Mineralogy and Texture of the Weathered Products from Silicic Lavas in the Sabga Area, North West Cameroon

Products of weathering usually clay minerals, are commonly characterized through mineralogical, chemical and geochemical analysis, emphasizing their implications in industrial applications, initiation of landslides and their im-pact on surface water geochemistry. In this study the vertical variations in textural characteristics and clay mineral type within weathered profiles in the Sabga area were investigated. Two exposed weathered profiles were logged from bed rock to topsoil and each horizon was sampled separately. Granulometric analysis on the samples indicated mixtures of clay, silt and sand size particles in all horizons. Smectite and kaolinite were identified by X-ray diffractometry (XRD) and scanning electron microscopy (SEM). The two logs show that the lower horizons display whitish colors with fine laminations, middle horizons are brownish and the upper horizons display dark brown colors. Graphic mean values for both sections gave values that range between 2.02 - 3.7 φ with an average standard deviation value of 1.9 φ , indicating that the grains are poorly sorted sand, silt and clay. SEM micrographs show laths of smectite in the lower horizons, flakes of kaolinite and laths of smectites in the middle horizons while the upper horizons show flakes of kaolinite with microlites of quartz + feldspars. XRD patterns show broad basal reflections for kaolinite at 2θ 36.8˚ (3.15 Å), smectite at 2θ 33.5˚ (3.34 Å), both contaminated with quartz at 2θ 36.5˚ (3.32 Å) typical for these minerals.


Introduction
The weathering of rocks is the most fundamental geomorphic process which modifies the Earth's surface, and it is one of the vital processes in soil mineral cycling. The intensity and products of chemical weathering in tropical regions are controlled generally by climatic factors (e.g., variations in temperature and rainfall), chemistry, mineralogy, texture and structure of parent rocks, drainage, topography and time [1] [2].
Despite being controlled greatly by climatic factors (variations in temperature and rainfall) [1], is afunction of the chemistry, mineralogy, texture of the parent rock, drainage system and topography, structures, vegetation andtime [2]. For instance studies carried out on weathered profiles of basaltic rocks from the French Massif Central indicated the presence of iddingsite from forsterite, iron oxyhydroxide from augite and halloysite from plagioclase feldspars [3]. [4] studied tropical weathering of recent basalts from Cameroon and described the evolution of the weathered product from parent rock through allophone, halloysite, metahalloysite to gibbsite, as a function of climate, drainage and time. On well drained, upper and middle sections of slopes on suitable rocks, kaolinite develops. On the contrary, downslope or in hindered drainage conditions, kaolinite is gradually replaced by illite/smectite [5].
With the dependence of chemical weathering on the parent rock, the products of weathering on silicic and mafic lavas will differ to a greater extend, since their chemical compositions also differ [1]. For instance study carried out on weathered basalts in Eastern Australia revealed that the minerals altered to a mixture of allophanes, iron oxyhydroxides and clay minerals [6]. [7] showed that Quartz, feldspars, micas and opaque minerals that make up the mineral constituents of silicic lavas (rhyolite to dacite) alter to smectite, illite, kaolinite, chlorite and epidote.
The variability of the geological basement of Cameroon and its location has favored the development of thick weathering profiles [8] [9]. These profiles are generally characterized by the presence of clay deposits which are some of the most important industrial materials as millions of tons are utilized in a vast variety of applications including ceramics, oil drilling and the paper industry. Numerous studies have been carried out on these weathered profiles particularly along the Cameroon Volcanic Line (CVL: e.g., [9]- [16]). These studies have explored the products of weathering, usually clay deposits, with focus on characterizing the type of clay minerals, emphasizing their implications in medicinal and industrial applications including ceramics, oil drilling, pharmaceutical and the paper industry. Most of these works focused more on geochemical and minera-

Geologic Setting
The Sabga region is part of the Bamenda Mountains an extinct volcano of Western Cameroon highland [19] which in terms of volumetric importance is the fourth largest volcanic massif along the Cameroon Volcanic Line (CVL) ( Figure   1). The CVL is a near 1700 km, SW-NE trending, African intraplate "fan shaped", alkaline volcano-plutonic rift zone of variable width (<200 km), with a ca. 66 Ma history of magmatic activity without any systematic internal pattern of age variation [20]- [25]. The CVL structure is a consequence of a series of parallel fissures oriented N30˚E and some transversal events [26]. The felsic lavas are more abundant than their mafic counterparts and are comprised dominantly of rhyolitic lava flows, trachytic, dacitic and rhyodacitic flows as well as welded ignimbrites. The mafic units are mainly alkalis basalts, hawaiites and basanites [25]- [32]. The rhyolitic ignimbrites consist of welded and non-welded massive lapilli tuff and lithic breccias [33]. The welded ignimbrites vary in colour ranging from welded grey, welded greyish brown and dark grey units [25]. The felsic lavas are massive, blocky, moderately to deeply weathered, occur as tongue-like lava flows (coulee), and as domes. The lavas vary in colour from light-grey, grey, dark-grey, dark green to black. They are highly lo-   Weathered profiles and rock outcrops are well exposed on slopes and along road cuts. Nonetheless, accessibility to some exposed profiles is limited due to

Materials and Methods
Two weathered exposed profiles (S1 and S2: Figure 1(d)) 500 m apart from silicic volcanic bed rocks were identified. The surface area for each deposit was to changes in thickness, texture, grain size, grain shape, structures and color. Log diagrams representing each weathered profile were subsequently produced using SedLog 3.0. Twenty samples collected from the two sections were air dried and crushed separately in cleaned mortar to avoid cross contamination. Weighted 200 g of each sample from different horizons were sieved through a 4 mm -63 µm stacked sieve with the help of a sieve shaker for 15 minutes. The grains retained by each size fraction were weighed separately and their values recorded. Their weight percentages and cumulative weight percentages were calculated. Log plots of cumulative percentages finer by weight against grain sizes/phi sizes for each sample were computed in Microsoft excel 2013. The <63 µm were analyzed for XRD patterns and SEM at the Electron Microscopy Unit (EMU) of the University of Botswana, Gaborone. The Samples for SEM analysis were prepared following the methods described by [34] and [35] and analyzed using the Scanning Electron Microscope model XL30 (University of Botswana, EMU). During sample preparation the <63 µm powdered sample was treated by dipping in cross-linked proteins to increase their stability, they were then removed and put in lipids to increase their conductivity and minimize image distortion due to charging. The specimen was then removed and dehydrated by incubation in a series of ethanol solution. To avoid micro ripping of the surface of the samples, the ethanol was replaced by carbon dioxide liquid for 30 minutes. The sample was then mounted on a metal stub using a sticky carbon disc and coated with a conductivity gold metal using a sputter coater. 2D images were generated over a resolution of 4.0 spots with magnifications ranging between 200 -10,000 X with a secondary electron detector at a voltage of 20 Kv. XRD analysis was performed on the dried < 63 µm powder sample using Bruker AXS D8 diffractometer and the pattern recorded over the 1 -70˚2θ range, using a scanning step of 0.05˚2θ at a step count of 10 seconds. The analytical conditions are as provided in [36].

Vertical Variations from Top to Buttom
The detailed logs for S1 and S2 are provided in Figure 3(a) and Figure 3(b). Corresponding field photographs ( Figure 4 and Figure 5) are also provided to reveal the details of the color and textural parameters (physical characteristics) of the various horizons that range from saprolite at the base to topsoil at the top.

Interpretation and Discussion
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Texture and Colour Variations within the Weathered Profiles from the Saprolitic Zone to Topsoil
Logs (Figure 3(a) and Figure 3  . Representative SEM micrographs of clays from Sabga (Section 1) (a) SEM micrograph of S1 Topsoil; non-diagnostic clayey material rich in microcrystic laths of quartz. (b) SEM micrograph of S1H4 composite; lath-like "balls" of interlocking dioctahedral Smectite with few flaky "booklets" of pseudo hexagonal kaolinite, with loose microcrystic laths of quartz and feldspars in the matrix. (c) SEM micrograph of S1H3 composite; wavy "ball" of smectite with few flaky kaolinite. (d) SEM micrographs of S1H2U; Flaky "booklets" of well crystallized kaolinite displaying stepped appearance with few poorly crystalized lath-like "ball" of hexagonal smectite. (e) SEM micrograph of S1H2M; desiccated wavy "ball" shaped flakes of hexagonal smectite. (f) SEM micrograph of S1H2L; Flaky "balls" of well crystallized polygonal smectite. (g) SEM micrograph of S1H1U; Wavy "balls" of interpenetrating and interlocking hexagonal smectite with few flaky "booklets" of poorly crystallized pseudo-hexagonal kaolinite. (h) SEM micrograph for S1 Saprolite, lath-like "balls" of interpenetrating and interlocking dioctahedral aggregate of smectite clay.  by [15]. The grain size distribution can be classed as clayey sand and clayey silt.
Clay sized particles were computed from the percentage of the <63 µm mesh fractions retained in the pan. This particle size distribution of sandy clays and silty clays has been described by [12] where the sand particles were separated by wet sieving and the clay fraction separated through successive sedimentation methods. The proportion of the fine particles especially the clay sized particles determine the plasticity of the weathered product. Plasticity and particle size are amongst the main geotechnical characteristics to determine a convenient choice of construction materials. The particle size distribution of this study conforms  [11] carried out in the Oku highlands and to that [15] in the Sabga area. Thus the Sabga clays can be used for civil engineering purposes, oil refining, heavy metal retention, foundry bonding, ceramics and waste disposal.

Mineralogical Characterization of the Weathered Profiles
Results from SEM micrographs (Figures 8(a)-(g)) revealed that the dominant clay minerals in the weathered profiles are the "ball" shaped interlocking hexagonal smectite and the flaky "booklets" pseudo-hexagonal authigenic kaolinite associated with microcrystic flakes of quartz and feldspars. The XRD pattern of S1H1U ( . The basal reflection in the above ranges at the 2θ angles for the above mentioned minerals are typical for these minerals. Combining SEM and XRD analysis result both gave smectite and kaolinite as the dominant minerals in association with some quartz and feldspars. According to [11], montmorillonite is the smectite type in the Sabga area after treating the clay deposit with glycerol, Li-saturation and heating (Greene-Kelly test). Similar to the results from textural variations there is progressive heterogeneity from the saprolite towards the topsoil in terms of mineralogy. The SEM micrographs for the saprolite and lower horizons (Figure 8(h), Figure 8(g), Figure  8(f) and Figure 8(e)) show that the mineral present is smectite (Figure 8). Midway within the profile (S1H2U and S1H3 composite: Figure 8(c) and Figure  8(d)) the SEM shows that smectite and kaolinite co-exist within the profile. The SEM data for the upper horizons (S1H4 composite and the topsoil: Figure 8(a) and Figure 8(b)) shows that the minerals present are kaolinite and quartz + feldspar respectively. This is in agreement with the work of [5] which stipulates that kaolinite is usually the dominant clay mineral formed on the upper and middle sections of slopes under a good drainage system on suitable rocks while smectite and illite will replace kaolinite as the dominant clay mineral formed down slope under poor drainage systems. [11] in unraveling the paragenesis of the clay stipulated that the Sabga clays are located in both the lower and upper slopes of the landscape with altitudes ranging between 1500 -1700 m above sea level (a. s. l). The mineral paragenesis according to Mache et al. [11] shows montmorillonite ± cristobalite ± feldspar ± kaolinite ± ilmenite ± heulandite. This mineral paragenesis is similar to that of [15]. This paragenesis suggests that clay formation is still in progress with continuous alteration of feldspars. The clay minerals according to both authors are geochemically derived from the weathering of trachyte parent rock. Compared to the minerals identified in the present study for weathered products developed from rhyolites, the mineral paragenesis is similar but for the oxides of iron and heulandite which have not been identified may be due to their very low content. The presence of smectites and kaolinite as well as fine particle sizes gives the Sabga clays the potential to be used in pharmaceutical industries as reported by [15], after treatment to reduce toxic chemicals and crystalline quartz. A combination of kaolinite and smectite, and fine particle sizes could contribute to the alleviation of gastrointestinal discomfort related to diarrhea and the elimination of toxins in geophagic individuals. Similar to previous studies, no hydrothermal minerals were identified. [9] suggested that the absence of hydrothermal alteration minerals such as pyrite, hydroxyl apatite or any other associated ore minerals indicated the residual origin of clay deposits. Therefore the Sabga clay deposits are residual in origin with the clay minerals formed from the meteoric weathering of feldspars since no hydrothermal alteration minerals have also been identified in this study. The kaolinites from Sabga are similar with the quartz-rich kaolinites from the Mayouom deposit in West Cameroon [38], kaolinites from the Douala Sub-basin [16]. The book-like nature of kaolinite flakes suggest their transportation and deposition in piles [39]. According to [11] the main smectite clays from Sabga area are montmorillonite. These smectites are different from the quartz-rich smectites developed in confined environments within Sahelian climate conditions as in northern Cameroon.

Conclusions
Textural and mineralogical characterization of rhyolitic weathered profiles carried out in the Sabga area, NW Cameroon has revealed that particle size distribution for the samples from the saprolite zone towards the topsoil range from poorly sorted sand, silt to clay at different percentages. The dominant clay minerals found in the weathered products are smectites and kaolinite with variable amounts of quartz and feldspar. Smectite laths occur in the lower horizon, flakes of kaolinite and laths of smectite in the middle horizon with the occurrence of kaolinite flakes, quartz microlites and feldspar in the upper horizon. No hematite, heulandite and hydrothermal alteration minerals have been identified. The presence of smectites and kaolinite as well as fine particle sizes give the Sabga clays the potential to be used in pharmaceutical industries after treatment.
The weathered profiles show clear vertical heterogeneity in color, texture and structure from the saprolitic zone (white mottled with grey, finely laminated clayey sand)-the middle horizons (brown, clayey silt with cooling cracks)-upper horizons (reddish brown, clayey silt with rusty brown leached zones) to the topsoil (dark brown, clayey).