kaolin indicates that the
Buwambo kaolin is more weathered.
Relative to the Singo granite, SiO 2 , MnO,
MgO, and, especially TiO2 and Fe2 O 3, are de-
pleted, and Al 2 O3 is enriched in the kaolins (Figs.
3(a) and (b)). Sodium and CaO are completely lost
from the Buwambo and Migade deposits, whereas
K2O is only moderately depleted and correlates
with the illite content.
Trace elements Compared to the Singo granite,
the trace element abundances show variability and
some depletion (Figs. 3(c) and (d)). Scandium, Cr,
Co, Zn, Rb, Sr, Y, Zr, Nb, Sb, Cs, Ba, and U show
concentrations below the abundances for the Singo
granite, probably due to weathering and
kaolinization processes. This is in agreement with
studies by Nesbitt et al. (1980) and Wronkiewicz
and Condie (1987), who conclude that smaller
cations, such as Na, Ca, and Sr, are selectively
leached from weathering profiles, whereas cations with relatively large ion radii, such as K, Cs,
Rb, and Ba, have been fixed by preferential exchange and adsorption on clays. Chromium, Co,
Ni, V, Sc, and Cs are mainly concentrated in clay
Fig. 4. Rare earth element plots of Uganda kaolin samples normalized to (a), (b): Singo granite, data from
Nagudi et al. (2000) and (c), (d): C1 chondrite data from Taylor and McLennan (1985). The kaolins show a
general LREE enrichment with a Ce negative anomaly.
Trace element data in ppm; Singo granite data from Nagudi et al. (2000); n.d. = not determined; Ce/Ce* = Cecn/((La cn2/3)·(Ndcn 1/3)) where subscript cn = chondrite normalized.
Table 2. Trace element composition of kaolins from Buwambo and Migade deposits, central Uganda
The Buwambo kaolin deposit in central Uganda
251
252
G. W. A. Nyakairu et al.
Fig. 5. Ternary SiO 2/Al 2O 3/total oxides diagram of
Uganda kaolins compared to several European commercially marketed kaolins (data from Ligas et al.,
1997).
minerals, whereas Ni, Co, and especially Cr other
factors may control their distribution (Nesbitt,
1979). Nickel is enriched in all the kaolin samples. In terms of trace element contents, the two
kaolin deposits are very similar. The differences
between the trace element contents in the
Buwambo and Migade deposits are so small that
it seems that the deposits were derived from parent rocks with similar chemical and mineralogical compositions.
The effects of kaolinization and weathering are
evident in the variable total REE contents of the
kaolins (Table 2). The REE abundances normalized to Singo granite and chondritic values are
shown in Figs. 4(a)–(d). The kaolins in general
show uniform REE patterns, being more enriched
in LREE compared to the HREE. The kaolin samples show a distinct negative Ce anomaly, which
is a result of weathering (see below).
SUITABILITY FOR CERAMIC APPLICATIONS
An assessment of possible ceramic applications
of Buwambo and Migade kaolins, was carried out
by comparing its composition with the kaolins
commercially marketed in Europe (cf., Ligas et
al., 1997). Figure 5 shows a comparison of the
Fig. 6. Ternary diagram: quartz/kaolinite/other minerals for the Uganda kaolins compared with the kaolins
commercially marketed in Europe. The Uganda kaolins
fall in the field of the Bretagne and Bavaria kaolins.
Ugandan kaolin major element composition with
those of commercially used kaolins from Europe.
This plot indicates that the Uganda kaolins have a
higher sand component compared to those from
Latium (Italy). In contrast, the mineralogical composition diagram (Fig. 6) shows a closer similarity to high quality kaolins from Germany and the
U.K. This comparison indicates that the Uganda
kaolins have a potential for high quality industrial use.
DISCUSSION
The mobility of chemical elements in natural
environments depends largely on their redox potential and the pH of water. Chemical weathering,
such as kaolinization of rocks, usually starts in a
weakly acidic environment, but liberation of alkalis and alkali earths’ results in a rapid increase
of the pH. Alkalis and alkali earths are quickly
removed from the kaolinization environment, thus
the last stages of kaolinization proceed in acid
environment. This change, from slightly acid to
strongly alkaline, and finally to acid conditions
is, of primary importance for the migration of several chemical elements in the course of
kaolinization. Progressive weathering and
The Buwambo kaolin deposit in central Uganda
kaolinization of the granite lead to decomposition
of all minerals except quartz in the Buwambo deposit. The decrease in the SiO2 content relative to
the Singo granite shows an advanced stage of
kaolinization for the Buwambo kaolinite, as the
solubility of quartz increases in an alkaline environment (Garbarino et al., 1994). Successive decreases in the SiO2 , MnO, MgO, P2O5 , and more
so in the Fe 2O3, TiO2 , CaO, Na2 O, and K2O concentrations shows their higher mobility. The reduction in these oxides is compensated by enrichment in Al2 O3 and higher LOI.
The presence of TiO 2, Fe2O 3, and MnO has a
negative effect on the quality of Buwambo kaolin. In kaolins, Ti occurs in small amounts, mainly
in the form of anatase, which in finely dispersed
form is difficult to determine by XRD (Fischer,
1984). However, during weathering and
kaolinization small amounts of Ti enter the
kaolinite structure (Murray and Keller, 1993) and
it enhances the coloring effect of iron in kaolins
(Fischer, 1984). In the Buwambo kaolin, iron occurs in form of hydrated oxides adsorbed on small
kaolin grains and in secondary iron minerals
formed in the course of weathering and
kaolinization. In small amounts iron enters the
kaolinite structure and replace aluminum in the
octahedral layer (Murray and Keller, 1993). The
MnO content in Buwambo kaolin may be correlated to biotite, hornblende, and, to some extent
in other iron-bearing minerals. During weathering and kaolinization, Mn is easily oxidized to
Mn4+ and immobilized in the form of oxides, along
with the rise in pH. In a high reducing environment, Mn enters the structure of carbonates, in
which Mn2+ may replace Fe2+. However, in an acid
environment, Mn is leached during kaolinization.
Similarly, trace element contents in both
Migade and Buwambo kaolins decrease with increasing extent of weathering and kaolinization
(Fig. 3) compared to the Singo granite. The higher
concentration of Ni in the kaolin samples may be
due to the mobilization of iron oxide. Trace elements are liberated from primary structures during granite weathering and kaolinization processes
and are subsequently removed from the rocks.
253
Leaching of these elements in Buwambo deposit
proceeded at varying rates, depending on the resistance of the elements-bearing minerals to
weathering, kaolinization, and on physico-chemical conditions (e.g., pH, Eh). Assuming that weathering proceeded under similar chemical conditions
in the Buwambo and Migade deposits, the difference in the elements composition in the kaolin
deposits resulted by kaolinization process. Strontium is more concentrated than Ba in all samples.
Strontium is mostly bound in plagioclase, where
it replaces Ca. Barium has a behavior similar to
that of K, and is mostly concentrated in Kfeldspars and micas. Both Ba and Sr are easily
mobilized during weathering and are readily removed from the environment, but leaching of Sr
starts much earlier and is more intense. Calcium
and K are almost completely removed by weathering, but significant amounts of Sr and Ba, which
have a close affinity to these elements, remain in
the kaolin. Presumably Sr and Ba are partly
adsorbed by clay minerals or were added to a small
extent by downward percolating solutions.
Potassium, Rb, and Cs are mainly hosted in
micas and K-feldspar (Heier and Billings, 1970),
thus alteration of these minerals will dominate the
fractionation of these elements. Rubidium has a
trend comparable with that of Nb and V and it is
less mobile than K, whose behavior it generally
follows. This has been explained in that Rb, in
contrast to K, is preferentially retained in weathered illite (Garrels and Christ, 1965), but Nb and
V are less mobile (Middelburg et al., 1988). The
concentration of these elements in the Buwambo
kaolin is controlled by the phyllosilicates. Feng
and Kerrich (1990) noted that Cr, Co, Ni, Ti, and
V behave similarly during magmatic processes,
but cautioned that they may be fractionated during weathering. The high Ni content in the kaolins
is related to its being easily mobilized during
weathering (cf., Turekian, 1978). Scandium has
been slightly concentrated in the kaolin during
kaolinization compared to other trace elements.
The tendency of Cr to be removed is probably related to Cr4+ oxidation, soluble as chromate ion
(Marsh, 1991). Thorium and U behave differently
254
G. W. A. Nyakairu et al.
during weathering, in that U, unlike Th, is chemically mobile as U6+ and is precipitated as insoluble U4+ compound. Thorium and U are mainly
accommodated in some heavy minerals, such as
zircon, which are resistant to weathering. The low
trace elements contents in the Buwambo kaolin
samples in comparison to Singo granite can be
accounted by their clay mineralogy with noticeable quantities of kaolinite, in addition to the fact
that these elements are preferentially fixed by illite
as K2O reveals. However, the kaolinite percentages in kaolin samples constitute an additional
evidence of intensity of weathering and
kaolinization extent.
The Singo granite-normalized REE patterns
(Fig. 4) show LREE enrichment, a slight positive
Eu and a negative Ce anomalies, and HREE depletion in the Buwambo kaolin with the exception of BW-3 and BW-9 samples showing higher
contents. Compared to Buwambo kaolin, Migade
kaolin shows no Eu anomaly. The chondrite-normalized REE patterns of the kaolins are identical
with LREE enrichment, a negative Ce anomaly
and HREE depletion with samples BW-3 and BW9 showing higher contents. However, Migade kaolin shows more REE contents than Buwambo samples. The LREE enrichment reflects the results of
extreme weathering while the slight positive Eu
anomaly is due to plagioclase or orthoclase presence in the kaolin samples. Weathering causes
fractionation between LREE-HREE and between
Ce and other REE. The REE are released from
primary minerals and taken up by the secondary
phases during kaolinization. The REE are leached
by local meteoric waters from the upper levels of
the profiles and concentrated in deeper zones,
probably in the form of complexes, or by mechanical transport of secondary phases (clay minerals
and oxides) (cf., Prudencio et al., 1989). However,
Ce has a different behavior, as it may be partly
oxidized to Ce4+, which is retained in the upper
levels of the soil profile relative to the other trivalent REE. This accounts for the observed negative Ce anomalies displayed in the Buwambo kaolin samples. Although kaolinite accommodates the
REE (Prudencio et al., 1989), and particularly the
LREE (Nesbitt, 1979), a basic pH environment is
also known to fractionate LREE-HREE, as the
HREE are preferentially retained in solution,
where they form soluble complexes (Cantrell and
Byrne, 1987).
However, some trace elements and the LREEs
are extremely concentrated in the kaolin samples.
For example, BW-3 and BW-9 shows extraordinarily high concentrations of Sr, Ba, Zr, Y, LREE,
and P2O5 . The high concentrations of the LREEs
and P2O 5 in these samples strongly suggest control by phosphate minerals, such as apatite and
monazite, during weathering and kaolinization
processes. The high Zr and Y contents are associated with heavy minerals such as zircon although
the zircon content hardly explains the low abundance of Hf. The high level of Sr in the samples is
due to the uptake of Sr in magnesite and aragonite lattice. High Ba content is due to the presence of feldspar and clay contents. However, their
distributions are not only controlled by zircon, clay
minerals, or phosphates, but a rather combination
of these minerals. The LREEs enrichments in the
Migade kaolin show that the Migade kaolin has
undergone more weathering and has been
kaolinitized to a greater extent.
C ONCLUSIONS
Kaolinite is the dominant mineral in the
Buwambo and Migade kaolins, followed by
quartz, with muscovite/illite as accessory minerals. The chemical composition of the kaolin is
essentially SiO2 and Al 2O3, with minor amounts
of TiO 2, Fe2 O3, MnO, MgO, K2 O, and P2O5 . The
decrease in SiO2 content relative to the Singo granite indicates kaolinization in alkaline environment,
and Al2O3 has remained immobile. As TiO2, Fe2O3,
MnO, and MgO are known to negatively affect
the kaolin color, the Buwambo and Migade kaolins
have almost a white color. The loss in MgO, CaO,
Na 2 O, and K 2 O indicate the extent to which
kaolinization of the Buwambo kaolin has undergone. The kaolin samples from Buwambo are quite
pure and are virtually equal to the chemical composition of pure kaolinite. From chemical and
The Buwambo kaolin deposit in central Uganda
mineralogical composition, Buwambo and Migade
kaolins are suitable for the production of earthenware, sanitary, white ware, and electrical insulators production.
Compared to average source rock abundances,
e.g., the Singo granite, the Buwambo and Migade
kaolins are depleted in all trace elements and only
Ni shows an enrichment. Assuming uniform
weathering conditions, Ni has been concentrated
by the kaolinization process. The Singo granite
normalized REE patterns shows LREE enrichment, with a slight positive Eu and a negative Ce
anomalies for the Buwambo kaolin. The chondrite
normalized REE patterns, however, show little
change, except a slight LREE enrichment and a
negative Cerium anomaly. The Migade kaolin has
undergone a higher degree of weathering and
kaolinization than the Buwambo kaolin.
Acknowledgments—The authors are grateful to the
Austrian Academic Exchange Service (ÖAD), which
provided a Ph.D. stipend and financial assistance for
fieldwork to G.W.A.N. We acknowledge W. U. Reimold
(University of Witwatersrand, Johannesburg) for XRF
analyses, and S. Gier (Institute of Petrology, Vienna
University, Vienna) for assistance with XRD analysis.
The laboratory work was supported in part by the Austrian FWF, project Y58-GEO (to C.K.). We are grateful to Profs. K. Suzuki and J. Matsuda, as well as two
anonymous reviewers, for constructive comments on
the manuscript.
REFERENCES
Bundy, W. M. (1993) The diverse industrial applications of kaolin. Kaolin Genesis and Utilization
(Murray, H. H., Bundy, W. and Harvey, C., eds.), Clay
Min. Soc., Spec. Publ. 1, Boulder, 43–47.
Cantrell, K. J. and Byrne, R. H. (1987) Rare earth element complexation by carbonate and oxalate ions.
Geochim. Cosmochim. Acta 51, 597–606.
Feng, R. and Kerrich, R. (1990) Geochemistry of finegrained clastic sediments in the Archean Abitibi
greenstones belt, Canada: implications for provenance and tectonic setting. Geochim. Cosmochim.
Acta 54, 1061–1081.
Fischer, P. (1984) Some comments on the color of fired
clays. ZI, Ziegelind. Int. 37(9), 475–483.
Garbarino, C., Masi, U., Padalino, G. and Palomba, M.
(1994) Geochemical features of the kaolin deposits
255
from Sardinia (Italy) and genetic implications. Chem.
Erde. 54, 213–233.
Garrels, R. M. and Christ, C. L. (1965) Solutions, Minerals, and Equilibria. Freeman and Cooper, San Francisco, California, 450 pp.
Geological Survey, Uganda (1962) Geological map of
Kampala. Sheet NA 36-14.
Harris, N. (1946) A note on kaolin supply near Bombo.
Report Geological Survey and Mines, Uganda, 2 pp.
Heier, K. S. and Billings, G. K. (1970) Rubidium. Handbook of Geochemistry (Wedepohl, K. H., ed.), 37B1–
37N1, Springer, Berlin.
Ligas, P., Uras, I., Dondi, M. and Marsigli, M. (1997)
Kaolinitic materials from Romana (north-west Sardinia, Italy) and their ceramic properties. Appl. Clay
Sci. 12, 145–163.
Marsh, J. S. (1991) REE fractionation and Ce anomalies in weathered Karoo dolerite. Chem. Geol. 90,
184–194.
Middelburg, J. J., van der Weijden, C. H. and Woittiez,
J. R. W. (1988) Chemical processes affecting the
mobility of major, minor, and trace elements during
weathering of granitic rocks. Chem. Geol. 68, 253–
273.
Mongelli, G. (1995) Trace elements distribution and
mineralogical composition in the <2 µm size-fraction of shales from southern Apennines (Italy). Mineral. Petrol. 53, 101–114.
Mongelli, G. and Moresi, M. (1990) Biotite-kaolinite
transformation in a granitic saprolite of the Serre
(Calabria, southern Italy). Miner. Petrogr. Acta 33,
273–281.
Murray, H. H. (1991) Overview: Clay mineral application. Appl. Clay Sci. 5, 379–395.
Murray, H. H. and Keller, W. D. (1993) Kaolins, kaolins
and kaolins. Kaolin Genesis and Utilization (Murray,
H. H., Bundy, W. and Harvey, C., eds.), Clay Min.
Soc., Spec. Publ. 1, Boulder, 1–24.
Nagudi, B., Koeberl, C. and Kurat, G. (2000) Petrography and geochemistry of the Singo granite, Uganda:
Interpretations and implications for the origin. J. Afr.
Earth Sci. 30, 65–66.
Nesbitt, H. W. (1979) Mobility and fractionation of rare
earth elements during weathering of a granidiorite.
Nature 279, 206–210.
Nesbitt, H. W., Markovics, G. and Price, R. C. (1980)
Chemical processes affecting alkalis and alkali earths
during continental weathering. Geochim.
Cosmochim. Acta 44, 1659–1666.
Newman, A. C. D. and Brown, G. (1987) The chemical
constitution of clays. Chemistry of Clays and Clay
Minerals (Newman, A. C. D., ed.), MonographMineralogical Society, 6, 1–128, Mineralogical Society, London, U.K.
256
G. W. A. Nyakairu et al.
Nyakairu, G. W. A. and Kaahwa, Y. (1998) Phase transitions in local clays. Amer. Ceram. Soc. Bull. 77(6),
76–78.
Nyakairu, G. W. A. and Koeberl, C. (2001) Mineralogical and chemical composition and distribution of
rare earth elements in clay-rich sediments from central Uganda. Geochem. J. 35, 13–28.
Pekkala, Y. and Katto, E. (1994) Reconnaissance field
trip to the Namasera and Buwambo kaolin occurrences and Kitiko clay deposit near Kajjansi. Report
Geological Survey and Mines, Uganda, 4 pp.
Prudencio, M. I., Figueiredo, M. O. and Cabral, J. M.
P. (1989) Rare earth distribution and its correlation
with clay mineralogy in the clay-sized fraction of
Cretaceous and Pliocene sediments (central Portugal). Clay Miner. 24, 67–74.
Roskill Consulting Group (1996) The Economics of
Kaolin. 9th ed., Roskill Information Services, London, 320 pp.
Schultz, L. G. (1964) Quantitative interpretation of
mineralogical composition from X-ray and chemical
data for the Pierre shale. U.S. Geol. Surv. Prof. Pap.
391-C, 33 pp.
Taylor, S. R. and McLennan, S. H. (1985) The Continental Crust: Its Composition and Evolution.
Blackwell, Oxford, 312 pp.
Turekian, K. K. (1978) Nickel-Behavior during weathering. Handbook of Geochemistry (Wedepohl, K. H.,
ed.), Vol. II, Sect. 28-G-1, Springer, Berlin.
Wronkiewicz, D. J. and Condie, K. C. (1987)
Geochemistry of Archean shales from the
Witwatersrand Supergroup, South Africa: source-area
weathering and provenance. Geochim. Cosmochim.
Acta 51, 2401–2416.