The results on the elemental and mineralogical compositions of clays from Central Uganda differed from those from the volcanic sediments of the Mt. Elgon in Eastern Uganda. Utilisation of the two types of clays should be strict after understanding their structural differences. Whereas elemental, mineralogical, DTA, IR, XRD and pH data on selected clays from Kumi, Nakawa, Seeta, Kajansi, Kawuku, Lwanda, Chodah and Umatengah indicated that they were kaolinites. Similar data on clays from Mutufu, Budadiri, Chelel and Siron indicated that they were largely smectites. The IR data accumulated on Kawuku, Kajansi, Lwada, Seeta, Chodah, Umatengah, Kumi and Nakawa clays revealed they were largely kaolinites yet that on Mutufu, Chelel, Budadiri and Siron clays indicated they were smectite-rich.
Clays are collectively called alumino-silicates, they contain aluminium oxide and silicon dioxide as universal minerals; and clays were classified into phyllosilicates and layers silicates [
Clays consist of small particles of dimension 3 mm diameter and are adsorbents used to filter or remove solids and color in oils [
The kaolinite has a single tetrahedral silica sheet and single octahedral alumina sheet, a combination which repeats itself indefinitely. The crystal structure consists of unit layers, which are stack on one another and held together finally by hydrogen bonding among the hydroxide ions of the octahedral sheet of one layer and the oxygen of the tetrahedral sheet of the adjacent layer. Kaolinite is a clay mineral with the chemical composition Al2Si2O5(OH)4 or Al2O3·2SiO2·2H2O [
Halloysites exist in two states of hydration: hydrated and metahalloysite, and are typically formed by hydrothermal alteration of alumino-silicate minerals [
The illite is a group of micas of the igneous and metamorphic rocks that has a unit layer composed of octahedral sheets sandwiched among tetrahedral sheets. Presence of potassium ions among these sheets results in increased thickness of the layer. The chemical formula is given as (K, H3O)(Al, Mg, Fe)2(Si, Al)4O10[(OH)2,(H2O)] but there is considerable ion substitution [
The montmorillonites are made of an alumina octahedral sheet between two silica tetrahedral sheets. Nontronite is the iron (III) rich member of the smectite group of clay minerals. Nontronites typically have a chemical composition consisting of more than ~30% Fe2O3 and less than ~12% Al2O3 ignited basis. A typical structural formula for nontronite is Ca5(Si7Al8Fe2)(Fe3.5Al4Mg.1)O20(OH)4. The dioctahedral sheet of nontronite is composed mainly of trivalent iron (Fe3+) cations, although some substitution by trivalent aluminium (Al3+) and divalent magnesium (Mg2+) does occur. The tetrahedral sheet is composed mainly of silicon (Si4+). On the basis of relative percentages of aluminium, silicon and alkali metals or alkaline earth metals, the clays studied can be asserted to satisfy the formulae and structures [
Clay are phyllosilicate minerals that impart plasticity to clay and which harden upon firing or drying [
The smectites are broadly dioctahedral smectites like montmorillonite and nontronite and the trioctahedral smectites like saponite and hectorite [
Geologically, bentonites transform to illites and kaolinites under hot wet conditions. However, hydrothermal alteration decreases the smectite content via replacement by kaolinite or halloysite and precipitation of various silica polymorphs, carbonates, sulphates and sulphides [
Diagenesis refers to the process and the changes (usually excluding cation exchange) that take place in sediments after deposition [
Kaolinites may form at the expense of granitoids, mainly granite rocks by means of in-situ alteration of feldspars on acid igneous rocks; hydrothermal alteration in hot wet climate of granite, quartz diorites, granodiorites, and rhyolites; and/or deuteric alteration of igneous materials involving reactions of vapors and gases with igneous mass [
Some Central Uganda clays have been excavated, analyzed and classified; kaolinite deposits were found at Namasera Hills, Kajansi, Katiko, Kitetika, and Ntawo [
To determine the elemental and mineral compositions of some selected clays of Uganda in order to establish their geological origin.
Samples of clay were collected from volcanic and non-volcanic areas in Central and Eastern Uganda. The clays were dug from virgin mines except for Kajansi, Seeta, Budadiri and Lwanda where clay mines were operational. The samples were collected at depths in range of 35 - 150 cm from the surface to reduce the effect of weathering and contamination.
Clay samples were collected from:
a) Samples of clay were mined from Kajansi, Seeta, Lwanda, Nakawa, Kawuku, Umatengah, Chodah, located in non-volcanic areas of Uganda;
b) Samples of clay were mined from Mutufu, Budadiri, Siron and Chelel in areas of past volcanicity of Eastern Uganda near Mount Elgon.
Raw samples of clays were separately soaked in distilled water, sieved to pass through a mesh 5.3 × 10−4 m diameter, dried at 105˚C and ground to powder using a rolling mill. The clay powders were stored for future use in desiccators.
The chemical analyses of elements like aluminium, iron, calcium, sodium and potassium in clays were carried out three times for every selected sample used in the study and done by decomposition using sodium carbonate fusion method [
The mineralogy of clays was determined using X-ray Powder diffraction (Philips diffractometer with PW1710 control unit operating at 40 kV and 30 mA using the Ni-filtered Cu Ka radiation). The diffractograms were automatically matched with JCPDS-cards in the computerized XRD CD-rom. Bulk mineralogy was studied with randomly oriented air-dried samples [
The clay powder (3 mg, 0.01 mmol) was mixed with KBr (100 mg, 0.08 mmol) ground to powder and pressed into discs. The infrared spectra were run using the KBr discs using B10RD FT540 Fourier Transform IR spectrometer in the frequency range of 3700 - 400 cm−1 [
A sample of clay (0.5 g, 1.2 mmol) together with inert reference sample of calcined alumina (0.5 g, 5 mmol), were placed in nickel block cavities and heated in parallel to 1100˚C at the rate of 12˚C per minute. The changes in mass and temperature of the sample against the reference were recorded in a table and drawn in graph.
14 samples of clay were collected from the regions containing mainly kaolinite and six samples from regions suspected to contain smectites. In all cases the clays were dug from virgin mines except for Kajansi, Seeta, Budadiri and Lwanda where clay mines were operational. The samples were collected at depths in range of 35 - 150 cm from the surface to minimize the effect of weathering and contamination.
The clay deposits sampled were observed to be stratified and most of them are composite, containing more than one distinct kaolinite or smectite mineral in the different horizons (shown in
It has been observed that the upper horizon for clay deposits in Central Uganda is black due to presence of organic matter and humus presumably brought into the swamps by surface water wash off of the top soil up stream. The upper horizons of the stratigram for the clay deposits from region associated with magmatic volcanism (
The upper horizons in clays from non-volcanic deposits do not show the orange or brown color because rainwater and/or fulvic or humic acids [
The clay samples showed that alteration of the parent rock is complete. The geological and spatial characteristics indicated that the deposits were formed by alteration probably of quartz, quartz diorites, dickites in acid conditions for clays in horizons in
All clays studied had high content of quartz and this leads to the conclusion that there is downward migration of quartz. The clays therefore had a high content of silica. The sample deposits of Kajansi, Kawuku, Lwanda, Kumi and Seeta formed principally from a crystalline basement of Precambrian gneiss, schist and/or granitized rock bases. Clay deposits in Chelel, Siron, Mutufu and Budadiri showed presence of smectites, feldspars and plagioclase which are clear cut indicators that they were formed from tertiary-quaternary volcanic ashes or glass and are therefore much younger than the rest of the clay samples studied.
Changes in colors of clays often suggest presence of characteristic elements, upon which prediction of surface and bleaching properties can be made. Records of colors of clays may serve as reference to nature of the environment from which the clay sediments were collected.
The clay deposits sampled exhibited different colors depicting differences in trace transition elements composition in the clays. The appearance of most clays depends on the quantity and oxidation state of iron and other transition elements present. While clays containing iron in oxidation state +3 are yellow or brown, green clays contain iron in oxidation state +2. However, changes in color could also arise from presence of manganese and titanium compounds in different oxidation states [
The colors of selected clays observed in this study are shown in
Weathering resulted in changes in mineral composition in these clays. There are two recognized categories of weathering processes: physical and chemical.
Physical weathering involves disintegration of rocks and minerals by a physical or mechanical process. Chemical weathering is chemical alteration or decomposition of rocks and minerals. However, both processes work together to break down rocks and minerals to smaller fragments of clay size or to clay minerals more stable near the earth’s surface. Among the conditions present near the Earth’s surface that are different from those deep within the earth are temperature, pressure, more free water and more free oxygen. Because of these differing conditions, minerals in rocks react with their new environment to produce new minerals that are stable under conditions near the surface.
Clay | Source | Color | pH ± 0.034 |
---|---|---|---|
A | Kajansi | Dark grey | 5.96 |
B1 | Seeta | Yellow-grey | 6.10 |
B2 | Seeta | Orange-brown | 4.95 |
C | Lwanda | Whitish-grey | 5.60 |
D | Nakawa | Reddish-brown | 5.75 |
E | Kawuku | Dark brown | 6.40 |
F | Umatenga | Grey-yellow | 6.30 |
G | Chodah | Grey-yellow | 6.80 |
H | Ngero | Dark-grey | 6.60 |
I1 | Budadiri | Dark brown | 5.60 |
I2 | Budadiri | Dark grey | 5.65 |
I3 | Budadiri | Dark grey | 6.50 |
J1 | Mutufu | Grey | 6.90 |
J2 | Mutufu | Dark grey | 8.40 |
K | Siron | Brown | 6.80 |
L | Chelel | Whitish grey | 7.90 |
Minerals that are stable under conditions near the surface, in order of most stable to least stable, are oxides of iron and aluminium, quartz, clay minerals, muscovite, alkali feldspar, biotite, amphiboles, proxenes, calcium-rich plagioclase and olivine.
Clay minerals are an important group of minerals because they are among the most common products of che- mical weathering, and thus are the main constituents of the fine-grained sedimentary rocks called mudrocks (including mudstones, claystones, and shales). In fact clay minerals make up about 40% of the minerals in sedimentary rocks. In addition, clay minerals are the main constituent of soils. Understanding of clay minerals is also important from an engineering point of view, as some minerals expand significantly when exposed to water. Clay minerals are used extensively in the ceramics industry and are thus important economic minerals.
Based on their structures and chemical compositions, the clay minerals can be divided into three main classes: kaolinites based on a structure similar to kaolinite; smectites based on a structure similar to pyrophyllite and illites based on a structure similar to muscovite. Each of these is formed under different environmental and chemical conditions. Kaolinite is formed by weathering or hydrothermal alteration of alumino-silicate minerals. Thus, rocks that are rich in feldspar commonly weather to kaolinite. In order to form, ions like sodium, potassium, calcium, magnesium and iron must first be leached away by the weathering or alteration process. This leaching is favored by acidic conditions (low pH). Granitic rocks, because they are rich in feldspar, are a common source for kaolinite. Halloysite, is also a kandite clay, with a structure similar to kaolinite. Kaolinite has the chemical formula―Al2Si2O5(OH)4·4H2O. Kaolinite does not absorb water, does not expand when it comes in contact with water, so it is preferred for ceramics.
The smectite group of clays has a structure similar to that of pyrophyllite, but can also have significant amounts of magnesium and iron substituting into the octahedral layers. Thus, the smectites can be both dioctahedral and trioctahedral. The most important aspect of the smectite group is the ability for H2O molecules to be absorbed between the sheets, causing the volume of the minerals to increase when they come in contact with water, smectites are expanding clays with a general chemical formula, (½Ca, Na)(Al, Mg, Fe)4(Si, Al)8O20 (OH)4·nH2O. And montmorillonite is the commonest smectites and it is the main constituent of Bentonite, derived by weathering of volcanic ash. Montmorillonite can expand its original volume by several times when it comes in contact with water.
However, the kimolian white bentonites were found to have higher brightness and whiteness index values, and lower yellowness index and the variations in white color were inversely related to the abundance of iron oxides and anatase [
The selected clay deposits at Siron, Budadiri, Mutufu and Chelel showed colors ranging from brown or orange through grey to white due to presence of iron in different oxidation states as well as difference in the mineralogical compositions in the different strata resulting from the levels of alteration and migration of silica through the mine. The clays from deposits at Kajansi, Seeta, Lwanda, Nakawa and Kawuku had pH values near 6 showing that they are acidic presumably as result of the natural acidity of the hydroxyl groups on silica sites [
The selected clay samples from Mutufu and Chelel had average pH ranging from 7.5 to 8.50 ± 0.034, due to the environment where they were mined was ultra alkaline. The alkalinity of the clay slurries is attributed to presence of alkali and alkaline earth metals whose silicates strongly hydrolyse raising the pH above 7. This showed presence of excess alkali metals like sodium and potassium as well as K-feldspars in these clays. The pH of slurries for bentonites or montmorillonites was reported to lie in the range between 7.8 and 8.5 [
As low pH of kaolinites is associated with presence of acidic water or ionisable hydroxyl groups on the surface of clays, it has been proposed that the slurries of kaolinites ionise. The pH of slurries of smectite-rich clays is greater than 7 due to hydrolysis of sodium or/and potassium silicates. The hydrolysis may be complete or partial. Therefore, the interlayer ions are structurally acting as counter ions to the silicate skeleton in nontronite.
Clays are classified in two major groups: kaolinites and smectites [
The selected samples used in this study are broadly divided into smectites and kaolinites basing on mineral compositions. While kaolinites are clays containing kaolinite, halloysite, metahalloysite, quartz and illite as dominant minerals [
The selected clays from Budadiri, Chelel, Siron and Mutufu showed presence of quartz, nontronite, illite, plagioclase, feldspars and kaolinite showing that these clays were of the phyllosilicate (smectite) type and will probably show high bleaching tendencies because their surface properties will be modified by acid leaching very easily [
The clays studied showed different compositions of the minerals. Despite the efforts to eliminate sand stones and quartz through filtering, clay deposits at Kajansi, Kawuku, Seeta, Lwanda, Nakawa, Umatenga, Chodah, and Ngero had large quantities of quartz as shown in
The selected clays used in this study showed presence of quartz, kaolinite, illite, halloysite and metahalloysite in clays sampled from Kumi, Ngero, Nakawa, Kawuku, Seeta and Chodah showed that these clays formed from granite in acidic medium. This showed that these clays differ from those formed from volcanic sediments and have different structural, chemical, mineralogical, surface and bleaching properties, age and diagenesis [
The mineralogical compositions above have been determined using the Reynold’s semi-quantitative method [
Sample | Kaolin % | Quartz % | Halloysite % | Smectite % | Illite % | Feldspars % | Plagioclase % |
---|---|---|---|---|---|---|---|
A | 31.3 | 50.0 | 6.2 | 6.3 | ND | 6.2 | ND |
B1 | 37.5 | 37.5 | 25.0 | ND | ND | ND | ND |
B2 | 40.0 | 53.3 | 6.6 | ND | ND | ND | ND |
C | 40.0 | 26..3 | 6.6 | ND | 26.7 | ND | ND |
D | 53.3 | 40.0 | 6.6 | ND | ND | ND | ND |
E | 40.0 | 53.5 | 6.5 | ND | ND | ND | ND |
F | 36.4 | 36.4 | 18.2 | 4.5 | ND | 4.5 | ND |
G | 47.1 | 47.1 | 5.8 | ND | ND | ND | ND |
H | 37.5 | 37.5 | 25.0 | ND | ND | ND | ND |
I1 | 23.5 | 23.5 | ND | 6.0 | ND | 23.5 | 23.5 |
I2 | 25.0 | 25.0 | ND | ND | ND | 25.0 | 25.0 |
I3 | 23.5 | 23.5 | ND | ND | 6.00 | 23.5 | 23.5 |
J1 | 23.5 | 23.5 | ND | 5.6 | ND | 23.5 | 23.0 |
J2 | 7.7 | 15.4 | ND | 46.2 | ND | 30.5 | ND |
K | 23.5 | 23.5 | ND | 6.0 | ND | 23.5 | 23.5 |
L | 7.7 | 10.4 | ND | 50.0 | ND | 30.0 | ND |
Key: ND refers to not detected as the peak for the mineral was very low or invisible on the X-ray diffractograms as it was less than 1%; Clay samples designated as A, B, C, D, E, F, G, H, I, J, K and L were selected from the respective sediments from Kajansi, Seeta, Lwanda, Nakawa, Kawuku, Umatengah, Choha, Ngero, Budadiri, Mutufu, Siron and Chelel.
The dominant clay mineral in clay deposits at Budadiri, Chelel, Siron and Mutufu is nontronite, a smectite and this coincides with studies on clays formed from volcanic sediments. The presence of bentonite among volcanic sediments all over the world was discussed by many authors [
The high abundance of quartz in all clay deposits sampled showed that the clays must have high silica content, and are bound to be strongly acidic as the silica provides several sites for the hydroxyl groups or water molecules to bind. Presence of high silica content in the clays does not serve as evidence for the migration of silica in the mines [
Feldspars are rare among Kajansi, Kawuku, Nakawa, Chodah, Umatengah, Lwanda, Ngero and Seeta deposits, showing that the weathering process is complete or near complete. The abundancy of quartz in these clays is an indication of the remains of the unaltered gneiss-granitoids parent rock [
The bentonite deposits of Kimolos Island, Aegean, Greece were investigated in order to determine their physical and chemical properties [
In this study, it has been found that kaolinite-rich clays from Seeta, Nakawa, Kawuku, Kajansi, Chodah, Lwanda and Umatengahmineralogically contain between 31% and 53% kaolinite, 5.8% and 18.2% halloysite and this has been used to propose that the structures of these clays is an amalgamation of kaolinite and halloysite structures. For the smectite-rich clays selected from Chelel, Mutufu, Budadiri and Siron contained clay minerals in the ranges of 46% - 50% nontronite, 23% - 30.5% feldspars and 1% - 25% plagioclase, we can propose that these clays have a di-octahedral structure [
Elemental analyses of clays have always revealed the class of alumino-silicates to which the analyzed material belongs [
The elements present in clays have been presented as relative percentages of the elements expressed as oxides in the entire sample in
Clay sample | A | B1 | E | F | H | I1 | I2 | J | K | L |
---|---|---|---|---|---|---|---|---|---|---|
LOI | 9.3 | 4.5 | 5.0 | 9.0 | 7.2 | 8.0 | 9.0 | 8.2 | 9.7 | 9.9 |
SiO2 | 49.4 | 63.1 | 65.0 | 71.0 | 66.0 | 55 | 46.0 | 48.0 | 45.0 | 44.3 |
Al2O3 | 20.1 | 10.5 | 11.1 | 8.4 | 9.0 | 15.0 | 12.0 | 13.0 | 18.3 | 19.4 |
Fe2O3 | 8.0 | 1.5 | 1.4 | 1.8 | 3.5 | 5.7 | 6.0 | 4.1 | 5.7 | 6.2 |
CaO | 0.2 | 0.1 | 0.1 | ND | ND | 2.0 | 2.1 | 2.0 | 2.3 | 2.4 |
Na2O | 0.1 | 0.1 | 0.1 | ND | 0.1 | 3.0 | 3.0 | 2.0 | 3.5 | 3.4 |
K2O | 0.3 | 0.4 | 0.1 | ND | ND | 1.0 | 1.2 | 2.1 | 2.4 | 2.6 |
Total | 87.4 | 80.2 | 82.8 | 83.0 | 85.8 | 89.7 | 79.3 | 79.4 | 86.9 | 88.2 |
Key: LOI is loss on ignition, signifying an estimate of matter lost when the clay was heated. ND is not detected. Readability was 0.045. A, B, E, F, H, I, J, K and L were respectively Kajansi clay, Seeta clay, Kawuku clay, Chodah clay, Umatengah clay, Budadiri hill clay, Budadiri river valley clay, Mutufu clay, Siron clay and Chelel clay.
showing that volcanic clays of Mutufu, Budadiri and Chelel should be less acidic than clays from Kawuku, Kajansi, Umatengah or Lwanda as the high silica/quartz content gives higher chances of exposing hydroxyl groups and adsorption of water on silica which causes the clays to be acidic. The differences in silica content indicated that the parent rocks in these sets of deposits are very different, silica readily dissolves in alkaline medium but does not in acidic medium. The Kajansi, Kawuku, Umatengah, Lwanda clays formed from acid igneous quartz diorites in a similar way to the Singo granite [
Similarly, the data adduced in the study on clays from the Ntawo, Kajansi, and Kitetikka valleys reveal that the valleys are filled with quaternary to recent alluvial and lacustrine sands, silt, and gravel, which are derived from underlying meta-sediments of Buganda-Toro system and granitoid rocks of the basement and the clays therein are composed of sandy sediment, with kaolinite, chlorite, smectite, quartz, feldspars and calcite as minerals that could be observed using X-ray crystallography [
Similarly, the clays from Kajansi, Kawuku, Seeta, Umatengah and Kumi have been found to contain these elements in nearly the same proportions. This showed that the parent rock was quartz diorites which weathered in acidic medium. However, clays from magmatic sediments in oceans were reported to contain major elements with relatively high magnesium oxide, ranging from 23.9% to 40.4% and low silicon dioxide and aluminium oxide, ranging from 12.8% to 29.0% [
The loss on ignition of the selected clays used in this study lie in the range of 6% - 10%, an indication that heating clays to 105˚C results in loss of structural water from the clay [
Apart from clay deposits at Kajansi, Mutufu, Budadiri, Chelel and Siron, all other deposits had higher silicon dioxide content revealing the diagenetic paths for these clays are different [
The percentages of iron, aluminium and silicon among bentonites worldwide are approximately 11%, 18%, and 60% respectively. Basing on this, clays from Mutufu, Budadiri and Chelel ressemble bentonites can serve as replacements for commercial bleaching earths and cracking catalysts. On the basis of relative percentages of aluminium, silicon and alkaline metals or alkaline earth metals [
Clay sample | Clay formula | Name | Reference |
---|---|---|---|
A | Al2Si2O5(OH)4 mixed with Ca5(Si7Al8Fe2) (Fe3.5Al4Mg.1)O20 (OH)4 and silica SiO2 | Kaolinite and nontronite | [ |
B1 | Al2Si2O5(OH)4 mixed with SiO2 | Kaolinite and quartz or silica | [ |
E | Al2Si2O5(OH)4 mixed with SiO2 | Kaolinite and quartz or silica | [ |
F | Al2Si2O5(OH)4 mixed with SiO2 | Kaolinite and quartz | [ |
I1 | Ca5(Si7Al8Fe2)(Fe3.5Al4Mg1)O20(OH)4 with SiO2 | Nontronite and quartz | [ |
I2 | Ca.5(Si7Al8Fe2)(Fe3.5Al4Mg1)O20(OH)4 with SiO2 | Nontronite and quartz | [ |
J | Al2Si2O5(OH)4. | Halloysite | [ |
K | (½Ca, Na)0.33(Mg, Fe+2)3(Si, Al)4O10(OH)2·4H2O and Na0.3(Fe.2)Si4O10(OH)24H2O | Montmorillonite and nontronite | [ |
L | (½Ca, Na)0.33(Mg, Fe+2)3(Si, Al)4O10(OH)2·4H2O4H2O and Na0.3(Fe2)Si4O10(OH)24H2O | Montmorillonite and nontronite | [ |
Key: Samples A, B, E, F, H, I, J1,J2, K and L were respectively Kajansi clay, Seeta clay, Kawuku clay, Chodah clay, Umatengah clay, Budadiri hill clay, Budadiri river valley clay, Mutufu clay, Siron clay and Chelel clay.
altered to give the clay; volcanic clay sediments I, K and L had smectites yet non-volcanic clay sediments sampled as A, B, E, F and J had kaolinites as clay mineral in mixture with quartz. Basing on the clay minerals present in the samples, it can be concluded that kaolinites occur in Central Uganda and Kumi as these areas were devoid of volcanism. The smectites occur in volcanic sediments of Sironko and Kapchorwa because the areas lie on the foothills of Mountain Elgon, a well known volcanic mountain.
IR studies have been used to identify smectites because they show broad absorption band at 3600 cm−1 due to OH, 3454 cm−1 due to inter layer water, 1664 cm−1 due to deformational vibration in the HOH group, at 1042 and 798 cm−1 due to Si-O vibration, the bands at 526 and 466 cm−1 show presence of Si-O-Al and Si-O-Si deformation vibrations respectively [
The clay deposits at Kajansi, Kawuku, Seeta, Umatengah, Chodah, Nakawa and Ngero showed strongly resembling infrared (IR) spectra (shown in Figures 4(a)-(c)) due to presence of identical groups of ions or minerals.
The band at 1040 cm−1 is assigned to (Si-O). The band at 3454 cm−1 is due to adsorbed water, and that at 3640 cm−1 is due to (Al-Al-OH, Mg-OH-Al). The broadening of the absorption bands in the kaolinite-rich clays indicates high acidity of the clays even when they are air dried. Kaolinite clays contain protonated water and hydroxyl groups on silica in their crystal lattices and their absorption bands tend to be broader. The absorption bands at 918 and 879 cm−1 in Figures 4(c)-(d) in the representative spectra for the clay deposits at Mutufu, Chelel, Budadiri and Siron characteristically contain smectites and band at 800 cm−1 is obscured by Si-O mode which is broadened. These bands are due to the bending mode of Al-Fe-OH bonds and smectites are also expected to show absorption bands at 845 cm−1 [
The IR spectra for the kaolinite-rich clays from Kajansi, Kumi, Nakawa, Kawuku, Seeta, Chodah and Umatengah showed peaks due to hydroxyl stretch at 3600 cm−1, Al-OH at 3490 cm−1, H-O-H stretch at 1640 cm−1,
Si-O-Si stretch at 1100 cm−1, Si-OH stretch at 900 cm−1, Si-O-Al stretch at 550 cm−1 and Si-O-Al at 470 cm−1, it has been proposed that these clays conform to the structure of kaolinite and halloysite [
The IR spectra for smectite-rich clays selected from Mutufu, Budadiri, Siron and Chelel showed peaks due to Mg-OH-Al stretch at 3490 cm−1, Al-Al-OH stretch at 3480 cm−1, H-O-H at 1800 cm−1 and 1620 cm−1, Si-O-Si at 1100 cm−1, Al-Fe-OH at 800 cm−1, Si-O-Al at 600 cm−1 and Si-O-Si at 490 cm−1 showed that these clays conform to the structure of montmorillonite [
Powder XRD diffractograms were used to characterize minerals and elements present in solids. Clay powders can be distinguished in two broad classes: smectites and kaolinites basing on XRD patterns. XRD analyses therefore form a core component in elucidating clay structures [
The X-ray diffractograms obtained on the different samples showed clear resemblances and differences. The representative X-ray diffractograms shown in Figures 5(a)-(c) revealed the presence quartz and kaolin in all the clays. All clays studied had abundant quartz because they were transported by erosion to valleys where they were mined. Quartz represents unaltered clay precursors. The distribution of the various clay minerals and clay precursors are shown as interpreted on Figures 5(a)-(c). The Central Uganda clays are represented by the sample from Kajansi (
The volcanic clays from Sironko and Kapchorwa are represented by the Mutufu and Chelel clays respectively (
As the XRD patterns in
The clays from Central Uganda had kaolin as the dominant clay mineral yet clays from volcanic sediments in Eastern Uganda nontronite as clay mineral.
DTA curves showed that clays from Central Uganda contained kaolinites as they showed peaks at 150˚C, 530˚C and 600˚C. As kaolinites show minimal bleaching tendancy and surface acidity, it can be concluded that DTA curves can be used to identify kaolinites and smectites. The basis that smectites gain bleaching capacity at faster rate than kaolinites, it can be inferred that DTA curves can be used to identify clays that can be very useful as bleaching earths. Generally, DTA curves showed a small change in the endothermic peak after acid treatment, corresponding to the loss of adsorbed water (at about 150˚C), and peaks corresponding to the loss of structural hydroxyl groups (530˚C and 600˚C) [
The endothermic peak in the range of temperatures between 550˚C and 660˚C is attributed to dehydration of the mineral leading to formation of γ-aluminium oxide in kaolinite clays [
DTA curves show no evidence of the difference between halloysite and kaolinite except for the difference in the initial endothermic peaks representing water loss at low energy levels [
Both kaolinite-rich and smectite clays showed exothermic peaks in the range of 900˚C - 1000˚C.
that the selected clays from Uganda give endothermic and exothermic peaks in differential thermal analyses [
Illites showed the endothermic peaks in the temperatures between 100˚C and 200˚C, 500˚C and 650˚C and at about 900˚C. This gives three endothermic peaks. The second peak in the temperature range of 500˚C to 650˚C represents loss of most of the water from the clay lattice. The third peak is associated with destruction of crystal structure [
The thermal stability of montmorillonites is related to crystalline structure (Lombardi et al., 2002). The loss of hydroxide ions from clay material causes irreversible modification of the crystal structure, producing the endothermic peak at temperatures between 650 and 700˚C [
The deformations in crystal structure of montmorillonite clay are initiated by different isomorphic substitutions which cause a temperature shift of the peak [
The DTA curves for the selected clay from Mutufu and Budadiri (
The results on the elemental and mineralogical compositions of clays from Central Uganda differ from those from the volcanic sediments of the Mt. Elgon. Utilisation of the two types of clays should be strict. Whereas elemental and mineralogical data on selected clays from Kumi, Nakawa, Seeta, Kajansi, Kawuku, Lwanda, Chodah and Umatengah indicated they were kaolinites. The XRD, IR and DTA data on clays from Mutufu, Budadiri, Chelel and Siron indicated they are smectites. So clays to be used to develop bleaching earths should be selected from smectites.
The pH data on Nakawa, Kawuku, Kajansi, Umatengah, Chodah, Seeta, Lwanda and Kumi clays indicated they are acidic kaolinites. The Mutufu, Budadiri, Siron and Chelel clays have been shown to be smectites.
The IR data accumulated on Kawuku, Kajansi, Lwada, Seeta, Chodah, Umatengah, Kumi and Nakawa clays revealed they were largely kaolinites yet that on Mutufu, Chelel, Budadiri and Siron clays indicated they were smectite-rich.
We thank Mr. Edward Ssekubunga for heating the clay samples studied in the furnace, Mr. Moses Nkolongo (rest in peace) for availing us chance to use the spectrophotometers in the analytical laboratories and Professor Ludwig of Institut für Mineralogie, Federal Republic of Germany for running XRD patterns of clays used in the study.