Clay is usually (but not necessarily) an ultrafine-grained sediment considered to be less than 2 µm in size on standard particle size classification. Chemically, clays are hydrous aluminum silicates, ordinarily containing impurities, for example potassium, sodium, calcium, magnesium,   or    iron,   in    small    amounts    and   are characterized by sheet-like silicate structures of composite layers stacked along the c-axis (Grim, 1968). The term can thus be applied both to materials having a particle size less than 2 µm and to the family of minerals that have similar chemical compositions and common crystal structural characteristics (Velde, 1995).

Clay minerals are products of long periods of the gradual chemical weathering of pre-existing rocks (usually silicate-bearing), by low concentrations of carbonic acid and other diluted solvents, particularly in warm tropical and subtropical regions of the world. In addition to the weathering process, some clays are formed through hydrothermal alteration of rocks. In terms of sedimentary processes, some clays are residuals that are formed in situ as a result of leaching (chemical), while others undergo substantial transportation before being deposited (clastic).

Clays have slaking ability and exhibit plasticity when mixed with water in certain proportions. However, when dry, clay becomes firm and when fired in a kiln, permanent physical and chemical changes occur. These changes convert the clay into a ceramic material. Because of these properties, clay is used for making pottery, both utilitarian, decorative and construction products, such as bricks, wall and floor tiles. Different types of clays when used with different minerals and firing conditions are used to produce earthenware, stoneware and porcelain.  Due to thegeometric increase in pollution indices of many towns in the developing countries, clays are being used for pollution control in engineered landfills as linings (Ibe et al., 2003). Solid heterogeneous catalysts and adsorbents derived from clay are also becoming relevant in pollution control (Okoye and Obi, 2011) as they have been associated with the removal of heavy metals.

Based on the foregoing and the high volumetric abundance of clay minerals in the study area, a thorough review of the properties of clay deposits in some parts of Southeastern Nigeria was undertaken in order to ascertain the quantity and quality of the deposits for economic purposes. In view of the current program of government to invest in the mining of mineral deposits to augment oil and gas revenues, this study aimed at ascertaining the possible utilization of the clays as raw materials will be of great value in the planning and exploitation of the identified clay resources in the Southeastern part of Nigeria.

The selected localities for the study are bounded by Latitudes 5°53' and 6°30' N and Longitudes 6°56' and 7°30' E. (Figure 1).Within this area are Uturu, Ikpankwu, Ezinnachi and Ohiya in the Okigwe-Umuahia axis and sections along the Awommama-Orlu axis Southeastern Nigeria. The towns can be accessed through the Enugu-Port Harcourt Expressway and other roads in the area.

 

 

Geology

The studied lithostratigraphic units include the Mamu Formation (Uturu and Ikpankwu), Ajali Formation (Uturu), Nsukka Formation (Ikpankwu), Imo Shale (Ezinnachi), Ameki Formation (Ohiya) and Benin Formation (Awommama and Orlu). These are the post-Santonian formations in the Anambra Basin and Niger Delta.  

Sediment deposition in the basin started in the Campanian with a short marine transgression followed by a regression. The Nkporo Shale and its lateral equivalents, the Enugu Shale and Owelli Sandstone (Nkporo Group), constitute the basal beds of the Campanian period. The broad shallow sea gradually became shallower because of gradual subsidence, initiating a regressive phase during the Maastrichtian that deposited deltaic foresets and flood plain sediments of the Mamu Formation (Lower Coal Measures). The Mamu Formation is overlain by the continental beds of Ajali Sandstone (False bedded Sandstone), followed by a return to partially paralic conditions and the deposition of the Nsukka Formation. The Nsukka Formation marked the onset of the Sokoto transgression and documented the return to paludal conditions (Murat, 1972). The Imo Shale that overlies the Nsukka Formation show shallow-marine shelf conditions in which foreshore and shoreface sands are occasionally preserved (Petters and Ekweozor, 1981) and consists of blue-grey clays and shales and black shales with bands of calcareous sandstone, marl and limestone (Reyment, 1965).

Regression continued throughout the Eocene (Jones and Hockey, 1964; Reyment, 1965) culminating in the deposition of Lower and Middle Eocene deposits including the clastic Ameki and Nanka Formations. The progradational Nanka Formation marks the return to regressive conditions. The prograding shoreface and river plain deposits are reflected in the subsurface deposits of the Agbada Formation in the northern depobelts of the Niger Delta, whilst the marine Imo Shale equivalent in the subsurface is termed the Akata Formation.

The uppermost lithostratigraphic units of the stratigraphic succession in the area are dominated by the Ogwashi-Asaba and the Benin Formations that are of Oligocene-Recent in age (Nwajide, 2005).

During the Miocene, the Niger Delta continued to build up and prograde seawards. There was lowering of sea level during the Pleistocene. The Niger River cut wide valleys through its own delta. These troughs are being filled today as the sea level gradually rises. The stratigraphic sequence shown in Figure 2 and graphically illustrated in Figure 3 generally show the lithic fill of the study area.

 

 

 

 

 

 

 

 

 

Chemical analysis

Chemical analysis was performed in the Geochemistry Laboratory of the Nigerian Geological Survey Agency, Kaduna. The main analytical equipment used for the analysis were the Energy Dispersive X-Ray Fluorescence (EDXRF) and Atomic Absorption Spectrometer (AAS).

Sample preparation for XRF involved the pulverization of about 100 g from each of the collected clay samples with a motorized agate mortar so that the grains of the pulverized fine powder would pass the 300 (53 µm) mesh sieve. 10 g each of the powdered samples was mixed with 1 g of stearic acid (binder) and thoroughly homogenized in an agate mortar. The mixture was then transferred into a hardened-steel disc with a diameter of 40 mm and pressed into a pellet at a pressure of 25 tons using hydraulic pressure press. The pellets were then used for XRF analysis.

About 0.5 g of the powdered samples was placed in a Teflon beaker and digested  with  10 ml  of  hydrofluoric  acid  and  4 ml  of perchloric acid before being subjected to sand bathing to near dryness in a fume cupboard. The digested sample was filtered using Whatman filter paper 541 into a 100 ml volumetric flask, which was later filled up to mark with distilled water. The obtained solution was analyzed using the Schimazu AA-7000 Atomic Absorption Spectrometer. Standard solutions of sodium and magnesium were prepared from stock solutions for instrument calibration.

 

The result of the diffraction analysis is presented as X-Ray diffractograms (Figures 4 and 5). For purposes of enhancement so that other clay minerals present in the samples  could  be   recognized,   the   “stick   pattern”  of diffraction signature was utilized to highlight the presence of iron, bentonite and dickite in some of the deposits (Figure 6).

 

 

 

 

The diffractograms revealed that only kaolinite, bentonite and dickite constitute the principal clay minerals while some iron minerals variants (fersilicate, hematite and bixbyite) are also present. Quartz was identified in all the diffractograms at basal reflections of 4.263, 3.348, 2.46 and 2.28 A°.

The results showing the major element oxide compositions and calculated Weathering Indices of the clay samples in the study area are as shown in Tables 1 and 2. Quartz (SiO₂) is the most abundant and ranged between 56.45 and 31.70%, in samples from Orlu and Ikpankwu, respectively. The tables also showed that the major oxides apart from SiO₂, include Al₂O₃ and TiO₃ while MnO, MgO, CaO, Na₂O, K₂O, and N₂O₅ were present in the clays only in small amounts. Iron as Fe₂O₃ is greater than 2% while the values of K₂O varied from 0.374 to 2.040% of the sample.

 

 

 

 

 

 

Mineralogy

Quartz was identified in all the samples indicating that it is the dominant mineral in the clay deposits in the study area. Its high dominance clearly explained its grittiness and suggestive that the clays could be of residual origin (Akhirevbulu et al., 2010).

Prominent basal reflections, strong and sharp peaks are indicators of moderate to well-formed crystalline mineral components (Jubril and Amajor, 1991; Obrike et al., 2007). Some of the samples  studied  (e.g. those from Njaba, Ohiya II and Orlu) showed brooding of kaolinite reflection (Figure 6) suggesting the presence of poorly ordered kaolinite (Grim, 1968). Significant quantities of bentonite were observed in samples from Awo-Omanma and Orlu. The presence of bentonite in these samples indicates that ceramic products derived from such clay types would have difficulties during drying due to the slaking properties of bentonites. Clays often contain considerable amounts of organic matter which also influences their firing properties (Robbins, 1984). Baylis et al. (1965) reported the widespread presence of dickite associated with kaolinite and illite as was observed in Ohiya II where dickite occurred with kaolinite (Figure 5).

The presence of non-clay minerals and high proportions of kaolinite suggests that the clays in the study area originated from the weathering of silicate minerals of Basement Complex rocks probably the nearby rocks of the Oban Massif under acidic condition (Odigi, 1989).

Geochemistry

Iron as Fe₂O₃ is one of the major oxides in the clay deposits in the study area. Its high values (>2%) in the samples from Uturu and Ikpankwu is probably responsible for the extreme changes in color of the clays from pink, yellow to red and the deposition of iron minerals as cementing materials that formed the concretions common in the Uturu and Ikpankwu sections. Other factors could contribute to color variations in clays (Kreimeyer, 1987), including the presence of some other constituents such as CaO, MgO, MnO and TiO₂, relative amounts of Al₂O₃, the temperature of firing and the furnace   conditions   all    play   important   roles   in   thedevelopment of color in the fired clay products (Fischer, 1984). The presence of iron minerals in the clays could also constitute a challenge to their use as industrial raw materials for paper and rubber productions (Obrike, 2012). Low iron content in samples from Ezinnachi, Njaba, Ohiya I and II potentially makes the dominant kaolin mineral useful in white body production (Murray, 2007).

The low values of K₂O that varied from 0.374 to 2.040% could be related to the absence of illite in most of the clay deposits. MgO values were also low and thus explained why smectite was absent in the deposits. Generally, the low amounts of MgO and CaO caused the absence of associated carbonate or dolomitization processes in the study area.

Provenance

The clay deposits in the study area are mainly clastics and not the product of in situ weathering, hence, the geochemical signatures used to identify provenance were those used for clastic rocks (Madharaju and Ramasamy, 2002; Armstrong-Altrin  et  al.,  2004). The ratios of most clastic rocks have also been used to infer the source rock composition: Al₂O₃/TiO₂ ratio increases from 3 to 8 for mafic igneous rocks, 8 to 21 for intermediate rocks and 21 to 70 for felsic igneous rocks (Hayashi et al., 1997). The application of these hypotheses in the analyzed clay samples deposited in the study area, showed that the calculated Al₂O₃/TiO₂ ratios ranged from 3.6 to 13.5, which is indicative that the clays originated from mafic to intermediate igneous rocks.

Ekosse (2001) also used of TiO₂ versus Al₂O₃ binary plots to distinguish between granitic and basaltic source rocks. Application of this plot using the recalculated oxide ratios for the discrimination of provenance of clays in the study area (Table 3) suggested that the deposits originated from basalt + rhyolite/granite provenances (Figure 7).

 

 

 

 

Tectonic settings

Plate tectonic processes impact distinctive mineralogical and geochemical signatures to sediments and as such clastic sedimentary rocks and sedimentary basins can be classified according to plate tectonic settings (Roser and Korsch, 1986). The SiO₂ versus K₂O/Na₂O  diagram  was therefore used for the discrimination of tectonic settings at the time when the claystone suites were formed in the study area. The SiO₂-K₂O/Na₂O diagram suggested that the sediments in the study area were deposited between passive and active continental margins (Figure 9).

 

 

The weathering indices of sedimentary rocks can also provide useful information of tectonic activity and climatic conditions in the source area. As demonstrated by Nesbit et al. (1982), a measure of the degree of chemical weathering alteration of sediments was constrained by calculating the Chemical Index of Alteration (CIA):

CIA = Molars [Al₂0₃/(Al₂O₃+CaO*+Na₂O+K₂O)],

where CaO* represents the amount of CaO in silicate minerals only (that is, excluding those of carbonates).

The proposed Chemical Index of Weathering (CIW) of Harnois (1988) was also evaluated with the formular:

CIW = Molars [Al₂0₃/(Al₂O₃+CaO*+Na₂O)]

Nesbitt et al. (1982) reported a CIA value of nearly 100 for kaolinite and chlorite and 70 to 75 for average studies whereas Taylor and McLennan (1985) reported a CIA value of 85 to 100 for residual clays. Condie (1993) reported that most post-Archean Shales show moderate losses of Ca, Na, and Sr from source weathering with CIW values of 80 to 95. The CIA and CIW of the sediments in the study area are both higher than those of the average shales (95.8 and 98.3), respectively suggesting relatively intense weathering of the source area, probably corroborating the presence of clay minerals and absence of detrital feldspar (Tables 3 and 4) suggesting chemical maturity of the sediments.

 

 

Potential industrial value of the clay deposits

The assessment of the clay deposits as potential raw materials was achieved by a comparative analysis of the chemical composition  of  the  clay  deposits  in  the study area with those of some notable clays and industrial specifications of some clays in other areas (Tables 3 and 4). It was observed that the average chemical compositions of the studied clay deposits were similar to the Afam clay as reported by Jubril and Amajor (1991) and plastic fire clay of St Louis (Huber, 1985), but for higher Fe₂O₃ content in the Uturu and Ikpankwu clays (Table 4).

The clay deposits in the study area were also compared with the Maastrichtian clay from the Bida Basin as reported by Olusola et al. (2011). The Bida Basin clay was observed to be more enriched in silica than those from the study area. The Fe₂O₃ with a value of 1.6 compared favorably with some of the clays in the study area, although in some locations like Ikpankwu and Uturu, the clays from the study area showed very high Fe₂O₃ content. The Bida Basin clay was observed to be depleted in MgO and CaO. The Al₂O₃ and alkali contents however showed similar value with clays from the study area  (Table  4).  The   clay   from   the   study   area  also compared similarly with Dukku clay from Kebbi State as reported by Salihu and Suleiman (2018), however, the Al₂O₃ and CaO were higher. The Dukku clay was also depleted in TiO_2. In comparison with standard chemical specifications for the requirements of kaolin to be used for refractory bricks (Parker, 1967), the average chemical compositions of the sampled clay deposits in parts of Southeastern Nigeria are suitable for refractory bricks production (Table 5).

 

 

The chemical data from the clay deposits in the study area were also plotted into the tenary diagram of Fabbri and Fiori (1985) for the classification of raw materials and industrial ceramic bodies. The plots gave ceramic compositional fields and reflected the overall chemical composition of the clays in parts of southeastern Nigeria (Figure 10). Considering the ideal composition for an optimum white body product: SiO₂ = 72 wt.%, Al₂O₃ and total oxide = 8 wt.% (Fabbri and Fiori, 1985), samples outside this specification would need processing in order to moderate iron oxide and quartz contents. It is based on these standard chemical compositions that clays could be considered as raw materials for use in structural ceramic products (Konta, 1995). 

 

 

The analyses have thus shown that the clay deposits in the study area could be used as raw materials for the production of refractory bricks (Parker, 1967; Malu et al., 2013) and pottery (Jubril and Amajor, 1991; Huber, 1985).

Iron oxide (Fe₂O₃) would be a limitation to their use in paper, rubber and white body production especially the samples from Uturu and Ikpankwu. Appreciable amounts of oxides of sodium, calcium and magnesium recorded in the deposits would also lower the vitrification of the clays (Obrike, 2012). High alumina-iron ratio in the clays will render them less suitable for the production of good quality cement except for samples from Uturu and Ikpankwu, where the alumina-iron ratio is low. An alumina-iron ratio range of 1.71 to 2.45 for clay or shale is suitable for the manufacture of good quality cement (Abatan et al., 1993). 

 

 

 

 

 

 

 

 

 

 

 

Mineralogical and chemical parameters of clays have been assessed and used in this study to ascertain the industrial potentials and provenance of clays from parts of Southeastern Nigeria.

Mineralogical analysis showed the presence of kaolinite, bentonite, dickite, quartz and iron minerals in the clay deposits. Kaolinite and quartz were the most dominant minerals as they occurred in all the samples. Bentonite occurred only in samples from Awomamma and Orlu indicating a possible source from the adjoining Cameroun volcanics. The presence of bentonite in samples from Awo-Omamma and Orlu with slaking properties would render a ceramic body prone to difficulties during drying. Dickite occurred in association with kaolinite, showing progressive replacement in the Ohiya II clay unit, while iron minerals were common in clays from Ikpankwu and Uturu sections. The iron minerals caused the occurrence of concretions and extreme coloration of clays from both sections and could ensure color effects on finished products. The presence of iron minerals in these clays would constitute huge challenges for their usage in industries, especially for paper and rubber production. On the other hand, low iron content in clays from Njaba, Ohiya I and II would potentially make the dominant kaolin mineral useful in white body production.

The geochemical analysis revealed that the samples from the study area are rich in alumina Al₂O₃ which corresponds with the kaolinite clays with major oxides like SiO₂, Fe₂O₃, TiO₃, MnO, MgO and CaO.

Comparison with some industrial specifications, the clay deposits in the study area would be used as raw materials for the production of pottery, bricks, ceramics, refractories, paper, earthenware and paints. The low alumina-iron ratio in the samples from Uturu and Ikpankwu deposits would render the clays suitable for the production of good quality cement.

 

Though the results from the studied samples show wide range of industrial applications, most of them are not comparable to commercially marketed European and Asian counterparts. It is therefore recommended that elaborate treatments should be carried out by a centralized clay mineral dressing and preparation plant, remediation process by proper blending, beneficiation with lime and refining processes. It is expected that processing will help overcome compositional deficiencies.

 

The authors have not declared any conflict of interests.