Mineralogy and Geotechnical Properties of Fine-Grained Sediments from Swali Community

The mineralogical and geotechnical properties of fine grained deposits from Swali community, in Yenagoa local government area of Bayelsa State, Nigeria were analyzed using X-ray diffraction (XRD) for the qualitative analysis of the mineral composition and the Atterberg limit using ASTM D 4318 standard for the geotechnical properties. Four representative samples that were analyzed using XRD show the deposit is composed of quartz, albite, orthoclase and muscovite with average values of 59.25%, 16.45%, 13.25% and 11% respectively. No typical clay mineral was observed in the analysis. The mineral suite indicates an acid igneous rock source and long distance and time of travel. Ten samples were analyzed for the geotechnical properties, and the Atterberg limits derived are: Liquid limit ranging from 29.7% – 50.4%, Plastic limit ranging from 22.9% – 36.1% and the Plastic Index ranging from 11.4 – 26.1. Four of the samples were non plastic and only six show a measure of plasticity. The deposits are classified as inorganic lean clay in locations SW6 and SW10 designated as CL as they fall above the A – Line and have Plasticity Index > 7, and those from SW2, SW3, SW4 and SW8 are classified as silty clay with designation ML because they fall below the A – Line on the plot of Liquid limit versus Plastic Index but have Plasticity Index > 4.  All the deposits have low plasticity because they fall below the Liquid limit separator line of 50% with only one on the borderline. Therefore, the deposit range from inorganic lean clay to silty clay with low plasticity derived from an acid igneous rock with long distance and time of travel.

Introduction

Clay minerals are very fine-grained hydrous silicate crystalline particles. They often occur in conjunction with other minerals such as quartz, feldspars, micas, and other detritus. Very fine-grained sediments with particle size below 0.002 mm are referred to as clay [1]. According to the definition given by ASTM International [2], clay is a fine-grained soil that passes through 75 µm (No 200) US standard sieve and has a Plasticity Index of equal or greater than four. Clay minerals are mostly formed from the chemical weathering of primary rock-forming minerals in processes where dilute acidic solution passing through the rock causes leaching and weathering of the upper region. Some clay minerals are also formed from hydrothermal processes [3]. Weathering and leaching effects on rock-forming minerals such as the feldspars and some ferromagnesian minerals such as olivine, pyroxene, amphiboles, muscovite, and other micas vary according to the stability or ease of weathering of the mineral. The stability of the ferromagnesian minerals is as follows: muscovite > amphibole (igneous) > pyroxenes > olivine > biotite. We have both primary and secondary clay deposits. Primary clay deposits are residual deposits that remain at the site of formation, whereas secondary clay deposits are those that have been transported from their original formation site and deposited elsewhere. Deposition of clay deposits usually occurs in low-energy depositional environments such as lakes and marine basins [4].

Clays have a lot of industrial uses, such as in making paints, paper, polymers, refractory drilling mud, pesticides, fertilizers, cement, ceramics, and applications in the pharmaceutical industries, and many other uses [5]. Therefore, the quality and quantity of clay deposits should be properly assessed.

Swali, is a community in Yenagoa Local Government Area of Bayelsa State. It is geographically situated within latitudes 4° 5′ N and longitudes 6° 15′ E and latitudes 4° 57′N and longitudes 6° 17′E. It is situated in the swampy areas of the Niger Delta sedimentary basin of southern Nigeria. The swamps are made up of vegetated tidal flats resulting from interconnected meandering creeks and tributaries of the River Niger (Fig. 1). The area is typically overlain with fine-grained sediments, which the locals call “Clay.” This work is aimed at evaluating the mineralogical composition and the geotechnical properties of the “clay deposits” in Swali community to provide information and data on its quality for geotechnical and industrial purposes.

Fig. 1. Map of the study area showing sample point.

Stratigraphy of the Niger Delta

The stratigraphy of the Niger Delta region is defined by three lithologic units, which are: the basal Akata Formation, which is overlain by the Agbada Formation, and the topmost unit, which is the Benin Formation. The Akata Formation is Paleocene to Recent and basically marine shale, which becomes sandier towards the top. It is the main hydrocarbon source rock in the region [6]. It is the pro delta mega facies. The Agbada Formation is the paralic delta front facies, which is made up of intercalations of sand and shale. It serves as the main hydrocarbon reservoir of the basin. It is from Eocene to Recent [6]. The continental Benin Formation, which occurs above the Agbada Formation, is composed of unconsolidated sand with lignite streaks occurring in places. It is from Late Eocene to the Recent [7]. At different locations and environments, Quaternary deposits, which are made up of gravel, sand, silt, and clay, occur above the Benin Formation in the region [8] (Table I).

Table I. Stratigraphic Column of the Niger Delta [8]

Geologic unit	Lithology	Age
Alluvium (General)	Gravel, sand, clay, silt	Quaternary
Freshwater backswamp, meander belt	Sand, clay, some silt gravel
Mangrove and salt water/backswamps	Medium fine sands, clay and some silt
Active/Abandoned beach ridges	Sand, clay, and some silt
Sombreiro–Warri deltaic plain sand	Sand, clay, and some silt
Benin Formation coastal plain sand	Coarse to medium sand with subordinate silt and clay lenses	Miocene
Agbada Formation	Mixture of sand, shale and silt	Eocene
Akata Formation	Shale	Paleocene

Literature Review

Several workers have worked on clay deposits in some parts of southern Nigeria. Chukwudi and Benjamin [9] worked on the clay and sand units of the Nanka Formation in the Anambra Basin, southeast Nigeria, using field techniques, XRD, and grain size analysis and observed that the grains are fine to medium-grained to coarse-grained sandstone and the relative abundance of clay minerals, quartz, muscovite, hematite is high and are good for industrial uses. According to them, the sands contain less impurity, therefore, are very good for construction purposes.

Also, Onyekuru et al. [10] worked on the clay deposits found in Ikpankwu, Okigwe, and Ohiya along the Okigwe–Umuahia axis of the Anambra Basin, using the XRD method. The clay minerals were found to be basically kaolin, with traces of bentonite and dickite, associated with non-clay minerals such as quartz and iron minerals. It was also observed that the chemical composition of the clays studied is made up of SiO₂, Al₂O₃, Fe₂O₃, TiO₂, CaO, MgO, Na₂O, and MnO.

Using XRD, Adewole and Modupe [4] observed that the residual clay deposits that occur in Papalanto, Ifo, and Imoto in southwest Nigeria were kaolin in association with non-clay minerals such as quartz, anatase, and hematite. The chemical data showed the average values of the following oxides to be: SiO₂ is 59.46%, Al₂O₃ is 22.16%, and Fe₂O₃ is 3.06%, making a total of 98.3% of the bulk compositions.

The study by Ike et al. [1] on the suitability of clay using XRD shows that the Mamu and Nkporo Formations in the Anambra Basin in southeast Nigeria are basically made up of kaolinites, with smaller amounts of mixed layer, illite, and montmorillonite. Using X-Ray Fluorescence for the oxide analysis, they observed a substantial amount of silica oxide, which, when combined with the low alkali oxide observed, will favor ceramic manufacturing.

Methodology

Ten samples were collected in the studied area at a distance of 100 m apart. Four of the samples were sent for XRD analysis for bulk chemistry in order to determine the mineralogy.

While all ten were analyzed for their Atterberg Limits–Liquid Limit (LL), Plastic Limit (PL), and Plasticity Index (PI). The experiments and calculations were carried out as outlined by ASTM International [11].

Presentation and Discussion of Results

Sample Description

Ten samples were taken from the geo-referenced spots in the Swali community. Some of the sediments were light brown to dark brown in color, while others were light grey to dark grey. The samples were fine-grained sediments that were smooth to feel. They were basically silty clay-to-clay sediments (Table II).

Table II. Description of Sampled Sediments and Location

Location/sample no.	SW1.	SW2.	SW3.	SW4.	SW5.	SW6.	SW7.	SW8.	SW9.	SW10.
Geo-reference	N4°54′59.2″	N04°54′58.6″	N04°54′58.0″	N04°54′57.5″	N04°54′57.1″	N04°54′57.1″	N04°54′56.7″	N04°54′56.8″	N04°54′56.1	N04°54′55.9″
	E006°15′98.0″	E006°15.9″	E006°15′55.3″	E006°15′55.7″	E006°15′56.0″	E006°15′55.9″	E006°15′56.2″	E006°15′56.1″	006°15′56.5″	E006°15′56.6″
Colour	Brown	Grey	Dark brown	Brown	Dark brown	Brown	Light brown	Brown	Light grey	Grey
Rock type	Silty clay	Clay	Clay	Clay	Silty clay	Clay	Silty clay	Clay	Silty clay	Clay

Mineralogical Composition

The bulk chemistry of the sediments was determined by XRD analysis using samples SW2, SW3, SW6, and SW9 as representative samples. Typical clay minerals were not observed in the deposits, but, the minerals observed were: Quartz (SiO₂) 51%–71%, with an average of 59.25%; Orthoclase [K(AlSiO₃O₈)] 3%–28%, with an average of 13.25%; Albite Na(AlSiO₃O₈) 3.8%–26%, with an average of 16.45% and Muscovite KAl₂(AlSi₃)O₁₀(OH,F)₂ ranging from 4%–26% and an average of 11% (Table III). Figs. 2–5 show the qualitative analysis phase data views of the minerals, associated diffractograms, and plots of the qualitative analysis as detected by the X-ray diffractometer for the analyzed samples.

Table III. Mineral Composition of Selected Samples from Swali Community

Sample points	Quartz (SiO2) (%)	Orthoclase (K–Feldspars) [K(AlSiO3O8)] (%)	Albite (NA–Plagioclase) Na(AlSiO3O8) (%)	Muscovte Kal2(AlSi3)O10(OH,F)2 (%)
SW2	59(3)%	28(2)%	3.8(4)%	9(3)%
SW3	51(5)%	9(6)%	14(4)%	26(6)%
SW6	71(8)%	3(4)%	22(7)%	4(5)%
SW9	56(12)%	13(15)%	26(8)%	5(8)%
Average	59.25%	13.25%	16.45%	11%

Fig. 2. Qualitative analysis results phase data view for SW2 (a) and Plot of results for SW2 (b).

Fig. 3. Qualitative analysis results phase data view for SW3 (a) and Plot of results for SW3 (b).

Fig. 4. Qualitative analysis results phase data view for SW6 (a) and Plot of results for SW6 (b).

Fig. 5. Qualitative analysis results phase data view for SW9 (a) and Plot of results for SW (b).

According to Glencoe Earth Sciences [12], chemical weathering occurs when carbonic acid interacts with granite over a long time. The feldspar in the granite is broken down into silica, a potassium salt, and the clay mineral kaolinite. Feldspars are major components of igneous rocks. The mineralogy of the studied samples is made up of quartz, albite, orthoclase, and muscovite, as observed from the qualitative analysis.

Quartz is a very stable mineral and very common in acidic igneous rocks. Due to its stability during weathering and transportation, most residual clastic deposits are enriched in quartz as unstable minerals disintegrate in transit. It has the highest proportion in the sampled sediments, with an average composition of 59.25%. Albite is next, with an average of 16.45%. Albite is plagioclase feldspar, which was originally found in acid and intermediate igneous rocks. Due to their instability, they easily disintegrate into secondary clay minerals. This is why their proportion is low, ranging from 3.8% to 26%, in the sampled deposit. Orthoclase is alkali feldspars and a major component of acid igneous rocks as well. It has a range of 3%–28% in the deposit. Feldspars are very unstable minerals and are usually affected by chemical weathering, with the end product being clay minerals. Muscovite is a primary mineral with origin from acid igneous rock like granite and pegmatite. Its presence is also observed in intermediate igneous rocks, metamorphic rocks, and, to a lesser account, in immature sedimentary rocks [13]. It is a stable mineral, and having an average proportion of 4%–26% indicates a long time of travel, causing its enrichment and the disintegration of the ferromagnesian minerals it would have initially been associated with.

The above mineral suit of the deposit and their proportion suggest an acid igneous rock source with long distance and time of travel, which has removed most of the unstable minerals. This is inferred because acid igneous rocks are not in close proximity to the Niger Delta Basin, which is a sedimentary basin.

Atterberg’s Limits

The Atterberg’s Limit is the measure of the water/moisture content of fine-grained sediment. The derived Atterberg’s limit included the Liquid Limit (LL), which is the moisture content at which the soil changes from plastic to liquid state; Plastic Limit (PL), which is the moisture content in which clay soil will rupture or crack and Plasticity Index (PI), is the moisture content in which clay become plastic. It is the difference between Liquid Limit and Plastic Limit (LL – PL). These limits were determined according to the standard of ASTM International [11], and the results are presented below in Table IV.

Table IV. Atterberg’s Limits of Sediments from Swali Community

Sample location and rock type	Liquid Limit (LL) (%)	Plastic Limit (PL) (%)	Plasticity Index (PI) (LL – PI)
SW1 silty clay	29.7	NIL	NP
SW2 clay	40.1	28.7	11.4
SW3 clay	48.3	36.1	12.2
SW4 clay	50.4	32.2	18.2
SW5 silty clay	34.0	NIL	NP
SW6 clay	38.9	22.9	16
SW7 silty clay	46.3	NIL	NP
SW8 clay	42.9	29.04	13.9
SW9 silty clay	48.2	NIL	NP
SW10 clay	49.9	23.8	26.1

Clay is a fine-grained soil that passes through 75 µm (No 200) US standard sieve and has a Plasticity Index of equal or greater than four. The values fall above the A–Line when its Liquid Limit is plotted against its Plasticity Index. Similarly, silt is fine-grained soil that is non-plastic or slightly plastic and passes through the same size of aperture with a 75 µm (No 200) sieve and shows little or no strength when air dry. The Plasticity Index is less than four, and it plots the A–Line when the Liquid Limit is plotted against the Plastic Index (Figs. 2–5).

Fine-grained soil with Liquid Limit value less than 50% (Separator line) is low plasticity fine-grain soil and designated ML, whereas those above the separator line of 50% are high plasticity soils and are designated MH [2].

Standard by ASTM International [2], classify soil as Lean Clay (CL) if the Liquid Limit is below 50% separator line and it plots above the A–Line. The clay is inorganic if the values fall on or above the A-Line on the Liquid Limit versus Plastic Limit plot, and the Plasticity Index is above 7 with little or no effect from organic matter. The soil is classified as silt (ML) if the values plot below the A–Line and below the Liquid Limit separator line of 50% and the Plasticity Index is less than four (Fig. 6).

Fig. 6. Liquid Limit versus plasticity index plot of fine-grained deposits from Swali community.

The analyzed soils have their Liquid Limit values of less than 50% separator line, with only one on the borderline. The Plastic Limit ranges from 22.9%–36.1%, and the Plastic Index ranges from 11.4%–26.1%. 40% of the analyzed soil showed cracking during the Plastic Limit test, indicating their non-plastic nature, while 60% went through the test (Table IV). Two of the analyzed samples that didn’t rupture fell above the A-Line and are classified as Clay (CL), while the other four that fell below the A-Line are classified as Silt (ML). Five of the analyzed samples plotted less than the Liquid Limit separator line of 50%, and only one was on the borderline (Fig. 6). Therefore, the fine-grained deposit in Swali community that the local people call “Clay” is inorganic lean Clay in locations SW6 and SW10 because they fall above the A–Line and have Plasticity Index higher than seven. The deposit can also be said to be silty clay (ML) in the other four locations, SW2, SW3, SW4, and SW8, because they plot below the A-Line but have a Plasticity Index higher than four. The plasticity of the deposit is low in all locations as they fall below the Liquid Limit separator line at 50%, with the exception of one on the borderline (Fig. 6).

Conclusion

The fine-grained deposit from Swali, a community in Yenagoa local government area of Bayelsa state, is mineralogically made up of quartz, albite, orthoclase, and muscovite with average values of 59.25%, 16.45%, 13.25%, and 11%, respectively. No typical clay mineral was observed in the analysis. The mineral suite indicates an acid igneous rock source, a long distance, and time of travel.

The determined Atterberg’s limits included the Liquid Limit (LL), Plastic Limit (PL) and Plasticity Index (PI). The Liquid Limit ranges from 29.7%–50.4%, the Plastic Limit ranges from 22.9%–36.1%, and the Plastic Index ranges from 11.4–26.1. The deposits are classified as inorganic lean clay in locations SW6 and SW10, designated as CL as they fall above the A–Line and have a Plasticity Index higher than seven, and those from SW2, SW3, SW4, and SW8 are classified as silty clay with designation ML because they fall below the A–Line on the plot of Liquid Limit versus Plastic Index and have Plasticity Index higher than four. All the deposits have low plasticity because they fall below the Liquid Limit separator line of 50%, with only one that was on the borderline. Therefore, the deposits in the Swali community of Yenagoa local government of Bayelsa state range from inorganic lean clay to silty clay with low plasticity derived from an acid igneous rock with long distance and time of travel.